Hi guys, basically I did a vagcom scan and got this:

16804/P0420/001056 - Catalyst System; Bank 1: Efficiency Below Threshold

Now I know I need to change the oxygen sensors and ross wiki told me the same thing but I don't know which oxygen sensor it is. Is it the one on the manifold or on the cat? Pre cat or post cat?

How do I find out?

I have a 1.4S 60bhp 6n2 polo, 2000

Its doing terrible mileage, I get about 50 miles from a tenner these days!

50 miles from a tenner isnt that bad!

oh it is when my car was doing about 70 miles for a tenner a few months ago!!

Saying how many miles you get for £10 is pointless as fuel prices rise and are different all over the country. You need to say how many miles you get to a set volume of fuel to get any accurate measurement.

But petrol has gone up in price in the last few months.. There for you get less miles for your money...

You guys still deviated from his original question, which was which sensor needed to be changed..

If cat efficiency is below threshold then you probably need a new cat. The reason you're getting bad mpg is because the ecu is compensating with putting more fuel in probably.

Hi guys, basically I did a vagcom scan and got this:

16804/P0420/001056 - Catalyst System; Bank 1: Efficiency Below Threshold

Now I know I need to change the oxygen sensors and ross wiki told me the same thing but I don't know which oxygen sensor it is. Is it the one on the manifold or on the cat? Pre cat or post cat?

How do I find out?

I have a 1.4S 60bhp 6n2 polo, 2000

Its doing terrible mileage, I get about 50 miles from a tenner these days!

Bank 1 is the pre-cat probe

Bank 1 is the pre-cat probe

yes true, but it's the bank 2 sensor which tests the efficiency of the catalyst

nevertheless that fault code doesn't necassarily mean the cat or oxygen sensor is broke, it depends on a few factors, running temps, readiness, etc etc