It fails at 2500rpm only with CO 0.5~0.6 and Lambda 1.04~1.05. How can it possible be that emission fails because of HIGH CO, and HIGH lambda reading... It means that engine runs rich and lean at the same time...

High co and high lambda could be because of exhaust leak, but I checked it, by blocking exhaust and no hissing or anything.

(Air filter changed, maf changed (previous was defected, bought new one (got maf'ed...) bought BOSCH OEM, works perfectly).

I tested O2 and AFR sensor with obd2 scanner, when car is at operating temp at idle, o2 sensors works nicely. Afr sensor I think works too, when holding 2000rpm, and instantly press to 4500rpm and let go, afr sensor reacts to that nicely. So sensors not a problem. (maybe they're laggy thats why..)

Another thing what I think might be a solution, but i'm not sure. Maybe it's spark plugs or ignition coil problem ? Maybe a one or a few cylinders misfires, while others works perfectly, and that causes Rich and Lean condition at same time. Before testing I gave car a good 40 min highway run at 4.5k - 5k RPM. So catalytic should be okey, or it's totally defected..

What do you guys think, I will appreciate all answers! (Car BMW,engine n42b18)

EDIT: I want to add a one more thing. When I increase rpm above 2500. CO and Lambda reading also increases. So when increasing engine RPM, CO increases and lambda increases. I think that means, that by increasing RPM, it gets richer and richer, but at the same time more air comes out of exhaust... -Looks like an exhaust leak I will investigate more about this. Or could be bad ignition coil or spark plug, because by increasing RPM not all fuel burns out and is thrown away into exhaust, but that would increase Hydrocarbon HC, but it is FINE... Maybe fuel is partially burned and not all of them, that's why CO increases, and if there is AIR in cylinder that is not used to ignite all of the fuel, it could show HIGH CO and HIGH LAMBDA at the same time ? Or i'm thinking wrong there.

  • 1
    What do you mean O2 sensors work nicely? Just because the computer thinks they are working doesn't mean they are reporting as they should. If you have a discrepancy between emissions data and lambda sensors the lambda sensors are the first place I would look.
    – GdD
    Commented Oct 12, 2020 at 7:33
  • I mean, O2 sensor moves up and down at idle and at high rpm 0,1 volt to 0,8 volt at every second. At idle responses quicker. So you saying that this could be a problem anyways ?
    – John
    Commented Oct 12, 2020 at 7:58

1 Answer 1


The high lambda and high CO seem to be contradictory, but it either means you have 2 different problems or there's one problem causing both. The high lambda means you have an air leak somewhere, exhaust plug test isn't faultless, you need to check more.

Thinking about one problem causing both: If your air leak is upstream of your pre-cat O2 sensor then it will think there's too much oxygen and the ECU will compensate by making the mixture rich. This could result in too much fuel and too much oxygen. You could have a leak in your exhaust manifold, or your manifold may be loose. You might be able to see a bit of smoke coming from it if you look while it's running.

You could also have 2 problems: a rich mixture and an air leak in the system. Checking for the rich mixture first, you seem to have checked the air supply and replaced the MAF, and the lambda sensors seem to be working okay (presuming you have checked the upstream one as well). That leaves spark and temperature sensors. There's an air intake temperature sensor, this was probably integral with the MAF so has been replaced. There's engine temperature sensors, coolant temp, oil temperature or both. If these aren't reading correctly you could be getting too rich a mixture if the ECU thinks the engine is cold when it's hot. So look at your OBD readings for engine coolant temperature, see if they make sense, and see if they change.

If it isn't temperature then it will be spark, you could have a bad plug, fouled plug, weak ignition coil, bad spark lead, etc. Plug condition may give you an indication. Generally a failed spark plug or failed lead will give you a code, but a weak spark may not give any indication.

As for the air leak, that's just a matter of methodically tracing the system.

  • Sorry for my English, but I mean lambda (not voltage). Where a guy puts a emission probe into exhaust and it shows emission readings (CO,CO2,Lambda,O2, and so on). I was talking about that lambda. And yes, intake air temp sensor was a problem at first, but I fixed that by replacing MAF. I'm 100 percent sure Maf is okey and IAT sensor is okey. Engine coolant temperature sensor is okey, when using OBD2 when it is at operating temp is shows 87-93. So question is, could a bad spark, or ignition coil cause High CO, and High Lambda (at emission monitor where probe is in exhaust end)
    – John
    Commented Oct 12, 2020 at 8:44
  • Your english is fine, I should have understood you. I have edited.
    – GdD
    Commented Oct 12, 2020 at 9:04
  • I checked exhaust manifold when doing exhaust plug method, but maybe I missed something. I will try doing a better test next time, but could a tiny little leak cause this? I understand that a bigger leak (a few mm) could cause this, but that leak I would have definetly saw when doing exhaust plug test. Anyways I need to do more inspection on exhaust manifold and all exhaust upstream sensor.
    – John
    Commented Oct 12, 2020 at 9:27

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