Engine light and OBD reader

[MakaveliX]

Two weeks ago the engine light came on.
Scanned with OBD reader as p0420 which is something to do with the catalytic converter.
Car runs fine with no issues. MOT is tomorrow.

If I clear the fault with OBD app, will the light just come back on again ? Or does the problem need to be rectified by the mechanic and then the light stays off ?

[Gary C]

It will almost certainly come back on, but it will probably stay off long enough to get through the MOT.

Its saying the cat is not functioning effectively (or the O2 probe is on its way out).

Some cataclean and a good thrash might help the cat pickup its efficiency.

[snotrag]

Yes, you need to fix the fault.

The light&code is not the fault - it's what tells you there is something to be investigated and fixed.


For the purpose of your MOT you could clear the codes in the car park and ig might not come back on again for a few minutes/hours.


But there's every chance it will fail on the emissions test anyway.

[Gary C]

snotrag said:

But there's every chance it will fail on the emissions test anyway.

Depends on the age of the car and where you take it.

I have a 2003 polo that I have been clearing the light before the MOT for about 8 years, but it always passes the test.