The car is a 2006 BMW 325i (N52B25 engine). The problem started out as a performance issue (car didn't pull quite that hard), after a while it turned into more serious issues like low rpm stuttering/jerking, increased fuel consumption (x1.5), the latest issue being 3-4 stalls on cold start, then very rough running for the first 2-3 minutes.

I've managed to monitor lambda values and voltages for the O2 sensors (it has 2 banks, 1 probe before- and one after the catalytic converter for each bank). Bank2 looks rich while bank1 appears lean. There's quite a difference between long- and short term fuel trims also. Inspecting spark plugs for each bank confirmed this - bank1 plugs were burnt out nicely, while bank2 plugs were black.

This first image was taken while cruising (partial load, cruise control on). The second one is full throttle at low RPM (heavy jerking/stuttering).

This is how a typical reading looks like This is a screenshot at full throttle

There are two things I'd need some advice on:

  1. I suspect one of the O2 sensors in bank2 are faulty - based on the values it's the post-cat probe, but it's important to note, that if the pre-cat sensor is faulty, it would be more likely to produce such dramatic results, then the secondary sensor.

  2. Can a bad O2 sensor cause cold-start issues? As far as I knew, O2 sensor information was not taken into account until they were warmed up (20-40 seconds at least), so for a cold engine it doesn't really matter if they work properly or not.

+1 - One more thing that can cause different fuel readings between banks - the catalytic converter. Can it be so faulty, that it completely screws up fuel management like this?

Thanks for any ideas or suggestions. Cheers!

1 Answer 1


It's the pre-cat sensors that are used to determine the mixture departing the engine, the post-cat ones are there to monitor cat function. Since the fuel trims adapt to maintain the value at the pre-cat sensor, it'll look good, if it's producing a fairly rational but off-calibrated to the lean side signal, even though the real mix is too rich, per the plug evidence.

On full throttle, the system is likely trying to achieve an impossibly rich mix, so the pre-cat still indicates lean, but the rich, near oxygen-free exhaust may be overwhelming the cat and leaving the post-cat sensor detecting a rich condition.

I'm not familiar with this injection system, but though the O2 readings aren't used at startup until the sensors are warmed and producing rational readings, the 'wrong' trims resulting from the faulty O2 readings are still going to overfuel the bank 2 cylinders even in open loop, probably causing it to flood rather than fire reliably.

A faulty cat should only produce bad readings at the post-cat sensor, which would set the SES light, but the fueling would remain correct.

  • Thank you for your answer, some really useful information in there! So - if I understand correctly - you suggest, that IF it really is one of my O2 sensors that is screwing the mixture up, than it's most likely a pre-cat sensor?
    – galingong
    Commented Nov 20, 2019 at 17:11
  • 1
    Yes, the system is hiding it by compensating the fuelling, but it looks like the bank 2 pre-cat sensor is off. While I don't know how they're used, the parameters "Adaptionsfaktor" and the "correction injection time" adjust the injector timing, and hence how much fuel is injected for a given load/speed, to make the pre-cat reading correct, these slowly adjust over time to make the closed loop error minimal, but if the sensor is producing the wrong voltage relative to the true O2, it'll be maintaining the mix at a wrong value.
    – Phil G
    Commented Nov 20, 2019 at 17:27
  • While what you describe totally makes sense, I swapped the two primary O2 sensors, reset the adaptations, and to my surprise, the fuel-mixture anomaly didn't follow (bank1 still lean, bank2 still rich, while injection correction timing at full throttle trying like hell to equalize the two [bank1 +30%, bank2 -10%, seems like these are min./max. values]). I even decided to disconnect the post-cat sensors to rule them out, and still, the rich side is always bank2, no matter what I do. This tells me that my O2 reading are correct, and something else is screwing up the mixture. I might be wrong.
    – galingong
    Commented Nov 22, 2019 at 19:27
  • It might come down to measuring the actual voltage at the sensors then. There's always the possibility that there's either a wiring issue, or that the input circuitry in the ECU that reads the voltage is misbehaving. It's not a DC voltage, so you need either an oscilloscope or some specialized equipment to measure it.
    – Phil G
    Commented Nov 22, 2019 at 22:18

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