My 99 T plate A6 Avant (Multitronic) will not idle at stable revs. Instead it hunts constantly from about 950rpm down to 500 (ish). Local dealership replaced a split vacuum pipe but this has made the problem worse (or at least different). The hunting is now stronger but comes and goes at 10 second intervals. Putting the car into P or N, or switching the climate control off (or into economy) reduces the problem dramatically. It doesn't hunt if I drive forward slowly, but does if I try and reverse slowly.

A similar problem was fixed last year by replacing the idle control valve (IIRC). Now, they want to replace the ECU for £££££ but don't seem to be able to offer a reason why. It seems like something that a broken ECU wouldn't cause. I could see the ECU causing the hunting but not being the SOURCE of the problem (if that makes sense).

Any opinions? I'm not really up to much DIY but could check a few things if required. I'd just like some options to discuss with the dealer.

As an aside, can the ECU not be remapped/reflashed, rather than completely replaced.

TIA,
bigC

Rather than that, I'd like to see you get a VAG done on the car so that you can read what the codes are revealing. It sounds to me that the dealer/technicians aren't using their VAG/OBD tool and getting true information.

See if you can find someone locally with a VAG that would be willing to test your ecu. You could offer a pint or two at his favorite pub as payment - that's the typical price for the goodwill of a VAG owner.

By the way... Welcome to the forum bigc!