Overview and Questions about P0491 and P0492 Secondary Air Injection codes

[Snowmonkey]

Since this is a continual problem that this engine faces and has yet to really find a solution simpler than tearing the front of the engine off and powerwashing inside the block, I've been trying to look into it a bit more. I know it has been discussed on the page before but i wanted to combine several threads and add more info I have learned about the system.



So if anyone isn't entirely familiar, these engines have a system where they pump air into a small passage that connects all the exhaust manifolds on each side. This adds extra oxygen to the fuel-rich start up gases, allowing them to burn more and heat up the cats faster. Heating up the cats faster decreases the amount of unburnt fuel and low-temp noxious gases from exiting the tailpipe. Its a clever, if not entirely well executed, system. The problem is that the passages that distribute the air from the pump into each exhaust header are not always pressurized, so exhaust gases circulate freely in normal engine operation. Exhaust gases aren't squeaky clean, so eventually carbon deposits buildup in these tiny (~3/16" i believe) ports and in the engine block passages. Each bank has it own valve to distribute the pressurized fresh air into the manifolds



The system can sense when these passages are pressurized and to what extent by comparing them to various other pressure sensors in the powertrain.



Or if you prefer to see it graphed




The problem I and many others are running into is that The system gets choked with carbon buildup and blocks the passages enough to kick out the code. Since this is an emissions issue, and Audi/VW tend to get sort of touchy about that sorta thing nowadays, they issued an extended warranty to anyone with this engine that says if you get BOTH bank codes before 120k miles, they will perform the cleaning under warranty. It is not a simple process, and the tools are very expensive. by no means a weekend warrior in his garage type job. It requires taking the entire front of the engine off as well as the manifolds, and manually cleaning the passages free of carbon. Audi does this with a power washing hose thing that gets inserted into the passage and blows the carbon out with water. This tends to shoot water into the cylinders through the exhaust valves which then have to be vacuumed out. This is the part that scares me the most. Even some Audi dealerships wont do the work since its so difficult to get right. IF you do decide to bring it in for the warranty work, make sure they have done it before.

The part about BOTH banks need to throw the code has been the biggest sticking point in my case. I always have P0491 as a CEL, but cannot for the life of me get the other bank to throw the code. If i remove the code via OBDII, it will come back within a week, sometimes within days or hours. Also, oddly, sometimes the CEL will go away after an arbitrary amount of time. Sometimes its hours after coming on, sometimes its weeks. (and don't worry, i check the code every few days to make sure nothing else has popped up.) According to a letter i got from Audi of America after i inquired about the warranty work, they will only honor the TSB (number citation needed, I have not been able to nail it down), both the codes need to be thrown at the same time

From what i have learned talking to other owners and my mechanic, having the issue of blocked SAI ports isn't a big deal, since the state i live in doesn't test emissions regularly. I just cant start having the CEL on all the time. I have gleaned that the system is so buried in the ECU that simply coding it out is impractical or impossible without setting off a ton of either problems. I have thought about ways to "fake" out either the codes or the entire system but haven't discussed them with anyone who knows more about these engines than I do. I have the full Audi service manual in pdf form but the section on SAI is a couple hundred pages and I have not been able to fully comprehend the extent of everything yet. I am willing to share it with anyone who is as determined to fix this as I am.

Please comment if you have either of these codes, how it behaves in your car, if you've talked to anyone knowledgeable about it, if they have a solution that doesn't involve a very costly cleaning job, if they had it done under warranty, etc. Any and all information regarding this issue is welcome.

thanks

[Robdukes]

Just a warning. Do not clear the codes and expect them to come back before an appointment with audi. The codes must be active when they diagnose..cant be in history. I had to reschedule my carbon cleaning for next week and hope the codes get thrown again. They were going off on me almost every time I started the car. Mostly after it sat for a while. I cleared them about 10 times. Then of course it goes 2 days without throwing the code right before my scheduled appt.

At least they will have a loaner on hand next week for me. If I had it done this week all the loaners were booked out. And they expect it to be a day for ordering the kit and 2 to 3 days for the work. So with the weekend i would be without my car for 6 days.

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[Snowmonkey]

Just a warning. Do not clear the codes and expect them to come back before an appointment with audi. The codes must be active when they diagnose..cant be in history. I had to reschedule my carbon cleaning for next week and hope the codes get thrown again. They were going off on me almost every time I started the car. Mostly after it sat for a while. I cleared them about 10 times. Then of course it goes 2 days without throwing the code right before my scheduled appt.

