Fault Code P0420

[dudley6623]

Had fault code P0420 CATALYST SYSTEM EFFICENCY BELOW THRESHOLD BANK 1 come up today, anyone had this before and what it could be? (54 plate vectra sri 2.0 turbo)

many thanks

[djlee]

This error code gets returned when the post cat sensor (the sensor AFTER the cat conv.) returns what the ECU regards as a bad value. This is usually either:

a) A bad cat.
b) a broken sensor
c) The emissions from the engine itself is unusally high (i.e. it's not recycling waste gas properly).

Easiest thing to start with is the EGR value. If that's faulty, then it might not be recycling exhaust gases properly, which will cause higher emissions and could produce higher than normal readings from the post-cat sensor

Then you really need to get the live sensor data read. Get the car up to temp, then get watch the values coming from the pre and post sensor. The pre sensor (before the cat), should have a varying return value, however the post-cat sensor, should have an almost static value readout (it MAY vary a little, but it shouldn't be anywhere near as much or as often as the pre sensor, if it's varying too much, then obviously the cat isn't doing its job).

I very much doubt its the EGR, as the system has no way of detecting emission gases, that's why it has two o2 sensors for comparing the oxygen levels, so in theory, as long as the comparison of those values is correct (i.e. varying output vs static output) then this fault code wouldn't get flagged up. But it's not beyond the realms of possibility that VX may have put some upper bound limits on the readout from the post-cat sensor as a "safety net", and its a simple enough thing to check and generally a good idea to do anyway if you haven't cleaned it already.

P.S: I'm not a mechanic, i could be talking through my backside