Rear O2 Problems

[GrassyKnoll]

This is the first problem that has me completely baffled.

So the way I understand it, oxygen sensors detect a voltage which corresponds to the oxygen level running through the exhaust. So, I have installed a new Random Technology High Flow cat. The front oxygen sensor is fine. However, the rear O2 sensor is detecting a high flow. This is also expected but dealing with the CEL is troubling.

The problem in full:

The default voltage for the sensor is ~.45 volts. With a high-flow cat, the voltage runs around .6 volts. This triggers a P0420 DTC because of hte high flow. So, I thought I would buy a check engine light eliminator. I bought a PEP eliminator and to my dismas, still got a CEL. Upon review, I discovovered that it was a P0136 DTC which indicates a low circuit voltage with the rear O2. After researching, I discovered that a CEL eliminator halves the voltage from the rear sensor. So, the .6 volts is halved to .3 volts which causes that CEL

It seems no matter which was I will get the P0136 or the P0420 but I cannot get it to stay off. Anyone have any suggestions? Is it possible to get an adjustable oxygen sensor or a way to disable the rear O2 sensor entirely??

[shazapple]

Determine the current going to the rear O2 sensor, buy a resistor that will lower the rear voltage to the desired amount.

Resistance = 0.15V divided by measured current

[GrassyKnoll]

good thinking. I should brush up on my electrical skills ;-) I should be able to buy everything I need at Radioshack, no?

[Wiscon_Mark]

V=IR (voltage = current (x) resistance)

the derived equation from what Lee said.

I <3 physics :)

[Huffer]

That's funny you get the same error P0420 I get, and all I did was remove the front cat - went with a semi-generic replacement catalytic convertor.

I get the CEL every 2 months now, I use a $50 OBDii Actron code reader to delete the codes, no problems.

Nice thing is that I can use the scanner in other vehicles too. :D

[GrassyKnoll]

yea I have an adaptor to plug it into my laptop. It reads codes + does performance testing.

www.digimoto.com (http://www.digimoto.com)

[Plays_with_Toys]

yea I have an adaptor to plug it into my laptop. It reads codes + does performance testing.

www.digimoto.com (http://www.digimoto.com)

Now that is pimp. If and when I get a laptop, that'll be on my list as well.[/i]

[GrassyKnoll]

OK > I have tried a couple of things and what I would be most interested in doing is eliminating the rear O2 sensor completely.

Can anyone think of a way to eliminate the rear O2 sensor?

Is there a way to simply divert the .45 standing power to the output of the sensor so that it always detects .45 (normal O2 flow)? If this is possible, is it good for the ECU to think that the O2 levels are normal coming out of the highflow cat?

[shazapple]

stick in a resistor that uses 0.45V

[BAC5.2]

Here is the story with the rear O2 sensor on OBD2 cars.

The second O2 sensor is a tattle-tale. Like most Check-Engine lights, it indicates a failure that could be an emissions problem.

What the rear sensor does, is read oxygen value. What the ECU does, is checks the signal from the front sensor, and compares it to the rear sensor. What happens during driving is the sensor signal follows a sine-path.

If the rear sensors signal mirrors this, then you have a cat inefficiency.

Here comes the problem. The Oxygen sensor ranges itself on startup. What this means, is the sensor runs through it's voltage range and then "tares" itself to the exhaust gas (I say tare, because I can't think of a good word right now. It's 1am and I took two percocet for my wisdom teeth).

If you use an electronic CEL eliminator the rear oxygen sensor never reaches it's upper limit during it's test, and so it never really operates. This makes it so the sensor doesn't trigger a CEL. It also never sets the readiness code, so a plug-in OBD2 emissions test will fail the car because the rear Oxygen sensor readiness code has never been set. Fail test.

The other option is a mechanical fix. We sell them at the shop (www.gearboxtech.com (http://www.gearboxtech.com), call us and we can hook you up). They are 20 bucks. What they do is they space the oxygen sensor out of the exhaust stream, and they have a small sampling hole in them. This effectively delays the sampling of the sensor. So it still sets the readiness code, but since the sampling is delayed, the "image" of the sine wave never matches the front sensor. No CEL, Pass Emissions, only 20 bucks and 15 minutes to install it.

[Wiscon_Mark]

I have that spacer for my Stromung cat :)