2001 VW GOLF 2.0 L Cat Converter?

Question

the check engine light is on, because " Main Catalyst Efficiency Below Threshold " " O2 Censor, circuit slow response " and cylinder misfire in 1 and 3, i know the cat needs to be replaced, and thats more or less the reason the o2 censor is bad, but could that cause the cylender misfire in 1 and 3?

Answer 1

Yes.

Answer 2

A bad cat could cause a misfire but it also works the other way around. A bad O2 sensor could also trigger a misfire and a Cat efficiency code.

Life's a bich, huh?

Let's address the misfire first. I'm not sure how the coils are set up on this car. I know from recent experience that a 1.8T Passat has individual coils for each cylinder that fit right over the plugs. If your 2.0 is the same setup you could try swapping the coil for #1 with say #2 and see if the misfire moves with the coil. If it does, that indicates a bad coil. These are pretty cheap from Volkswagon - probably a reason for that.

Possibly you could have coil packs that may work on more than one cylinder, in which case I would expect one coil pack to work on 1 & 3 while the other works on 2 & 4. If you have a setup like that swap the coil packs and see if the misfire moves. If it does, then again, you have your answer.

Barring that, since we have a code for a lazy O2 sensor you would need a scanner to check how the O2 sensor(s) (probably more than 1 in this car) are switching. If they are moving through their normal range and not "sticking" then its not a sensor problem either.

You should also check temps at the before, at and after the Catalytic Converter while the engine is fullly warmed and running. If the Cat is too hot that's not good (seen em' catch fire!). Also, if the temp is too low after (more than 50-100 degree drop after the cat ompared to before the Cat) that's a good indication of a Cat failure.

That's just the real, basic basic stuff that I can think of late at night after a long hard day. How deep into this do you want to go?

Answer 3

How do you know the cat. needs replaced ?? NEVER replace any part till you know the problem. You got it all backward. The misfiring cylinders causes a rich mixture in the exhaust, this in turn messes up the cat. sensor and also causes slow o2 response. The ecm is trying to remedy the rich mixture but is getting bad readings from the sensors. Correct the misfire and I would guess the other problems will go away.