At least they will have a loaner on hand next week for me. If I had it done this week all the loaners were booked out. And they expect it to be a day for ordering the kit and 2 to 3 days for the work. So with the weekend i would be without my car for 6 days.

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Exactly, I have been hoping and praying 0492 pops at some point, since only 0491 has ever thrown. its pretty consistent so I'm not toooo worried about not coming back, and ive got 20k miles to go until the TSB expires. I have tried to think of ways to FORCE 0492 to get thrown but havent had any feasible ideas yet.

If you can, ask the dealer to take some pictures of the process, we can add it to the collectice knowledge

[cadguy77]

Does the cleaning prevent it from happening in the future?

More to the point, IS there a way of preventing this from happening?

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[wes8398]

But SAI cleaning and the infamous carbon cleaning (of valves) are two different things; no?

[Snowmonkey]

Does the cleaning prevent it from happening in the future?

More to the point, IS there a way of preventing this from happening?

If not forever, at least for another few years. Most seem to run into the issue between 50k and 100k miles, but I'm sure tuning and where/how you drive will contribute. Anything that would contribute to dirtier exhaust will have an effect.

And no, unfortunately there doesn't seem to be a way to prevent it, just a symptom of how it was designed. Im guessing when they were designing it, the testing engines were using perfect fuel in controlled weather and they underestimated the amount of deposits that would form in the passages.

But SAI cleaning and the infamous carbon cleaning (of valves) are two different things; no?

Yes, they are different, and often confused on this page when people just refer to carbon cleaning. I'm guilty of hijacking a thread about intake cleaning with information about exhaust system cleaning.

[weave]

Guys funny timing, I have my car in for 85k km service via Audi Care today and im getting this exact job done with only code p0491. They agreed on fixing it even tho i cleared it they told me it was on memory. I have never seen 0492. Not sure if the TSB is the same for Canada but its getting done car should be ready tomorrow and I will update you.

Car software is stock and under CPO currently.

[wes8398]

Yes, they are different, and often confused on this page when people just refer to carbon cleaning. I'm guilty of hijacking a thread about intake cleaning with information about exhaust system cleaning.

So is there an extended warranty for BOTH of these issues? I've heard rumblings of a marginally extended warranty on carbon build up on the valves, but never heard of one concerning the SAI. Is it only a TSB (not recall/extended warranty)?

[Sleipnir]

I've gotten both codes, intermittently, since I've bought the car about 2 years ago. I clear it and it comes back eventually. It doesn't throw a CEL, but it pops up during the OBD Eleven scan. I can't say I've noticed any performance deficits or other erratic behavior (like an unstable idle).

I've been trying to diagnose the culprit, but information on the code is sparse. This thread has actually added quite a bit to my understanding. I originally thought it may due to a broken vacuum line.

I recently replaced my PCV valve and ended up replacing a couple of breather hoses and clamps. Drove her through some canyons (got some new tires, too ) and checked for codes.

Nothing.

...well, I got that stupid impedance converter code for the radio, but fuck that code haha...

I have a smog check coming up in the next few weeks, so I suppose I'll find out how bad the impact is on the vehicle's emissions.

Attached is a screen shot of the code. The other code is right above it, but I don't have a Screenshot of the details.

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[Snowmonkey]

Guys funny timing, I have my car in for 85k km service via Audi Care today and im getting this exact job done with only code p0491. They agreed on fixing it even tho i cleared it they told me it was on memory. I have never seen 0492. Not sure if the TSB is the same for Canada but its getting done car should be ready tomorrow and I will update you.

Car software is stock and under CPO currently.

Honestly not sure, although i have heard from other Canadians that it is easier to get it done up there than in the US. Audi of Canada is probably just nicer.

So is there an extended warranty for BOTH of these issues? I've heard rumblings of a marginally extended warranty on carbon build up on the valves, but never heard of one concerning the SAI. Is it only a TSB (not recall/extended warranty)?

I am not sure if there is a TSB/warranty extension for the intake valve carbon buildup. Im assuming if you are still under powertrain warranty and your engine is stock, and your car begins to show definite signs of intake carbon buildup (bad idle, wobbly engine, misfires, rough startup) a good dealer would honor it.

