VCDS code: 16804 - Catalyst System...

[codyS4]

My B6 A4 1.8T has thrown a CEL for the past week so I decided to buy a VAGCOM setup. I did a full system scan and found this:


16804 - Catalyst System; Bank 1: Efficiency Below Threshold
P0420 - 002 - Lower Limit Exceeded - MIL ON

Readiness: 0000 0000


Ross-Tech wiki says it could be from an intake or exhaust leak, bad catalytic converter, or O2 sensors. The engine's power is fine and the car runs as it normally does aside from the CEL. I think this rules out intake or exhaust leaks. Catalytic converters typically glow red hot when they're bad and make the car run like crap? None of this is happening.

I have noticed that if I gently tap the throttle on my car at idle, just enough to make it jump maybe 50 rpm at the most, the engine will sputter for a half second or so. It doesn't come anywhere close to stalling and the engine runs perfectly everywhere else. It's just this tiny, almost unnoticeable sputter right at idle.

Do you guys think this would be indicative of O2 sensor failure? Is there any type of diagnostic test I can run to confirm this (either or VCDS or with a multimeter?)

I also found this:


Readiness Status: 0110 1101
Exhaust Gas Recirculation: Passed
Oxygen Sensor Heating: Failed or Incomplete
Oxygen Sensor(s): Failed or Incomplete
Air Conditioning: Passed
Seconday Air Injection: Failed or Incomplete
Evaporative Emissions: Failed or Incomplete
Catalyst Heating: Passed
Catalytic Converter(s): Failed or Incomplete

These are all readiness codes. I don't fully understand readiness codes, but I'm lead to believe this isn't anything to freak out about? Something about needing to clear readiness codes then drive 50 miles to really check them? Can someone confirm this or explain it more clearly?

Thank you. [:)]

[codyS4]

For what it's worth, I noticed that little sputtering issue months before a CEL ever came out. Maybe it's completely unrelated?

[MikTip]

How old are your 02's?

Air filter dirty?

MAf sensor dirty?

[codyS4]

How old are your 02's?

Air filter dirty?

MAf sensor dirty?

I've never changed the O2 sensors, so at least 50k miles old. I haven't checked the air filter since my last oil change but it didn't look too bad. MAF I haven't checked at all.

[codyS4]

Could someone with a Bentley manual please verify if block 034 is the O2 sensor readiness test and 046 is cat converter readiness test?

[old guy]

Clicky click (http://www.ross-tech.com/vag-com/m_blocks/030-049.html)

[codyS4]

Clicky click (http://www.ross-tech.com/vag-com/m_blocks/030-049.html)

I'm really new at using VAGCOM...that file says 034 is pre-cat and 044 is post-cat, correct? But there's probably no point in running a post-cat test since a bad catalytic converter will make the post-cat O2 sensor read incorrectly?

[old guy]

Code P0420 is almost always a deteriorating catalytic converter. first thing I would do is to reset all codes and drive the car. If it's the cat, depending upon its condition the CEL will probably come back on in a month or so. After that it will start to come on in a decreasing interval to the point where it just stays on all the time. An inefficient cat doesn't necessarily have to glow red or cause performance problems. It simply loses it's effectiveness. It can be from crappy gas, oil fouled, misfiring plugs over time, running leaded race gas (ask me how I know about this one) etc. If the rear sensor is defective (bad heater, etc) there are different codes to indicate that condition.

[codyS4]

So the cat readiness test won't really show anything unless the cat is truly on its way out? I have noticed that the code comes and goes. When I first noticed it, it stayed on for a couple days then went out on its own. The next time it came on (after refueling my car, probably unrelated) it came on and I reset the code with VCDS. It's been off for about 80 miles now. I guess this could point to the beginning stages of cat failure?

[old guy]

That is what typically happens when the cat starts to lose its efficiency. The cat isn't monitored continuously. The ECM runs an efficiency test and if the cat fails you get a CEL. If it passes the next several test the readiness code will reset and the CEL will go off on its own. This will occur on an increasing frequency as the cat gets worse.

[smyleysjoint]

So Old guy, this subject pops up all the time with bad Cats. What would you suggest for replacements? Audi dealership, millitek, tectonics, 034, universal? I've read that most of these high flow converters set off the code again. I've had this code for a year because I couldn't find an answer and don't want to throw money at a cat that doesn't fix the problem!

[old guy]

I guess it depends upon your expectations. If you want to increase your performance then options like Miltek, 034, Techtonics Tuning, Fast Intentions etc will give you a noticeable performance increase but you will probably have to deal with a CEL at some time or other. The one exception seems to be the Miltek HFC (which also happens to be the most $$ by far). Some guys have used O2 spacers with good results, others still deal with a CEL.

If you just want to remain stock then an OEM cat will be CEL free (but quite $$). I do see where ECSTuning has an "OEM style" replacement for $320 that should work. Clicky click (http://www.ecstuning.com/Audi-B6_A4-Quattro-1.8T/Exhaust/OEM/ES1831735/)

[smyleysjoint]

Thanks Old Guy! I've read the exact same thing in many diffrent threads. Guess Ill be getting one from the dealership! Got a quote and they said $1200 installed. I have the cvt and don't care to mod her, just want to keep her running and pass emissions!

[Antekaudi]

When i had this code i turned out to be defective cat ( it was loose inside )

I ended up getting 034 high flow cat and stage 2 tune that eliminates CEL and lets me pass emissions.

Total cost close to 1k$ which sounds better then 1.2k for oem cat.

[Charles.waite]

When i had this code i turned out to be defective cat ( it was loose inside )

I ended up getting 034 high flow cat and stage 2 tune that eliminates CEL and lets me pass emissions.

Total cost close to 1k$ which sounds better then 1.2k for oem cat.

Yea but would you do that with a CVT?

[Antekaudi]

Stage 1 is not that aggressive.

My stage 2 is still slow as f*ck. lol

[Charles.waite]

Stage 1 is not that aggressive.

My stage 2 is still slow as f*ck. lol

Haha, I agree, my stage 1 is merely adequate. I just think you might as well throw a stack of bills in the blender as go stage 2 in a CVT.

[codyS4]

Ok my CEL has been on for the past week now so I thought I'd run some readiness tests.

I did a throttle body alignment/reset the throttle adaptation (040/060.)
MAF airflow (002): Looked fine at idle, 2.50-2.53 g/sec.
O2 sensor regulation (030): B1S1 fluctuated between 00011 & 00111. B1S2 fluctuated between 0110 & 1110.
Pre-cat O2 aging test (034 basic settings): Gave a B1-S1 OK. Field 3 value was 1.788.
Post-cat O2 sensor control (036 basic settings): Didn't initialize. It just said B1-S2 OK. 0.840 Volts.
Catalytic converter (046 basic settings): Ran and gave a "CAT B1 OK." Field 3 value was 3.16.

I did some Googling and apparently the field 3 value on 046 should be <0.50. This is supposed to be the catalytic conversion efficiency. But the test say OK so I'm not sure what to do about this? The fluctuating values on 030 also weren't clear. One source says 111 is heater on and 011 is heater off. The last two digits can fluctuate between 1 and 0, but should be mostly 1.

Oh and I've noticed for a while that if I just barely blip the throttle my engine will hesitate like it wants to stall. It runs perfectly fine at every other RPM. I checked the misfire counter (aggregate and per cylinder.) Every time I blipped the throttle at idle to make it hesitate, the aggregate misfire counter would go up by one and the per cylinder misfire would increase by one on a random cylinder.

Why can't I have nice things? [=(] Any ideas on what could be wrong? [:(]

[codyS4]

For what it's worth I've never gotten a code or a CEL for the misfires. And again it ONLY happens when I just barely blip the throttle off idle. Still, I don't think it should be doing that.

[old guy]

You might be over thinking this. As I mentioned previously the cat isn’t being monitored in real time. The ECM performs an array of test on the cat under different driving conditions. If the cat fails the test you get P0420 MIL for "efficiency below threshold". The light will stay on until the cat passes a predetermined set of conditions via additional tests. If it continues to fail the tests the light will stay on. You are monitoring the cat performance under one specific condition in real time and it is apparently checking out OK. That doesn’t necessarily mean that the cat will pass all subsequent tests under the various different conditions.

As to the “blip” problem. Do you have a boost gauge? What is your vacuum reading at idle?

[codyS4]

You might be over thinking this. As I mentioned previously the cat isn’t being monitored in real time. The ECM performs an array of test on the cat under different driving conditions. If the cat fails the test you get P0420 MIL for "efficiency below threshold". The light will stay on until the cat passes a predetermined set of conditions via additional tests. If it continues to fail the tests the light will stay on. You are monitoring the cat performance under one specific condition in real time and it is apparently checking out OK. That doesn’t necessarily mean that the cat will pass all subsequent tests under the various different conditions.

As to the “blip” problem. Do you have a boost gauge? What is your vacuum reading at idle?

I do not have a boost gauge. Do you think it could be a vacuum leak somewhere causing these misfires? Now that I think about it my car might be misfiring during cold start ups. It hesitates and shakes during start up (for a few seconds until it settles down), just like when I barely blip the throttle.

[old guy]

Possibly. The ECM can adjust for a small vacuum leak. That might be causing the problem when you transition from idle to a slight rev of the motor. That's were a boost gauge can come in handy. You can perform a pressure test and see if anything shows up.

[codyS4]

I think I may have found a vacuum leak. I didn't have enough time to tear it apart and find out, but I heard a faint leaking sound in the mess of vacuum lines near the intake manifold. At least I think it's a vacuum leak. I don't think I would be able to hear just air rushing through the intake manifold making that noise, right?

[AlyAudi03]

reviving this thread. could a bad maf cause this dtc? first time it's popped up. well besides from when the crank sensor went out......