S3 Engine issues

My S3 has spent the last 3 months on my parents drive gathering dust. My Dad drives it every week for a few miles just to keep things ticking over. While I'm back home for Christmas I thought I'd take her out for a spin. All was well until about 5 miles in and the car developed what feels like a misfire. I have a liquid gauge on board that is giving me 2 fault codes; 16396, 1670, I've googled the fault codes but I'm not a mechanic and don't have a clue about engines. The car pulls fine until 3500 rpm which sadly suggests a turbo issue The engine revs cleanly when not under load. I've tried searching the fault codes on the net and forum but I'm not finding anything with similar symptoms or just the two codes being kicked up. The car is a 2001 model, with a B5 tip and remap, not sure if that information is relevant or not but it can't hurt. Anyone have any thoughts? Thanks in advance,

Sandy

Already been on Ross-Tech. The idle is fine, it's when the boost kicks in that the problems begin

Hi Tufty,

Thanks for the response, you are correct with your guess, it should've read 16705. The car has 125000 miles on it but the timing belt appears to be in good condition, no teeth missing etc, I'll go down the speed sensor route first and see if that resolves anything. Is this an easily fixed problem, I know a few people handy with a spanner, or one for the professionals?

Ok, had a look at the guide. I'll get it booked into the garage tomorrow, might as well get the timing belt checked out. Thanks for all the help