CEL came and dissapeared
 

Guys a few weeks ago the CEL came on. The code indicated problem with the rear O2 sensor. I reset the ECU applying the brake pedal procedure and the CEL dissapeared. However, after 50 miles it came back on. Therefore, I assumed that my rear O2 sensor went bad. I purchased another sensor and scheduled an appointment with my mechanic to go and change it. The appointment is in 2:30 hours from now. The funny part is that I just went to get some gas and after I started the car the CEL was not there. Should I still go and change the sensor or wait? Further, is there a way to test the O2 sensor and see if it is bad or not? Is it possible this CEL to be a false one? By the way the first thing I did when the CEL came on was to double check my gas cap and make sure it is appropriately closed. The car is MT, 54000, AEM intake, oil catch can, redlined frequently, cat replaced at 47000 miles. Guys any help would be greatly appreciated as always!

So the new CAT has approx. 7k miles on it? Was the O2 sensor changed then as well?
Here's what I would do based on what you wrote.
I think I would wait on changing it out.
However.................next time the CEL comes on I would immediately go get it read and not reset it. You really need to know what that code is....don't guess.
Autozone will read it for free as will some other places.

Alternatively, you could go ahead and replace it(I would do it myself....it's not hard at all) and keep the old one as a back-up until you see if that was really the issue.

Guess I responded the way I did at first because I have a code reader. If I didn't, I might be inclined to go with my alternate opinion.

Mazurfer thanks for replying!

Before the CEL coming on I forgot my cabin lights on and I had to jump start the car. To help the car start I pressed the gas pedal for like two seconds. The car started and run with no problems. The only thing is that after I started the car an awful smell came out for just a few seconds. I assume that this smell was related with me pressing the gas pedal. I run the car for about 100 miles and no problems appeared (no CEL or anything at this point). Further, for most of these miles I pushed the car to see if the cat is glowing red. Nothing, cat looks fine, no smell is coming out, no rattling.

After a few days the CEL appeared. The first thing I did when the CEL came on was to go to advance autoparts and have it read. The code as I said was P0037 = rear O2 sensor. After having it read I reset the ECU to see if it will appear again. 50 miles after I reset the ECU CEL came on again. As soon as it came on I went again back to advance parts to see if it was the same code (and it was again P0037). So I assumed that since the same code came back my rear O2 sensor is bad and it needs to be replaced. I purchased a sensor, closed the appointment and today after getting some gas the code is not there.

The new cat has 7k miles on but not sure if they changed my sensors. The receipt does not say anything about O2 sensors.

They likely wouldn't have changed the sensors.....

You might want to check the wiring if you semi flooded it...when it floods..it gets damn hot when the CAT burn off the fuel..and if they weren't careful when they re/re the CAT the wiring may have been melted...

Are you talking about the sensors' wiring? If they were fried shouldn't the CEL stay on? Is there a possibility the sensor got messed up because of me pressing the gas pedal (I did it for just a second) while trying to jump start it? What if the unburned fuel I sent got the sensor confused but after everything was burned the sensor started operating again. Is there a possibility for something like this?

For now there is no CEL so I have postponed changing the sensor. By the way I own the car for a year and a half and I have never flooded, except that freaking day that I had to jump start it due to letting the cabin light on:(It started right away though and no problems have been observed since then.

Yes, he talking about the wiring.
Well, if it got hot enough that some of the insulation on the wires got exposed, and the wires moved close enough to some other grounded surface, it could from time to time short. This could also happen if both wires got exposed, they could short together at some times and not others. It's probably worth a look since it's easily seen and gotten at.
I guess it's possible conecrning what you said about getting too much fuel in the when jumping, but I really don't think so. I hope that was it, but not sure how likely that would really be.
Well..............if/when it comes back on, get it read immediately, and then change it out.

Thanks for the input guys. I will check the wires see how they look.