P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Technical Area Engine, Transmission, Exhaust etc
2

Hi

After some advice on this, my MK2.5, has the engine management ligght coming on and what is saying is the code P0420.

The car had for a short while had a random misfire, but changing the a coil pack seems to have stopped this. I know the obvious answer is the misfire has goosed the cat, but since the misfire the car has recently sailed through its MOT, so the CAT appears to be working.

 

Any advice before i start replacing expensive parts?

 

Many Thanks

Paul

3

Disconnect the battery for 20 minutes.

Before reconnecting it push down the foot brake for 20 secs.

Reconnect the battery.

Is the engine check light still on.

As an engine check light is an MOT falure I cannot understand how it sailed though an MOT if it was on.

4

P0420 does seem to indicate that the cat is indeed ducked - a case of neat fuel getting on a very hot cat, superheating the core and melting it, I would hazard to guess.

Try Drums recommendation of resetting the error code making sure that it has cleared.  If it recurs it’s new cat time, sorry.     

5

The engine light being on is not an MOT failure.

 

6

Hi

 

Thanks for the reply, I have a fault code reader that will reset fault, hence why light not on for MOT. Sorry should have made that clear.

 

So new cat rather than busted sensor? I suspected that to be the case as it seemed linked to the misfire

 

thanks

 

Paul