lambda and afr

after watching this course (choosing the correct afr) I was curious so went to read afr and lambda on my car (mk7 gti stage 3), and I noticed that lambda is fluctuating between 0.2 and 1 at idle and the short fuel trim was reading about +14, I didn't understand how lambda is showing rich fuel mixture and short fuel trim is showing lean mixture. Just to give you more information 2 weeks ago I started getting a o2 sensor circuit bank1 sensor1 low voltage every couple days after clearing the code, so do you guys think that my o2 sensor is faulty and reading wrong values?

also i didn't feel anything wrong with the car, she idles and run normally.

Can you double check your values. A Lambda value of 0.2 does not make sense unless that is an error code from the wideband controller. Not sure what the +14 means, but if it's +14%, then you are right, the tune is lean, and the short term trim is adding 14% fuel. The Lambda reading will be after the fuel trim has been applied, so you would expect it to read close to the target value probably Lambda 1.00. Lambda values need to be provided with at least 2 decimal points of precision for any meaningful work.

If it's a factory tune that is off 14% in steady state operation, I would investigate things like a clogged fuel filter, or failing fuel pump that could cause the car to run lean.

I checked again, went for a short ride to get it to normal operating temperature, after checking the live data I noticed that lambda was stuck at 1.00 and not moving at all, same for the STFT stuck at 0% even when giving it gas, i shut off and started again the engine, the two got back to reading normally but still had the same issue, lambda fluctuating between 0 and 1.00 at idle i have some screen shots that you can check. and noticed this time that the STFT was reading about +5% which is ok unlike the first time when it got up to +14%.

you can see in one of the two screen shots that I sent that at WOT lambda dropped to about 0.24 which doesn't make sense like you said.

and no my car isn't running a factory tune as I mentioned before I have a mk7 gti stage 3 but she was running perfectly fine with a STFT of 0.8% before the o2 sensor circuit bank1 sensor1 low voltage code showed up.

so do you think it's a o2 sensor problem?

when a o2 sensor go bad they start giving wrong values or they just freez and stop giving values?

thank you for your help.

So the scale of the supposed Lambda reading, looks more like an Air-Fuel-Ratio scale (0 - 13.2), or maybe battery voltage. Could this really be a narrow band O2 sensor, and just acts like a switch when > Lambda 1.0 (lean) and < Lambda 1.0 (rich). OEM closed loop just adds fuel while lean, then removes it when rich -- causing it to bounce back and forth around Lambda 1.0

The Lambda trace is certainly not a Lambda value. Since it only seems to operate on overrun (when the Engine is slowing), I suspect there is an issue with the O2 Sensor heater or the wiring that drives that.

The data sampling with that scan tool is very slow (like once per second). You will have to be very slow / deliberate when trying to log data to help you tune.

A good first step would be to replace the O2 sensor with a new or known good one, and see what happens.

ok thank you for your help I'll go on and replace the o2 sensor.

just wanted to make sure, so lambda values should be stable around 1.00 at idle for cars with oem camshafts and at WOT should be around 0.84 for FI engines?

thank you again,

Those would be reasonable values. Why don't you start by measuring what it is, then we can decide if changes should be made. There should be some tuners on here with more experience with your specific vehicle. I was trying to provide a range I would expect to see.