[chi123]

I'm going to attempt to do the SAI cleaning myself this summer. I'm currently at 148K miles. I only had one code for a while and then the code for the second bank appeared like half a year later. I cleared them as well since it was annoying to see a CEL but I'm in no rush to fix it as it doesn't affect the engine performance after startup and I'm not due for emissions testing. It comes back on, but usually randomly. It might come back on after a few start ups or might take weeks. But usually very cold weather causes it to come back on faster. Anyways, I know of an experienced Audi technician and he said the dealer usually charges around $2K for the cleaning as it involves removing the whole front end and dropping the downpipes. No need to remove the manifolds unless you're scared of water running through your cats. It does require both codes for them to do the cleaning but my friend said the technicians can just claim that both codes are present. So maybe befriend an Audi technician if you have only one code? I have both codes but out of luck due to mileage as the dealer has record of a visit where I was over 120K miles. Nonetheless, the part needed for the cleaning costs about $900 along with a pressure washer capable of at least 1900 PSI and a vacuum. My friend offered to do it for half the price of the dealer but that's still more than the cost of the tool so I'm going to tackle it myself. Looks pretty straightforward. Like you said, the scariest part if you don't get all the water out of the cylinder.

[Sjc2014b8s4]

I was wondering, would a catch can system help with this (like the one from ECS)? I'm not an expert but have done a little research, and it seems like the location of a system like that probably wouldn't help with the PCV/oil separator issue, but might help with the carbon build up on intake valves, and with carbon build up in the SAI, since it should help keep the exhaust gas a little cleaner. Is my understanding right, or am I way off? Just wondering because I was thinking about getting a catch can system soon to hopefully push back the service needed for this stuff, I'm at 60k right now.

[Snowmonkey]

I'm going to attempt to do the SAI cleaning myself this summer. I'm currently at 148K miles. I only had one code for a while and then the code for the second bank appeared like half a year later. I cleared them as well since it was annoying to see a CEL but I'm in no rush to fix it as it doesn't affect the engine performance after startup and I'm not due for emissions testing. It comes back on, but usually randomly. It might come back on after a few start ups or might take weeks. But usually very cold weather causes it to come back on faster. Anyways, I know of an experienced Audi technician and he said the dealer usually charges around $2K for the cleaning as it involves removing the whole front end and dropping the downpipes. No need to remove the manifolds unless you're scared of water running through your cats. It does require both codes for them to do the cleaning but my friend said the technicians can just claim that both codes are present. So maybe befriend an Audi technician if you have only one code? I have both codes but out of luck due to mileage as the dealer has record of a visit where I was over 120K miles. Nonetheless, the part needed for the cleaning costs about $900 along with a pressure washer capable of at least 1900 PSI and a vacuum. My friend offered to do it for half the price of the dealer but that's still more than the cost of the tool so I'm going to tackle it myself. Looks pretty straightforward. Like you said, the scariest part if you don't get all the water out of the cylinder.

Its not the water I wouldn't want running through the cats, its all the chunks of schmutz that would get trapped in the matrix and block flow until they burnt off. from the one video i saw of the process it released a bunch of crusty bits that i wouldn't want in my cats. If you know what you're doing and have the tools then you might as well.

I wonder if there's a way you could add some lines and just bypass the manifolds, straight into the exhaust. Less chances for gas blowback and clog up, and if it does just remove the line easy and clean it out.

[Snowmonkey]

I was wondering, would a catch can system help with this (like the one from ECS)? I'm not an expert but have done a little research, and it seems like the location of a system like that probably wouldn't help with the PCV/oil separator issue, but might help with the carbon build up on intake valves, and with carbon build up in the SAI, since it should help keep the exhaust gas a little cleaner. Is my understanding right, or am I way off? Just wondering because I was thinking about getting a catch can system soon to hopefully push back the service needed for this stuff, I'm at 60k right now.

I dont know if anything intake related would help. I dont know of any additives that would have any effect downstream of the cylinder. Would definitely help with intake valve build-up, but thats a whole other can of worms.

[Robdukes]

I got the codes again. Whew!
Ill ask the service manager if they can snap some pictures and or video of the process.
Im also considering asking them for rates on swapping the cats while theyre in there. I have a used set thats in decent shape I picked up off an AZ member. Ive been putting off doing the cats because i just did the mechatronic repair and just like my cats im a bit burned out.

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[wes8398]

I got the codes again. Whew!
Ill ask the service manager if they can snap some pictures and or video of the process.
Im also considering asking them for rates on swapping the cats while theyre in there. I have a used set thats in decent shape I picked up off an AZ member. Ive been putting off doing the cats because i just did the mechatronic repair and just like my cats im a bit burned out.

