P0420 Catalyst System Efficiency Below
Threshold (Bank 1)


ive got this code showing.. its the second time in a month now. the first time it was read, was by a main dealer, he suggested it was due to the map being on its limit??? the code was cleared by said main dealer.

I decided to purchase a DTC reader. the CEL has come back again.. ive read the code and its the same one.. ive cleared the code for now.

any idea why its showing?? is it due to the map?

ok to drive?? and just keep clearing the codes?

does it need mapping again?

 

My first port of call would be checking for any leaks/cracks in the exhaust system from the headers to the cat. Then check the condition of the wiring of the sensor.

Did the main dealer not investigate first time round?

 

P0420 is a tricky code on any make of car.

Lots of faults can cause this.

Air leak (be it in the exhaust or inlet )
Misfire resulting in excessive hydrocarbons overwhelmingthe catalyst(dodgy plugs, leads, coilpacks, or injector/compression issue).
Excessive oil burning (as above)
Faulty pre-cat sensor (upstream)
Faulty post-cat sensor (downstream)
Sports cat/aftermarket exhaust (a common bodge on these aftermarket systems is to relocate the post cat sensor out of the main exhaust stream).
Map issue.
Dead or ineffective catalyst (rare unless its a non-genuine aftermarket cat (i.e like React) or a sports cat).

The latter point on cats made by React I can't stress enough - Euro car parts supply these and every one I've fitted has had issues with either quality or fitment. Not fun when its on a car where you have to remove the subframe to replace it because of a leaky weld...only to find the replacement was worse!....their warranties don't cover the wasted labour.

If the fault is a faulty xoygen snsor I always advise replacing both, even if you "think" the other one is OK. I have seen cars with faulty post cat sensor throw a tissy when just the one sensor is replaced...becuase the ECU uses it as part of the methods for calculating the aging of the pre-cat sensor and cat efficiency, one of the flaws with this is the calucations can assume both sensors age in the same manner and rate. Can't say if this is the case on Scoobs - certainly older ones aren't that clever (thankfully), well not yet anyway (wait for Subaru to get the direct injection engines to come out, then things will be fun ).

 

In an ideal world with O2 sensors working OK and no leaks anywhere this code is saying that the catalyst is past it either via age, or damage from misfire etc. or poisoning (contaminated)

Sometimes you can go out and give it a good blast and it can recover.

 

isnt this code removed during a remap anyway that has had an exhaust change ?

 

i used to get it a lot if I was getting the cat quite hot, get it removed at remap as it will come up a lot

 

I've got that code on my bug wrx but only because its had a full decat and aftermarket exhaust. The problem is, if you leave it and something else goes wrong you may not know because the CEL is already on.