P0420 / P0430 Error Codes

[iwaters]

I have been getting the MIL coming on recently. Especially after a long run. Initially it was just the P0420 code, now I am getting both. I ran through the proceudre in the repair manual to produce the readiness code. The catalyst test failed. The manual said that if no fault codes were stored after this failure then replace the cats. The 0430/0420 codes had re appeared so maybe it means any other codes.

I tried connecting an oscilloscope to the lambda sensors using an insulation piercing probe but couldn't get a decent signal. I earthed to the chassis. Perhaps I needed to earth to the gray wire on the sensor?

I used and OBD-II scope to record the voltages from both the upstream and downstream sensors. The pictures are below:

Upstream Cat:

Untitled

• iwaters
• 12 Mar 2011
• 1

Downstream Cat:

Untitled

• iwaters
• 12 Mar 2011
• 1

The front is switching ok and the rear is steady at about 0.6 V. This was at idle.

It looks to me like both cats are faulty? Can anyone confirm or deny this? The cats from Audi are about £900 each. I've heard that aftermarket cats are no good as they tend to trigger these codes as well because they don't perform as well as the OE ones.

The only thing I can't understand is it was only MOT'd 4 months and it sailed through the emissions test.

The only other fault is it tends to stall at idle when the engine is hot.

 

[Jimmyjaffa]

don't just assume that the cats are U/S, the codes are only a guide. it could just as well be airflow meter (should be change after about 60,000 miles due to unstable readings), coolant temp sender etc. The first scale looks very busy even though the voltage is increasing/decreasing it should be a lot slower and smoother but this may be caused by another faulty part. The second scale looks about right with the voltage reading quite steady. you need to do a bit more investigation first as spending £1800 just to find it runs the same is a bit of a gamble.To me the cat seems to be doing its job so look else where.

 

[iwaters]

Thanks skipster. The CTS looks fine, the dash reading and the ECU are similar and normal and the resistance on all the connectors are the same.

The MAF is still the original I think and it has done 118K so I will look at that.

I hope it is something else as the car isn't worth £1800.

 

[Spark123]

iirc sometimes the lambda sensors are not activated until the engine is hot, don't know if audis do that though.

 

[iwaters]

iirc sometimes the lambda sensors are not activated until the engine is hot, don't know if audis do that though.

They are heated to speed up them working but yes I did wait till they are hot. The tests I did with the scanner don't activate until the exhaust has temp is at least 350.

 

[Mursal]

Try the following for "ideal traces" from the scope

http://www.picoauto.com/tutorials/emissions-components-actuators.html

Switch sensors around and see if the trace stays the same? We had a Polo with the same it was a dodgy Lambda sensor.

If you could let us know how you solve the problem, thanks

 

[iwaters]

Try the following for "ideal traces" from the scope

http://www.picoauto.com/tutorials/emissions-components-actuators.html

Switch sensors around and see if the trace stays the same? We had a Polo with the same it was a dodgy Lambda sensor.

If you could let us know how you solve the problem, thanks

Thanks, looks like a lot of tests to run through there. Hard to find the enthusiasm when the engine runs fine and hasn't yet failed an MOT. I will update when I have done some more tests