Went for an MOT, first test failed on high CO AND HC but normal lambda. Got new air filter, cleaned EGR and throttle body and added a cat cleaner into petrol tank. Second MOT failed due to high CO (nearly x2 as last time) and high lambda (1.037, max 1.030) on second fast idle. Although HC were within limits this time on all idles.
Anybody have any idea what this can be? 2002 Seat leon 1.4 petrol.


It's difficult to say definitively without having the car to hand but I'd suggest the next course of action would be to clean up the air flow meter (MAF sensor) and throttle body. You might also need to force the ECU to perform a throttle position relearn. Have you had the car plugged in to a diagnostic tool like VCDS? If it hasn't been scanned for faults, I'd start here.

If you do decide to clean the air flow meter, don't use brake clean or a similar solvent as this can leave deposits on the mesh. Use electrical contact cleaner as this will leave it clean without any deposits remaining.

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  • I don't know where the MAF is on my car, only managed to find the MAP sensor. I can try cleaning that, and the throttle body I already cleaned. I was just wondering if I messed something up since the CO readings were nearly twice as big as my first tests, before all the stuff I've done. I didn't plug it into a VCDS because I don't have anyone with that tool. I will try looking for garage that can do throttle body alignments near me. – LionLeon Jun 13 '19 at 17:35
  • MAP and MAF are pretty much interchangeable terms. You need VCDS (or similar) to get the car to recalibrate the throttle body after cleaning. – Steve Matthews Jun 13 '19 at 17:43

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