Thread: CEL Codes P0138, P0139

2000 G20T 5 speed.

Uhm, I'm a bit confused, I don't want to replace something that doesn't need to be replaced and end up wasting more money.

If it's the o2 sensor thats messed up I don't mind replacing it I just want to know that it's the o2 sensor first.

I've done some research on google / forums etc.

People say it could be the MAF that needs to be cleaned?

also something about an o2S Simulator?

maybe theres a short somewhere, i'm trying to pass smog and this is causing it to fail.

======================
Rear O2S, Bank 1 Minimum Voltage Monitoring

P0138

======================

Rear O2S, Bank 1 Maximum Voltage Monitoring

P0139
======================

If i replace the rear o2S will the CEL go away?

Any help much appreciated,

Thank you!

99.9% of the time yes. If not you have bigger problems. You have 2 rear o2 sensor codes, the chances of you not having a bad o2 sensor are slim to none.

Thank you for the reply.

I'm just lost on what it is, I think I will clean up the MAF first, then if it doesn't fix the problem I will get the rear o2 sensor replaced!

the smog was all straight besides the "NO (PPM)" for 15MPH.

15 MPH
Max 437
Ave 57
Meas 746
FAIL

25 MPH
Max 724
Ave 50
Meas 276
PASS

I also found a good chunk of info again. So I will start diagnosing tomorrow !

The rear o2 ONLY reports the catalytic converter efficiency. If you receive a CEL for the rear o2, replace it. The MAF has NOTHING to do with the rear o2. It's about as retarded (the rear o2) and useless as a TPMS sensor.

You can use an o2 sim, but it's not as likely to clear the code as just replacing the sensor. Especially if you need to pass emissions.

Get a universal Bosch sensor, they are cheap and should do the job (with some wiring).

Hop on the Bosch Parts Finder, then go to RockAuto and get one. We also have a RockAuto Discount Code thread.

Look to spend about $50 for the sensor.

I know everyone says it's always the MAF (for the 00+ P11's), but that's more of when you have mysterious issues not a definite O2 sensor code.

Thank you for the response!

I'm gonna order it from here.



Another question, anyone know where the real o2 sensor is located?

I'm on FSM and I can't find it, its suppose to be after the cat right?

FSM: Page 9.

It will either be on the pipe after the cat, or on the cat itself on the rear 1/4 of it. Most likely to be facing 45deg from straight up.

Thank you very much for the response, Gonna order the sensor and get it finished!

On 00+ g20's its on the down pipe facing up. Right by the rear motor mount. Those years have 2 cats, so O2 is put after the first one.

Sounds like you got some bad emissions equipment. The rear oxygen sensor is supposed to monitor that. But it's either not connected, or *really really* broken.

That NO (PPM) reading is for nitrogen oxides.
You have REALLY high numbers. Usually resulting from something like a missing or malfunctioning EGR.

I suspect the previous owner tampered with the emissions equipment. How long have you owned this car?