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Are you under 'standard' warranty still? Or is this something that you're having covered outside of that?

[Robdukes]

Are you under 'standard' warranty still? Or is this something that you're having covered outside of that?

Im at 113k. Just under the 120k extended warranty for the SAI carbon cleaning. The car was covered by original owner with audi care and i purchased audi care plus. But im out of warranty now. I used this audi dealer for all the scheduled services since 70k miles. But purchased at a used car dealer.

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[S4'ed]

I am pondering an electronic "dongle" to plug into the SAI system which always simulates normal airflow signals, to prevent DTC codes if your air passages are clogged.

[Sleipnir]

What is the likelihood that these codes might be related to the PCV valve?

I ask, because I used to get these codes pretty consistently and they haven't reappeared since I changed the PCV Valve.

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[S4'ed]

Not at all related to PCV.

[C6Avant]

I've been driving with these codes for a few thousand miles. Currently at 141k miles. Quoted $1,200 by the local Audi shop to have the cleaning done. No thanks.

One question I do have. Is there any chance this could cause a loss in MPG? I highly doubt it, but strangely enough my MPG has dropped by about 4-5 (new avg 18-20 and used to avg 22-24). I don't see any way the SAI system would cause the ECU to add fuel, but I'm no expert.

Another odd thing is I never had these codes until i had the PCV service and walnut blast of the intake valves done about 6 months ago. As soon as I had that done I started throwing the SAI insufficient flow codes. I'm sure it is possible the mechanic got a vacuum line or some other hose screwed up somewhere, but they can't find anything.

I may get the MIL coded out at some point, but I don't really mind the light being on. I constantly monitor for other codes anyway and it doesn't affect performance at all. In fact the thing has been running great since the carbon cleaning and PCV service.

[Snowmonkey]

I am pondering an electronic "dongle" to plug into the SAI system which always simulates normal airflow signals, to prevent DTC codes if your air passages are clogged.

Ive thought about a dongle, and it could be possible. Would take someone who has very intimate knowledge of the ECU software, of which I am not. I was thinking more of a bypass line that just runs air from the Valves into somewhere slightly downstream of the header flange.

[S4'ed]

No need to know the details of the ECU software. I have a circuit design concept, using a PIC microcontroller, that simulates the five values of G609 pressure sensor output voltages (analog, 0 to 5VDC signal), depending on which of the six "Phase" states it is in of the diagnostic test sequence. (see figure in post #1). The states are easily detected by looking at three inputs: the pump motor voltage, and the two SAI solenoid voltages. You would unplug the pressure sensor, and plug my circuit into the connector where the sensor was (goes to the ECU). Then connect the 3 wires to the pump motor, and each of the two solenoids. The most difficult part of the design is determining what typical "good" pressure values are (the green bands in the figure), from a car that has a properly functioning SAI system (not clogged), but that should not be too difficult to measure with a scope, or a DVM if the duration of the diagnostic states last long enough to take a measurement.

[Snowmonkey]

No need to know the details of the ECU software. I have a circuit design concept, using a PIC microcontroller, that simulates the five values of G609 pressure sensor output voltages (analog, 0 to 5VDC signal), depending on which of the six "Phase" states it is in of the diagnostic tests. (see figure in post #1). The states are easily detected by looking at three inputs: the pump motor voltage, and the two SAI solenoid voltages. You would unplug the pressure sensor, and plug my circuit into the connector where the sensor was (goes to the ECU). Then connect the 3 wires to the pump motor, and each of the two solenoids. The most difficult part of the design is determining what typical "good" pressure values are, from a car that has a properly functioning SAI system (not clogged)., but that should not be too difficult to measure with a scope, or a DVM if the duration of the diagnostic states last long enough to take a measurement.

Quick, someone who has just had the cleaning done grab your scope and take signal (NOT pressure) readings (ahem @robdukes)

That's much less complex than I envisioned. I was imagining it was all done directly in the ECU and not as just signals back to the ECU. shows how much i know about vehicle electronics

Also someone quickly patent this idea so we can then sell it to all the vehicle dongle manufacturers

[Culi]

It is 90mbar of pressure at Phase 1.

Capture.PNG

[Snowmonkey]

Incidentally, my CEL turned itself off today. I think the cold temps have something to do with it.

Will run a scan and see if the code is still buried, just hasnt thrown the code enough.

[Robdukes]

Incidentally, my CEL turned itself off today. I think the cold temps have something to do with it.

