fault code P0420

**laur** · 10-09-2011,11:32 AM

hi guys,i am geting this code every 300-500 miles with engine management light on,it has been there since i bought the car 1 year ago,every time i clear it and it s fine for another 300-500 miles then the light comes back again,the car drives ok even with the eml on,looking thro the service history it had both o2 sensors replaced and the cat in 2009,i really don t know where to start,car is an audi a4 2.0 alt on a y reg her s a copy from vcds:
VCDS-Lite Version: Release 1.0
Thursday, 28 July 2011, 12:47:18:0
Control Module Part Number: 8E0 907 557
Component and/or Version: 2.0L R4/5VS G 0005
Software Coding: 0011501
Work Shop Code: WSC 63351
1 Fault Found:
16804 - Catalyst System; Bank 1: Efficiency Below Threshold
P0420 - 001 - Upper Limit Exceeded
regards laur

**benjie** · 10-09-2011,11:40 AM

You may find this page useful.

**laur** · 10-09-2011,11:48 AM

thanks,i think i can rule out the cat and the sensors,come to think about it i do hear like a hissing sound at high reves(4000 ish) it dose sound a bit like a leaking exaust,is ther an easy way to check for leaks (exhaust and intake) ??
regards laur

**benjie** · 10-09-2011,11:28 PM

To be honest, I don't know. I would take it to a VW/Audi specialist and tell them what fault code you have, what you've ruled out and ask them to check the exhaust/intake. It may be a common thing that they can identify right away!

**laur** · 21-09-2011,11:56 AM

right,so car keeps failing mot now on emissions,attached a scan of file,vag com is not showing any fault code now,really don t know what to do
please

**TheMatador** · 21-09-2011,03:07 PM

High Carbon Monoxide in combustion products is usually due to insufficient oxygen in the fuel/air mix. This could be due to too much exhaust gas recycle perhaps. Interestingly it passed on the second go round so could you have a sticking EGR valve?
Is the car stock or have you got any mods/remaps that could be affecting it?

**laur** · 21-09-2011,04:06 PM

Car is 2.0 petrol,alt engine,I'm not sure if it has an egr valve,car is totally stock

**TheMatador** · 22-09-2011,09:05 AM

I checked my Haynes Manual last night and there's no EGR valve on the petrol. If the car drives ok it could be the cat has gone - though that doesn't really explain the fact that the test passed on the second run. On the first run it does look like you've basically got too rich a mix going in - high CO and hydrocarbons in the outlet = incomplete burn. Could be the inlet air mass sensor?

**laur** · 22-09-2011,09:16 AM

Cat and o2 sensors are 1 year old,car drives perfect,could a leak in the exhaust cause this??

**TheMatador** · 22-09-2011,12:29 PM

I wouldn't have thought so - plus you'd most likely be able to hear it. The fact that the MoT failed suggests to me that the lambda sensors are ok (as they are indicating the upper limit exceeded) but the cat may not be (was it replaced with a genuine OEM part?). The air mass meter on the air intake may also be at fault. Can you smell fuel in the exhaust at tickover? Unburnt fuel kills a cat converter so unfortunately it might not just be a case of one problem - you could have both a faulty air mass meter which has in turn killed your catalytic converter