A few months ago the CEL came on. Dealer said that the fault code suggested "cat efficiency low" or similar.

Jim at Star (who supplied the cat) hooked the car up to VAG-COM which showed that the front Lambda probe wasn't heating - hence the inefficiency code.

At the MOT emissions were on the high side.

Then the CEL went out and has been out for 2 months - until yesterday when it came on again. Today on a long run I struggled to get the car over 28 mpg - normally approaching 38.

Problem - duff cat, duff Lambda, or dodgy connection?

The cat efficiency is measured using amongst other things data from the probes. If a probe is showing a heating fault, get it replaced, another symptom of a dodgy probe is poor fuel consumption. By all means check the connections first.

You have 3 lambdas on the vRS. When you replace a lambda with a new one the CEL will turn itself off after some miles. Get the error codes ... it will tell you exactly what is wrong.

My CEL came on yesterday morning.

VAG Com states lambda number 1 but hoping it is wiring (looking at it on Sat) as I hit a large pothole on Sun night :lol: