O2 Foolers, The Economical, Electrical Way!!

[GW_2004_S4]

First off let me say hello, this is my first post here at Audizine! I've taken much useful information from this site, and now I feel like I have something worth contributing in return.

I just installed the Magnaflow 16586 exhaust system (2.5" catback with x-pipe and resonators) on my 2004 S4 (LOVE it, by the way), and decided to gut my pre-cats while I was at it. I'll leave the miserable broken manifold stud story for another post...

So anyway, with the pre-cats gutted, you'll obviously throw a CEL, and I have found a fix that works great for me. I've been roughly 250 miles, with about 20 start/stop cycles, and no codes :-)

I was hesitant to use the spark plug defoulers, as I've heard of about 50% success rate. I did some preliminary reading on a diode modification where the diode is in series with the signal wire, and it sounded good so I tried it. That doesn't work either, as it often doesn't have enough voltage to turn on, and creates an open circuit... My mod uses a shunt diode/resistor combination, which is where it differs from the series diode in the other mod.

Here is my solution, which works for me, and I give no guarantees, but the electrical principals show no reason why it shouldn't work across the board.

NOTE: This modification is easily performed under the hood on the green and brown O2 sensor plugs. Locate the plugs at the firewall, and make sure you are modifying the wiring on the O2 sensor side, not the ECU side. This allows you to easily revert back to stock if necessary. I used solder connections for the black wire, and a crimp (with shrink wrap) on the grey wire, because it would not take solder...



This schematic shows my solution, using a 1N4148 axial-lead diode, and a 10k 1/8W axial-lead resistor.

The explanation:

Voltage from the (+) terminal of the O2 sensor ranges from 0.2V-1.2V. The acceptable range (from what I've read) is 0.2V-0.8V. The 1N4148 diode has a turn-on voltage of roughly 0.3V. This is when the diode begins to conduct, and it conducts more as the voltage increases, until around 0.7V where it acts like a short.

When the voltage on the (+) terminal (black wire) of the O2 sensor reaches 0.3V or higher, the diode begins to conduct, and redirects some current (and voltage) through the 10k resistor to ground (grey wire). It does this at an increasing rate as the voltage increases, until it reaches maximum conductivity, around 0.7V. In simulation, this circuit does not allow the voltage at the (+) terminal (black) to be any higher than roughly 0.7V (when the O2 is trying to put out 1.2V).

This means that the voltage varies as it needs to, never reaches zero, and never goes over an allowable 0.7V. Also, there is a direct path from the ECU to either pin on the sensor, if it needs to access it for impedance measuring.

Let me know if this works for you, I hope it does!!

[getslideways]

keep us posted as you log more miles

[tampas4]

bookmarked :)

[svra42.8]

....that software looks familiar for your schematic. Looks like stuff I had from Devry when developing circuits. It appears that it "should" work based on the information you provided. I’ll have to do piggies and give this a go before winter. I was waiting since I just did my emissions test.

[Jorgen]

Mustang guys have been doing something similiar for years. I used to run a set of MIL Eliminators that I made (similiar circuit) when I gutted the precats on one of my old cars. Worked well for me, probably 50-80k miles with no issues.

[GW_2004_S4]

I've logged 11 days, at least 50+ on/off cycles, and about 800 miles. Still good :-)

....that software looks familiar for your schematic. Looks like stuff I had from Devry when developing circuits. It appears that it "should" work based on the information you provided. I’ll have to do piggies and give this a go before winter. I was waiting since I just did my emissions test.

svra42.8: It's MultiSim. Great program for simulation too!

[PashaS4]

Do you have any live pics of the set up or parts?

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[AussieDan]

Neat idea, bookmarked!

[PashaS4]

I will be trying this tonight!

Wish me luck!

[GW_2004_S4]

I'm sorry, but unfortunately I don't have any pictures of the setup... the parts should be:

http://www.ventor.co.in/images/categ...ode-1n4148.jpg

http://ferpinheiro.files.wordpress.com/2012/01/10k.jpg

Sorry I'm sure this doesn't help much....

On a side note, another 12 days or so, about 1500 miles total, still no CEL :-)

[jr1415us]

So this is done on the rear cat wiring?

[PashaS4]

I did it as well but I think i messed up one side so i need to fix it.

[CoolHeadS4]

Have you installed the diode and resistor with the O2 sensor harness still on the car or you removed them?

[guopal.joyce]

keep us posted as you log more miles

[GW_2004_S4]

28 days, 1600+ miles, still good! :-)

This is performed on the rear sensor wiring (the sensor which is installed in the beginning of the post- or rear cat). It can be done at any point on the wires between the sensor and the connector, so there is no real need to remove the sensors. It can be a little difficult to perform with them installed just due to lack of slack in the wires, but there is enough; I did it without removing the sensors.

Just a little advice: if your grey wire is a stubborn bi*** like mine and will not take solder, don't be afraid to use a crimp. Make sure you get the wires deep into the crimp, make sure to crimp completely (do a tug test), and use shrink wrap over the connection. This is what I had to do on my grey wire. The black wire soldered like a champ.

Good luck!

[CoolHeadS4]

I've installed them yesterday morning. They have been installed close to the connector. Not to difficult but need a supplementary pair of hand to solder the one's on the driver side harness. Didn't had any problem soldering on the grey wire.

