What is likely culprit when O2 pegs at .9 volts and stays there indicating engine is rich. Then as this continues for 15 - 30 seconds, the LTFT eventually climbs to +25% or more. Computer calculates mileage that should be 30+ instantaneous mpg to be around 22 - 24. STFT eventually follows within seconds of the LTFT to being high readings. This consistently starts when car is under light load. Typically around 5 - 7% throttle.

O2 will remain flat at .9 volts until I stab the throttle or coast at 0% throttle. O2 will eventually resume sine wave and LTFT and STFT numbers will drop to 1's and 0's and adjust to negative numbers. Idle, Medium load or WOT I have no issues. It seems to be at very light throttle that will send O2 into flat line .9 volt reading first, with fuel trims to follow a few seconds later. Why would fuel trims try to add fuel when the O2 says its already rich? Car is lightly modded V6. Supercharged, with headers, exhaust, CAI, Rockers.


Sorry this is so long getting back to everyone. Still same issue for this 2003 Buick GS 3800 Supercharged. I get the suggestions. Checked them all. I thought the CAI may be the culprit, but the stock filter box doesn't improve the issue. This only happens when the outside temperature is 80 and above. Gets worse the hotter it is. Car runs perfect when its 50 and below. hardly an issue, however, occasionally I still see the O2 "stick" on .9v. However it appears the fuel curve moves rich before the O2 sensor reacts. Its very quick, but I think the computer goes rich and commands a richer mixture before the O2 reacts. Got new injectors, cleaned the throttle body, all new vacuum hoses. Could the hotter air affect the resistance in the MAF and its sending wrong data to the computer to go richer than needed at very low throttle positions? At 12 -15% throttle, there are no issues. But below that it is hit and miss if the car goes rich. What I don't understand is why the O2 can go up to .9v but the fuel curve never reacts to go lean?

  • Could be the o2 is faulty try inducing a rich or lean condition at idle. Capture some data at idle and while driving around and add it to your question. Include stuff like MAF (if it has one) TPSV ECT IAT etc... exclude any PIDs that aren't relevant to fuel control to try to get a higher sample rate.
    – Ben
    Commented May 27, 2017 at 12:33
  • What kind of car?
    – SteveRacer
    Commented May 28, 2017 at 4:49
  • Did you ever resolve the issue? I have a similar problem but my car actually throws the code p0140 = O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2). At idle, the data fluctuates as it should but the moment I start driving the car, it reading stays stuck at about .750 volts.
    – Paul
    Commented Apr 30, 2018 at 11:05

1 Answer 1


The O2 reading is odd given the LTFT, but if it were me I would start with the obvious basics... The KISS rule is and always will be in full effect.

Check for vaccuum leaks. Even small ones. That would explain most of your story, with the exception of the O2 pegging rich.

Be careful of CAI setups - they often change the MAF curve in a bad way, because altering the plumbing may change the laminar flow in the very small sensor cross section where the actual sensor is, usually creating a false lean condition. If you have been properly tuned this should not be an issue.

Check for exhaust leaks as well, although given the LTFT I would concentrate on the intake side at first.

Lastly, check for proper IAC/ISC operation.

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