Audi A4 B7 1.6 Petrol failing emissions

[Ralphh]

Hi All,

My first time posting here and any help appreciated.

I have an Audi A4 1.6 Petrol B7 model. It has 210,000km on it.

The car is failing the emissions test but only just, but it has failed 3 times. I have tried diptane and catclean.

Engine management light is showing code P0420 which indicates a malfunctioning Cat I believe.

I brought the car to a mechanic, he tested the oxygen sensors and we replaced the one after the CAT but this did not do the trick. The reason for replacing this was because the resistance was well off when tested. Previously, another mechanic changed the Egr valve, it made no difference.

Any ideas on what the problem is? I’m told that it could be the cat but might not be either. The sensors reporting CAT problems are the oxygen sensor’s I believe, we have changed one and the other tested fine.

other than that the car is mint, she’s a Petrol, hard on fuel and I must admit, is a bit Smokey at the exhaust.

Any help appreciated.

Thanks.

[Steve Q]

What if you change the cat? 

[Ralphh]

39 minutes ago, Steve Q said:

What if you change the cat? 

Thanks Steve. I’m not sure it will solve the problem, it could, but it’s an expensive procedure that may not be the solution

[Sid2020]

My first thought was the same as Steve Q’s. At that mileage the cats best days are far behind it

[Steve Q]

Alternatively change the other oxygen sensor to see if it helps. But you might be wasting money if the diagnostic test has said it's the cat. 

[Ralphh]

Thanks Steve, that could be done but where does the ECU get its data for the CAT, off the oxygen sensors I believe, as there are no actual sensors on the CAT itself?  So the fact that the ECU is throwing up the code means that the oxygen sensors are probably working…