So, I've had engine light for a while now and it's time to do something for it.

Code is :

16804 - Catalyst System: Bank 1: Efficiency Below Threshold

P0420 - 35-00 - -

Have had no problems at summer, but at winter it was running sometimes quite roughly for a minute when cold starting...

I somehow tough it was the rear oxygen sensor and without looking with VAG-COM, I today put a new one in... checked for exhaust leaks wich could not hear/feel/see any.

And after short testdrive, code is there again. After looking the values on VAGCOM I'm thinking maybe it's the front one(O2 sensor)? Heres some pics...

As you can see, PostCat sensor(new) voltage is in the specs, but Precat is sticking at 1,5V...

Also coolant temp@vagcom doesn't match the one on dash, normal? On the dash it goes quite fast to 90C(when you start driving) , on vagcom a lot slower...

So? Am I right with the guess on PreCat Oxygen sensor or any other thoughs?

Specs: Stage 1 remap , DIY 2,75" Downpipe with Magnaflow catalytic converter.

Cat and downpipe is under two years old and doesnt rattle or anything and car still pulls hard so shouldnt be any problem with that...

Edit: And we can't have any engine fault codes or the MOT won't pass, also thinking about emissions....hmm

Edited 3 July, 201213 yr by tuplavee

see http://wiki.ross-tech.com/wiki/index.php/16804/P0420/001056

Yes I know, rear sensor changed and checked for leaks so only ones left is front sensor and cat(wich i dont believe faulty).

I was just hoping a straight explanation for that 1.5V @ Front sensor and then go and change it to new...

change front sensor out of range !!

Bit off topic but found a nice cheat for any engine lights come MOT time, torque android app set to auto delete codes stops the light coming on lol

Sent from my Galaxy S2 not a Crapple!

Check fuse 43 is OK. The heater resistance is between 2.5 and 10 @20 degC between pins 3/4 of the sensor.

Your engine isn't really up to temp. The gauge on the dash has a large pull in range - anything between 80 and 97 will read 90. The VCDS temp is the actual coolant temp.

Well, I just changed my pre-cat sensor(Bosch). After ½hour test drive I didn't get any fault codes/light. Earlier it came up after less than 5min.

pre-cat o2 V is still showing 1,5V @ Vag-com as earlier

Looking at Readiness(what does that mean any way?) everything else PASSED, except Secondary Air Injection.

MOT is reserved on wednesday...

edit: and if i understand right, 1,5V is meaning rich conditions, so i should not be looking leaks at downpipe? Could it be caused by intake leak? MAF?

Edited 9 July, 201213 yr by tuplavee

for readiness see http://www.ross-tech.com/vcds-lite/manual/readiness.html

pre-cat o2 V is still showing 1,5V @ Vag-com as earlier

Does it change when you rev it?

If I was you I would just stick it in for the mot and see what happens. From experience I have only ever seen 1.8T engined cars fail emissions test when the front probe is completely knackered or the coolant temp sensor is reading the wrong way.

Yep with full throttle goes somewhere below 1V and hard engine braking 3-4V

Well cleared MOT today. No remarks.

Emissiontest results (with elevated revs)

CO% = 0,16

CO2 = 15,4

HC ppm = 45

O2 = 0,19

Lambda = 1,00

Didn't find last years results, but two years and 30000kms ago emissions were bit lower, but back then I had OEM downpipe and cat

edit: Limits in Finland are CO% max = 0,2 and HC ppm max = 100 , +2000rpm

Edited 11 July, 201213 yr by tuplavee