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.

What if you change the cat? 

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

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

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. 

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…

You should have a sensor upstream and another downstream of your cat, ecu compares the readings between the 2 sensors. That’s how it works on most cars but not exactly sure what setup your car may have 

3 hours ago, Ralphh said:

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…

Yep I appreciate that, but if both sensors are working and picking up a fault between them then the natural assumption will get the cat. Hence why the car believes it's the cat. 

Feel free to change the other sensor and let us know the outcome. 

I think I will remove the pre cat sensor myself and clean it, seems pretty straightforward to remove.

 

Any tips on what to clean it with?

If your doing this at home soak it in white vinegar for a few hours 

Thanks for this, might seem a silly question but do you reckon I’d be able to unscrew the sensor and leave the electrical connection connected and soak the sensor there in the engine bay? Only reason to do this is to save hassle of disconnecting electrical connection and re routing cable etc

Can’t see a problem but just don’t turn ignition on