Audi A4 1.8T P0420 problem

pippip

Hi all,

Car is a 2002 Audi A4 B6 1.8T. About a week ago the engine mgt light came on. Car driving fine. VAG gave me a P0420 002 error

16804 - Catalyst System; Bank 1: Efficiency Below Threshold
P0420 - 002 - Lower Limit Exceeded

I put out the error and it came on again over the weekend. From reading online this seems to point to either a sensor or the cat is gone. Obviously the sensors are the first to rule out.

My only clue is the following - Normally from a cold start the engine would idle at 1100rpm and work its way down to 900rpm after a few minutes. But over the last month or so when I start the car, 2 out of 3 times, the car will go straight to 900rpm and the engine fan is on. Once I drive it for a short while the fan goes off.

Everything else is normal, driving performance, engine temp etc all fine.

Questions is does the funny starting with the fan point to any sensor in particular that could be causing it?

Many thanks.

pippip

Found a identical scenario on an Audi UK forum and coolant temp sensor was the issue and cheap fix so I'm going to change that first.

Will update once I've given it a few days.

danod226

P0420... is a problem with a catalytic converter. It is clogged, and/or a lambda sensor would be possibly burned.