I get these readings as showed in the graph below from both o2 sensors and I am having a hard time to interpret them, especially sensor 2 (downstream). The readings are from a short drive around town (grey line = engine load). It's not throwing any codes, but that could have something to do with the ecu remap it has. It doesn't pass s/DTC / readiness though, which is my main concern.

Important notice: it's a heavily modified car (Alfa Romeo MiTo TB Multiair) with a catless downpipe. I use a lambda extender/eliminator of the mini-cat type on the second o2 sensor (which worked flawless before).

I tried several things:

  • changed 2nd o2 sensor (downstream, post cat) for a brand new one
  • checked for leaks between the two sensors (after sensor 1, before sensor 2). There don't seem to be any, but changed the flange gasket before sensor 2 to be sure. Get the same readings.
  • Very shortly and quickly unplugged sensor 1 while at idle just to see/check if sensor 1 is reacting and maybe there is something going on over there.. Voltage rises to almost 1V immediately on sensor 1. Which surprises me... shouldn't it be reading a lean condition, thus a low voltage near to 0V?
  • changed the lambda-extender/eliminator for a brand new one.
  • unplugged ecu to clear it

Nothing seems to have any substantial impact... I hope someone could help me out interpret the graph correctly and maybe point into the direction of a fix.

enter image description here

/Edit: I made another graph from an extended test-drive:

enter image description here

/Edit #2:

I called a (reputable) tuning-garage today which specialises in Alfa's. They gave me the following information:

  • Does not suspect a leak
  • Does not suspect a faulty sensor 1 (or 2)
  • Uses lambda-fakers/eliminator/spacers of the same type I have on my car all the time (Wagner-tuning mini cat type) without any problems
  • Disabling the second lambda-sensor in the ECU could cause problems and thus they always use a faker (I asked if they could just disable it for me)
  • Lambda 1 is wideband
  • He had the same car model (slightly different engine) on the dyno at the moment we called and he checked the idle voltage for sensor 2 on that car for me. This car gave 0,4 - 0,6V at idle.

So, this now feels like I am set even a little further from home.

I also tried numerous drice-cycles without any luck. The readiness test checks everything except the catalytic converter. The catalyst monitor should check when:

  • there are no MIL's and DTC's (check)
  • fuel level between 15 - 85% (check)
  • ECT above 70F or 21C (check)
  • Engine must have run at least 90 sec. (check)
  • Engine rpm between 1350 - 1900 (check)
  • Idle the car for 5 minutes (check)
  • Drive 2 minutes between 30 - 45 mph (check)


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