AudiForums.com - Audi a3 vag com errors

Unfortunately the A3 wasn't available in the USA until 2005. So you probably find that my knowledge about this car is somewhat limited. However I can give you some pointers as most Audi's are the same when it comes to sensors and the dreadful check engine light ;)

16806 - Main Catalyst; Bank 1: Efficiency Below Threshold
1. This one Could be a bad Oxygen sensor (or in Europe better known as lambda sensor). Relatively simple fix and compared to the catalyst converter cheap. I would go OEM on this one - you don't want to deal with splicing cables and heatshrink wrap (trust me on this one).
2. If the Lambda doesn't do the trick I would start looking at the catalytic converter for Bank 1. I would however sit down before you start looking at prices those things are expensive! You might want to look at after market one if available but beware that some are so weak it will still trigger the error since Audi's specs are a little tighter when it comes to these checks

16795 - Secondary Air Injection System: Incorrect Flow Detected
1. As always start checking the easy,cheap and most common causes first. In this case I would start with vacuum hoses. I had a 2002 Audi and when I got the errors I found a damaged vacuum hose going from my secondary air pump to the back of the manifold causing incorrect flow readings. Easy 10 fix and the alarm was gone for good
2. If all the hoses check out for your secondary air system - check the pump itself. Normally when they go bad they sound like a dentist drill and get very loud. However there is always a chance you get one that is silently dying

00562 - Sensor for Oil Level/Temperature (G266)
1. Seeing how old your car is and this alarm is only intermittent I would hope you only have a bad sensor. Replace it with the correct part - should be fairly simple as it only holds on by a few bolt on the underside of your oil pan (at least on most Audis).
2. The error indicates some electrical fault only after the sensor replacement I would start checking the wire all the way up to the EPC. Since there is no mentioning of pressure or flow I would think your oil + pump is good.

Good luck