vag com code p0420

**kellya** · 27-01-2010,07:29 PM

mk4 golf gti,code read p0420. What does this mean and what do i need to fix the proble. Have replaced lambda sensor as was told by rac who done the diagnostic test first.eml came on again after 2 days. not sure if it could be maf sensor or cat. Help would be much apreciated as car is really annoying me.

**dafydd** · 30-01-2010,05:40 PM

hi there, Google the fault code, then at least youll know what the problem is!!!!

**Crasher** · 30-01-2010,05:50 PM

I am guessing that this is a 150PS model with an AUM engine from around 2001 to 2004? The P code P0420 is VAG code 16804.

16804 (P0420) Cat. Conv. System, bank 1 not effective enough 035 – 001 Sporadic fault

Basically I suspect you left it too long running it with the engine management light on and now the catalyst is faulty and the lambda sensor behind the catalyst is picking this up. In some cases you would need a new catalyst and two new lambda sensors and genuine parts and labour would cost you around £1k. If you run it around for a few weeks the cat may clean itself up and come back to life. I also often see this when people fit cheap aftermarket Chinese Air Mass Meters bought “ont tinterweb”, I had to change one of these on an Golf 4 Anniversary (almost identical to an AUM model) this week for exactly this reason, hopefully his cat will recover but at the moment it stinks so much it smell like some with a bad flatulence issue.

**kellya** · 06-02-2010,07:28 PM

I have replaced the cat,both sensors,maf and air filter all from vw and the light still comes on reading the same fault code.Really stuck for what to do now

**Crasher** · 06-02-2010,07:37 PM

Is it an AUM engine?

**kellya** · 06-02-2010,07:38 PM

What is aum,sorry im female,know how to drive a car thats it lol. It's a 2001 y reg 2.0l petrol gti if that helps

**kellya** · 06-02-2010,09:53 PM

It is not an aum engine,it's 115bhp gti

**Crasher** · 06-02-2010,10:25 PM

Look in the service book for the “motor code”, it will be AQY or APK.

**kellya** · 07-02-2010,05:24 PM

Originally Posted by Crasher

Look in the service book for the “motor code”, it will be AQY or APK.

it is aqy

**herby0001** · 07-02-2010,09:32 PM

Bit of a side track, but this is a useful wiki page, which details engine codes and more for the Mk4.

http://en.wikipedia.org/wiki/Volkswa...d_Golf_Variant