Now, need to clear the code. Will keep you posted.

[PashaS4]

Hold on do I need to reset my ECU after this?
Or does disconnecting the battery do the trick?

[GW_2004_S4]

I used VAG com and cleared the codes when I did mine. Not sure if resetting the battery is good enough.

[rxbike]

I used VAG com and cleared the codes when I did mine. Not sure if resetting the battery is good enough.

Can this setup be done with inline wire taps between the gray and black wires ? This allows for an easy "kit" to be prebuilt and crimps.

[GW_2004_S4]

Originally Posted by GW_2004_S4 View Post
I used VAG com and cleared the codes when I did mine. Not sure if resetting the battery is good enough.

Can this setup be done with inline wire taps between the gray and black wires ? This allows for an easy "kit" to be prebuilt and crimps.

Certainly. With the understanding that these are low voltage DC signals and over time the crimp splitter may develop corrosion if not done perfectly.

[rxbike]

Certainly. With the understanding that these are low voltage DC signals and over time the crimp splitter may develop corrosion if not done perfectly.

I'll try to make some and shrink wrap the heck out of them.

[GW_2004_S4]

I'll try to make some and shrink wrap the heck out of them.

Great man good luck! If you can, at least solder the diode to the resistor. Then a crimp can be used on either end of the combination.

[S4King]

Never mind found my answer.
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[Renga]

So far so good?

[GW_2004_S4]

So far so good?

So it's been about 48 days and 2100 miles. Still good! At this point, I honestly don't see how I could get a CEL related to this issue anymore. I'm positive the system has done all it's checks plenty by now. I think we need others to give this a try and report with results! I hope we have finally found a solution!

[nrowensby]

If only we could figure out a way to fool the ECU into overlooking a bad heater circuit...?

[GW_2004_S4]

If only we could figure out a way to fool the ECU into overlooking a bad heater circuit...?

That's not an issue you're having with this modification, right? This should not manipulate the heater circuit at all.

I'm assuming you're talking about a completely separate issue?? I could look into the electrical side of things as far as what the ECU is looking for.

[nrowensby]

That's not an issue you're having with this modification, right? This should not manipulate the heater circuit at all.

I'm assuming you're talking about a completely separate issue?? I could look into the electrical side of things as far as what the ECU is looking for.

Yeah, separate issue, my heater circuit is bad, but I also have an HFC, so I'm really not interested in replacing the sensor. If you can figure out what the ECU is looking for, I'd be willing to be a guinea pig.

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[GW_2004_S4]

Yeah, separate issue, my heater circuit is bad, but I also have an HFC, so I'm really not interested in replacing the sensor. If you can figure out what the ECU is looking for, I'd be willing to be a guinea pig.

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Judging by your "Garage" on the site and your profile pic, this is for the B5 model? S4 or A4?

[nrowensby]

Judging by your "Garage" on the site and your profile pic, this is for the B5 model? S4 or A4?

B5 a4, uses same o2 sensors though

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[talkaudi]

Let me know if this works for you, I hope it does!!

Hi, Does any1 know if you need to cut the o2+ and o2ref wires for this diode resister fix? I dont understand how the diode short will stop the o2 from sending over .8 volts to the computer when the o2 is still wired up.

this should work on other cars besides audi right?

[mikeg2]

I know this thread has been inactive for a while, I'm just wondering if anyone else has used this fix and how it worked for them. GW_2004_S4, have you had any issues with this fix since you did it 5 years ago? People still blow up these threads with the spacers and tune options, but I don't see anyone talking about this.

[jr1415us]

I did this, stripping back the insulation, then soldering the parts between the two wires on the sensor. For some reason it doesn't work as it should.

[2004B6S4]

I did this, stripping back the insulation, then soldering the parts between the two wires on the sensor. For some reason it doesn't work as it should.

are you sure you got everything in the right order/direction? The resistor has to be connected to the grey wire, and the diode has to be connected to the black wire. One of those parts (I think the diode but not positive off the top) is also directional as current can only flow one way through it. I was planning to do this and put the pieces together but haven't spliced them in yet, but iirc from googling, the side of the diode with the black band is the negative side and current can only go from positive to negative through the diode, so the side with the black band needs to connect to the resistor and the positive side of the diode needs to connect to the black wire. That is all just based on memory and I could be mistaken so pls double check before trusting that.

edit: I think the way I originally described the diode was wrong, re-worded after looking more closely at the original diagram. If someone more knowledgeable about electronics could critique this it would be great, this was one detail that I don't think was explained well enough in the DIY

[kingkhalilz]

Lol do it right, get a tune

[VinnysS4]

A few bucks vs. $600. I'd try this before a tune.

[mikeg2]

Yeah, I think I'm going to give it a go. The older these cars get, the less I'm inclined to push the engine to it's limits with a tune.

[koolade9]

Lol do it right, get a tune

This

[DarrenOman]

Thought I would bring this thread back to life.

Does anyone have a picture of what the set-up should look like. Its a bit challenging to wire this mod without knowing what it should look like?

[neilpaku]

Does this technique accomplish the same outcome as just adding O2 spacers to the probe in the main cats?