Oxygen sensor - the ongoing saga

GPFred · 03-26-2007, 11:47 AM

I have a 2000 GP SE with 103k that has been having an on-going issue of a low-voltage code being set on the bank 1 sensor 1 O2 sensor. So far, total repairs are closing in on 2K over the last couple of years. The most recent was the dealer claimed it was the MAF sensor being in the bottom end of its range that was causing the code to be set, this after my having to pay to replace the Cat that I insisted wasn't bad. Anyway, after 3 weeks of driving the car, the same code popped again on Saturday. The dealer has me bring the car back this morning and they were going to call in some tech support to see if they can solve this issue. I was so angry that I really considered trading the car this weekend for a new Taurus. I have paid the equivalent of 2 years of loan payments in repair bills for this piece of sh!t. I like the car, it rides nice and runs great when the sensor is running properly, gets great milage as well. I am just at my wits end with this. I will post again what the dealer says this time. I wonder if this would fall under the "lemon law".

richtazz · 03-26-2007, 02:53 PM

can you get the exact code (p-0xxx) and post it here? that way we can see if you're being sent in the wrong direction.

GPFred · 03-26-2007, 04:07 PM

Same code as before P0131, low voltage bank 1 sensor 1. I forgot to mention that the dealer had also in the past replaced the wiring bundle that had the connection for this sensor in it.

richtazz · 03-26-2007, 04:53 PM

I would check the reference voltage from the PCM. You may have a pcm or wiring issue from it causing this code.

GPFred · 03-26-2007, 05:15 PM

As I understand it, the wiring has been changed out. I hope its someting simple as a PCM is expensive whether I go thru parts yard or not. One things for sure, once its fixed, there isn't much left in the way of things to be replaced on it. (Knock on wood!!)

GPFred · 03-30-2007, 12:21 PM

Ok, I'm skeptical, but according to the dealership this has been resolved. Apparently, the PCM would on occasion send an open-loop signal to the O2 sensor after the engine was at operating temperature. Well, the O2 would then trip a low-voltage code. When the dealership would look at the snapshot, what they saw was everything was running fine, the numbers were correct. SO finding what was tripping the code was difficult. They ended up clearing the codes, hooking up a portable computer to the car and taking it out on the road and waiting for the code to trip. They found that the PCM would send an open loop to the heated sensor and then send a closed-loop after a restart. SO the PCM has been replaced and we'll see if this solved the problem.

GPFred · 04-04-2007, 01:41 PM

Well, guess it wasn't the PCM after all. Had the same code pop again this past Sunday and the car is back in the dealership. I'm not sure there is anything left to be changed out. I've researched on the internet about the issue and can't find anything other than changing out the O2 sensor. Thank God that it is covered under warranty. But I am about ready to trade or sell this...car. Anybody have any ideas? Again, this is the bank 1 sensor 1 getting or sending a low-voltage code P0131 code. Can the sensor after the CAT be causing this? I'm am running out of patience and cash. Thanks.

BNaylor · 04-04-2007, 05:17 PM

I highly doubt the post CAT 02 sensor is the underlying cause. Basically, with that DTC setting the engine is running in a power enrichment mode - lean fuel trim. It looks like most of the usual suspects have been replaced.

Here is the basic theory - The PCM module supplies approx. .45 volts to the heated oxygen sensor HO2S 1. The sensor then varies voltage from 1 volt when exhaust is rich to approx. .10 volt when exhaust is lean. The PCM monitors and stores sensor voltage information and evaluates the voltage samples to determine the amount of time sensor voltage is out of range. If the PCM detects HO2S 1 voltage is less than the predetermined voltage, DTC P0131 will set.

Although the 02 sensor was replaced in the past was it replaced again? The other possibility is a wiring problem mainly in the form of a short on the 02 sensor signal line. The final checks would be to ohm out the wiring bewteen the PCM module and Sensor 1 - HO2S 1. Also, a check of the grounding circuit should be made.

GPFred · 04-05-2007, 12:24 PM

Thanks for your response bnaylor. The O2 sensor before the CAT is new and so is the wiring bundle between the PCM and the sensor. A few weeks back the dealership found that there was a ground issue with the Transmission. Apparently the previous owner put a new tranny in and the installer screwed up the grounding wire. That's why its such a stumper with this. I did do some research on the internet for the P0131 code and learned that there is something called the fuel meter assembly and a short term fuel trim. Not sure what these are, but I did relay the info to the dealer. I was told the tech would look into this aspect. I'm pretty mechanically inclined until I run into something that really throws me for a loop, and these two items I am not familiar with. I discovered these were associated with other GM products.

BNaylor · 04-06-2007, 12:10 PM

Really don't know what to say further GPFred but you have some idiosyncracy problem that may or may not be resolvable. How did the dealer go about replacing the wiring between the PCM and 02 sensor? They must have hacked at the main wiring harness from the engine to the PCM to do that. That is alot of work.

GPFred · 04-06-2007, 02:04 PM

Bnaylor, I'm not sure what wiring bundle they changed out, just going by what they told me. The service manager and I are now on a first name basis and he too is stumped with this. They are going to look into a fuel metering issue, something about the short term fuel trim that was mentioned above as well. Running gasoline with the ethanol shouldn't cause this, can it?

BNaylor · 04-06-2007, 07:10 PM

Your running regular unleaded with Ethanol? Really not sure what the side effects would be because I don't run it and don't even have it available. What is the Ethanol mix percentage?

It is a possibility I guess. What I did note is one of the final steps in the FSM stated check for fuel contamination for any P0171 or P0172 problem related to fuel trim.

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