Will run a scan and see if the code is still buried, just hasnt thrown the code enough.

The cold temps turned mine on. I got the codes again after sitting overnight in the garage. But didnt get the CEL until the car sat out in the below freezing temps for 8 hours.

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[Snowmonkey]

The cold temps turned mine on. I got the codes again after sitting overnight in the garage. But didnt get the CEL until the car sat out in the below freezing temps for 8 hours.

Interesting.... When is yours going in for the cleaning?

[street2gen]

Would a test pipe file just not take care of this? like 034 and IE have? I know those are designed for test pipes, but just curious if going that route would not be alot cheaper then the cleaning.

[wes8398]

Would a test pipe file just not take care of this? like 034 and IE have? I know those are designed for test pipes, but just curious if going that route would not be alot cheaper then the cleaning.

Test pipes or not, secondary air injection is still a thing though... No?

[Snowmonkey]

Would a test pipe file just not take care of this? like 034 and IE have? I know those are designed for test pipes, but just curious if going that route would not be alot cheaper then the cleaning.

As far as i know about Test pipe tunes, they just fake the O2 sensor inputs, where the SAI pressure sensor inputs would still be there. not 100% sure though. Someone mentioned on a thread a while ago that since its emissions related the coding for it in the engine management is hard to get around. could be wrong, that's just what i remember reading. The SAI system would still try and function, so unless you completely dupe the sensor readings back into the system it would still want to run the pump and open the valves.

[wes8398]

. Someone mentioned on a thread a while ago that since its emissions related the coding for it in the engine management is hard to get around.

We're taking about a Volkswagen product here, right?

[Snowmonkey]

We're taking about a Volkswagen product here, right?

The irony is real

[street2gen]

034 test pipe file disables cold start which is basically sai no??..... I tried asking them about sai specifically but no response.

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[Robdukes]

Interesting.... When is yours going in for the cleaning?

This Friday. They'll order the kit. Likely get it Saturday or Monday. Said Ill probably get the car back on Wednesday.

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[TheKenR]

That's interesting because I had code P0491 set and I assumed it was carbon buildup in the passages of bank 1. When I checked the mode 6 data it showed bank 1 clearly had an issue but bank 2 was perfect (right between min/max of the various parameters). When I took it to the dealer for cleaning they found the PCV regulator was also bad which I did not detect with my scan tool. They changed the PCV regulator and performed the carbon cleaning and now all is good. I always wondered if perhaps it was somehow related to the PCV regulator because only one bank was throwing a code. I haven't read that anywhere until I seen your post.

What is the likelihood that these codes might be related to the PCV valve?

I ask, because I used to get these codes pretty consistently and they haven't reappeared since I changed the PCV Valve.

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[S4'ed]

There is no connection or relationship between the PCV system and the SAI system.

[Snowmonkey]

That's interesting because I had code P0491 set and I assumed it was carbon buildup in the passages of bank 1. When I checked the mode 6 data it showed bank 1 clearly had an issue but bank 2 was perfect (right between min/max of the various parameters). When I took it to the dealer for cleaning they found the PCV regulator was also bad which I did not detect with my scan tool. They changed the PCV regulator and performed the carbon cleaning and now all is good. I always wondered if perhaps it was somehow related to the PCV regulator because only one bank was throwing a code. I haven't read that anywhere until I seen your post.

Veeeery interesting. Multiple sources seem to accuse the PCV of being a part of this. Does anyone with engine knowledge know if a bad PCV can cause dirty exhaust gasses?

[TheKenR]

Veeeery interesting. Multiple sources seem to accuse the PCV of being a part of this. Does anyone with engine knowledge know if a bad PCV can cause dirty exhaust gasses?

Well the PCM will set the 0491/0492 codes based on what the O2 sensors see. So if there is a link then perhaps the faulty PCV valve is changing the exhaust mixture on the passenger side bank and the PCM mistakenly thinks the SAI system is not flowing properly? Only a guess because I don't know how the PCV is plumbed.

[Snowmonkey]

Well the PCM will set the 0491/0492 codes based on what the O2 sensors see. So if there is a link then perhaps the faulty PCV valve is changing the exhaust mixture on the passenger side bank and the PCM mistakenly thinks the SAI system is not flowing properly? Only a guess because I don't know how the PCV is plumbed.

hmm, I didnt think about the oily gases to throw the wrong signal, I was thinking more about oily intake charges causing excessive carbon buildup in the exhaust side ports.