P0420 Catalytic Converter Conversion EML On

[Alan Patterson]

Hi
I was wondering if anyone else has had this and solved it? It's an M43 engined 1.9 2000 W plate.

I keep getting a P0420 Catalytic Converter Conversion error. I have checked the induction side and replaced:
Cam Sensor genuine BMW
New post cat O2 Bosch sensor
Known good MAF Bosch
New Exhaust good quality from Germany including catalyst.
Plugs recent
Clean air filter

Weirdly the EML light always comes on at the same place about 40 miles from home after a mix of open country roads and city stop start.

One thing is that both old and new post cat 02 sensors show 0 volts. Flat line on graph. Which is why I replaced the sensor but now wondering if it's a wiring issue or if the cat was super efficient its using all the oxygen?
It passed its most recent MOT no problems.

Pulling my hair out and I don't have much to start with!!

Any advice much appreciated.

Cheers

Alan

 

[colb]

Good explanation on youtube here

View: https://www.youtube.com/watch?v=Jwh4hJly4hQ

In three parts diagnostics, fitting new cat then check using diagnostics.
Post cat sensor should be a relativly flat line on a diagnostics graph, the pre cat sensor should be ranging up and down the voltage range. Worth looking at what your fuel trims are doing if its adding fuel then start looking for an air leak. A smoke test would be the quickest way of checking for air leaks, it will save you hours.
BMW explanation here re cat replacement and diagnosis method

View: https://www.youtube.com/watch?v=aTNhxvyesPs

 

[Alan Patterson]

Thanks for the video. Mine is closed loop pre cat and flatline post but the problem looks like it's the post cat 0.0 volt output via the code reader. Since my first post I tested the old post cat sensor with a blow torch and it produced the expected voltage so now suspect a wiring issue. Is there a common place to look for breaks? I am assuming the wires to the connector plug into the car?

 

[colb]

double post

 

[colb]

Sensor end check out the wires and socket the sensor plugs into, its hot down there so any heat damage should stand out. Gunged up internals in the socket may be harder to see, spray in some electrical contact cleaner. Othe end of wires end up at the ECU box, check all of the connectors for dirty contacts and see if that makes any difference. You could test the heater circuit in the sensors with a volt meter set to continuity and buzzer setting. If you connect a probe to each of the circuit wires and thy are good you will get a buzzer sound indicating a good circuit.

 

[Alan Patterson]

By way of an update, as I hate it when you read posts and the OP gets loads of feedback and they then don't report back with any solution ( I always assume they just traded the car in or sold it on!) So after replacing vacuum pipes, PCV pipework and valve, , checked wiring, new coil and both Lambda sensors with original Bosch. Drive 40 miles - again P0420 comes up!! Check the MOT emissions report from last MOT (with this Cat ) and its all in spec. Car runs like a dream, pulls well, flat spots all gone, mpg where is should be in the high 30's, rock steady idle, I have reached the conclusion that the IMASAF Cat I fitted last year is the weak link and the ECU is programmed to trigger a code if the Cat output is slightly below 100%.
I plan to refit the original BMW Cat into the new exhaust over this winter using stainless joiners. In the meantime, I have fitted a post Cat Lambda Spacer with some wire wool inserted for good measure. Car runs just the same and over 100 miles - NO P0420 codes or EML! Not a fix in the true sense (as far as the MOT is concerned) but it keeps the EML off for now until I change the Cat!