02 sensor help

[J.R.]

I have an 04 gxp and need some help. How many 02 sensor are there? I was under the impression it was 3. On my scanner both banks are operating normally. Looking at my stft levels for both banks are normal. However my stft for sensor 12 shows 99.21%. So I replaced the downstream one after the cat along with a new converter and it still shows the same. It's such on that number% and doesn't fluctuate at all. Any ideas and help is appreciated. Scanner shows 11 12 21

[nos4blood70]

The one after the cat only serves to tell whether the cat itself is operating.

Not sure if the N* has one per bank, but the 3800 only has one reading off of the front bank. I would check the upstream ones as those are the ones that affect how the car is running.

Hopefully one of the N* gurus see this soon. Paging Dr. Lane!

[CMNTMXR57]

There are three.

B1S1, B2S1, and B1S2. So two pre-cat (upstream), and one post-cat (down-stream).

Unless there is a specific DTC that requires STFT and LTFT monitoring, this usually isn't as important. What is important is watching the sensors and their "switch rate" in mV. Both pre-cat sensors should be making large, quick, coarse adjustments. A properly operating converter and post-cat HO2S should be making small, finite and slow adjustments, which would be in line with why your LTFT's are at 99% and not moving much (at idle I presume).

The problem with monitoring fuel trim cells, while at idle, is that they usually are trimming anyway.

[J.R.]

Thanks for the reply. Both upstream banks are doing their jobs well looking at stft right around 0. +or-1 to 2 and the voltage switch rates are good. I replaced the cat and the post cat 02. The post cat 02 is showing stft at 99.21% and not moving and its voltage is stuck at 0.045 volts. Is that where it's supposed to be?

[J.R.]

[CMNTMXR57]

Hard to say. Each vehicle is different, has different tolerances, etc...

The key takeaways;

1) It's switching at small, fine increments and at a slower rate than the pre-cat ones.
2) It's within tolerance for the vehicle's PCM, so as not to be throwing a slow response or low efficiency DTC or other I/M system test fault.

[J.R.]

Got the P0173 code today for low voltage to that sensor. Guess I'm going to have to take it in and have them look for shorts or if the new sensor is dead etc. Thanks for the help I appreciate it!

[CMNTMXR57]

I think you mean P0137.

It's not voltage TO that sensor, it's voltage reading FROM the sensor.

Your HO2S's read voltage in millivolts (mV). They have a range of 0 - 1,000 mV. .450mv is stoich. Higher than that, it is running rich (you would expect that in power enrichment mode, or WOT), anything lower is lean.

The HO2S's then "switch" bouncing higher and lower in terms of their mV reading, the mean point of all that switching should be around .450mV or stoich.

This code is when it falls below .025mV for a predetermined period of time.

The testing for this will have you testing both the high and low signal circuits and then the heater control circuit and checking all terminals and connections or an intermittent connection issue. Ensure that there is no silicone/oil/coolant contamination on the sensor, that there is no damage to the sensor or harness.

Then if all that checks out, replace the sensor, and then if that doesn't work, replace the PCM.

I know you said you recently replaced the sensor as well as the cat. Did you use an AC Delco HO2S? Sometimes GM PCM's can be very particular about the sensor (typically it's resistance as part of the circuit), and if it isn't an OEM sensor, issues can arise. When you replaced the converter, ensure that there are no exhaust leaks.

[J.R.]

Yes sorry that's the right code you posted. No exhaust leaks. I put a Bosch sensor in. Do you know the correct sensor I need with the name or part number from rock auto?

[J.R.]

Got the densos from rock auto and things are better. Has anyone ever used an 02 simulator for the post cat? Wondering which one I should try. Thinking about just removing the cat entirely. .

[CMNTMXR57]

I didn't think you could get O2Simms anymore. Been around for years. I used them many times on varying vehicles with no issues.

However, as tuners became more prevalent, I just had them change the DTC type class in the tune from a Class A, set the DTC, turn on the MIL, to a Class D informational code. Not sure if this is an option with this N*.

[Archon]

ZZP still sells the simulator.
http://shop.zzperformance.com/store/p/4 ... lator.aspx