About a year ago, I had my ECM flashed by J-Fonz Tuning (now Epic Tuning Group) with a Stage 2 tune. This included a rear O2 sensor code-out, since I knew my rear O2 sensor was bad and I was planning some exhaust upgrades anyways.

After 2 weeks of running the tune, my engine light came on... P0420. Very odd, since that's supposed to be coded out. I contacted J-Fonz Tuning about it, and I was asked to send in my ECM again. Josh at J-Fonz Tuning mentioned that maybe he had missed a block on the O2 codeout. At the same time, I had the tune adjusted for Bosch EV14 550cc injectors because my stock injectors were on their last leg and the Bosch ones were way cheaper.

After I received the flashed ECM a second time, the engine light seemed to stay off, so I assumed all was well. However my planned exhaust upgrades got delayed by the realities of life, and I never gave it a second thought. So I've been running the same stock exhaust this whole time, but I installed a manual boost controller in parallel with NV75 to limit boost to about 8psi and prevent any issues with exhaust backpressure. The tune actually allows boost to build to over 15psi.

Over the last year, I've been driving the car mostly without issues. I noticed that idle speed could be unreliable, sometimes sticking at 1100+rpm or wavering +- 25rpm while sitting at a red light. I also noticed that sometimes when I suddenly changed from coasting to accelerating, the engine would stumble a bit before accelerating. I assumed this was due to a dying front O2 sensor, since that's the one that handles fuel mixture.

Then 4 months ago the engine light came on... P0420 again!!! I thought this had been coded out?!? It was accompanied by another code which seemed to imply the front O2 sensor was bad as well (something about the front O2 sensor not heating up fast enough). So I installed a new front O2 sensor, cleared the codes, and it never came back on again. Except that none of the drivability problems were fixed. The idle was still unsteady, and the stumble when going from coasting to accelerating was still present. So I attributed those issues to the tune, which may not be taking my 550cc injectors into account properly. Oh well...

Fast forward to yesterday, I had a new 3" exhaust put in, including a 3" high-flow cat. I kept the rear O2 sensor in the stock location to keep up appearances. Now the unsteady idle is much more noticeable because the exhaust is louder. And it only took 2 driving cycles for the engine light to come on... P0420 AGAIN!!! There's no doubt about it, the rear O2 sensor is NOT coded out.

Here's where it gets weird. I figured since it's SUPPOSED to be coded out, maybe unplugging it would solve the problem. So while the engine was idling, I unplugged the rear O2 sensor at the firewall.

Lo and behold! The idle speed smoothed right out, became a rock-steady 840rpm. This happened the very moment I unplugged the sensor. So I took it out for a drive, and my car suddenly drove smoother than it ever has. The stumble between coasting and accelerating is completely gone, I can't even force it to happen. Throttle response is buttery smooth. Idle returns to 840rpm EVERY TIME without fail. And I haven't lost any power at all.

Only problem is, the engine light turned right on due to the unplugged rear O2 sensor, and I can't make it go away short of plugging it back in.

So 2 mysteries here:

- since when does the rear O2 sensor affect fueling? I thought its only purpose in life was to keep tabs on the health of the catalytic converter. Why does unplugging it affect how the engine runs?

- what the hell is up with my rear O2 sensor codeout? I know the tune works, boost ramps right up as it should and my 550cc injectors work fine. But the ECM behaves like the rear O2 sensor is still required.

 

I understand what your are saying. I think even though the first sensor is more important, the second one must be important too because if it wasn't why would they install it. I know the 02 sensor heater core is important because it helps heat up the catalytic converter and keep things running smooth.

And in regards to your tune.... I hope they did their job and tuned your ECU properly but you never know.

 

well no, actually. You've got some misunderstandings in how the system works. The heater core does not heat up the catalytic converter. That's what the secondary air injection is for. The sensor heater core is solely to warm up the sensor itself, because otherwise it can take over 30 seconds for the exhaust to warm it up naturally where it gets an accurate reading after you start the engine. And the catalytic converter has absolutely nothing to do with the proper running of the engine, its sole purpose is to reduce a specific type of emissions (and turn it into a different kind of emissions that's deemed "less harmful"). The rear O2 sensor's only function is to check the health of the catalytic converter. Other than that, it SHOULD have no effect on fueling in any way.

 

They have a rear O2 sensor because it is required by law for emissions purposes. The rear O2 sensors job is to monitor the health of the cat and throw a CEL if it is not working properly. It has no bearing on the fuel trims.

No. You really think that a dinky heating element in an O2 sensor is going to be able to heat up the cat? The cat heats up from the hot exhaust gasses flowing through it, not some tiny heating element.

O2 sensors have a heating element because they are designed to operate within a specific temperature range, namely the temperature range when the car (and exhaust) are fully warmed up.

The resistance in the O2 sensor changes based on the temperature, that change in voltage is the signal interpreted by the ECU. The heating element in the O2 sensor is designed to bring the sensor up to the proper operating temp more quickly than the exhaust on its own.

OP, it seems pretty clear that the rear O2 has not been coded out. If you would rather leave it in place, you could use a spacer to hopefully prevent the CEL from coming on.

 

Oh I see. Well what I don't understand is why don't they install the secondary 02 sensor after the CAT, not right before it?

If it is supposed to solely monitor the emissions and CAT health, why is it before and not after? How can it tell what is going on with the CAT if it is located before the CAT outlet?

 

I was considering that, but I'm kind of annoyed since I paid a lot of good money for this tune and it took 2 tries for them to continue getting it wrong... the initial tune (before adjusting for the 550cc injectors) was a package they offered, nothin custom or fancy. You'd think they could at least get their damn cookie-cutter tunes right.

 

The front O2 sensor tells the engine what the air/fuel mixture is and whether it needs adjusting. That's the sensor that sits between the turbo and the cat. You can see it right there under the hood.

The rear O2 sensor is immediately after the cat. It reads the exhaust to see what the cat did to it. The ECM compares the readings from the exhaust before the cat and after the cat and then determines whether the cat is doing its job. You can see this sensor under the hood farther down the exhaust pipe, just after the hump of the catalytic converter.

 

Well, I have no idea what it means, but I noticed a while ago that the Ross Tech article on fuel trim says this: "The rear O2 sensor is used mostly to monitor the condition of the catalytic convertor, although in some applications it also contributes to trim information."

Link: Fuel Trim Info - Ross-Tech Wiki

 

Yeah I saw that too a while ago and asked about it. What that means is that in some specific vehicles, the rear O2 sensor can contribute to the fuel trim. This is not the case for the B5 A4.

I did find a hint on that page you linked to, though... when no fuel trim tables are generated yet, the ECM defaults to the built-in ones without adjustment. It can take a few driving cycles for fuel trim tables to be generated. The page states that fuel trim tables are deleted when a code is cleared. I'm pretty sure they get cleared when something else happens, like when the rear O2 sensor suddenly disappears (ie. when I unplugged the sensor while the engine was idling). So most likely the ECM spontaneously switched to default fueling values instead of the fuel trim tables, which is why the idle stabilized and the engine ran smoother. It would probably eventually go back to unsteady values after a few driving cycles.

I won't be able to confirm for a while, since I've sent the ECM back to Epic Tuning.

 

as far as I know, the rear o2 sensor does not adjust fuel trims on our cars. I've heard it does from some people, but it just doesn't seem like it would.

for what its worth, I have had no issue deleting my rear 02 with maestro (I have a ch box). no code, no cel with the sensor removed.