Last week my garage fixed a 2010 Audi A6 with check engine light on. The car was around 190,000 km with BDW engine and 01J transmission.
According to the trouble phenomenon, I thought that the check engine light was on because the pollutant emission was substandard. I used VAS5054A scan tool, a professional VAG diagnostic tool. There was a trouble code 05137. It meant that secondary air system in 2nd cylinder had fewer throughputs.
Based on the trouble code, I analyzed the reasons for this trouble.
(1) The secondary air system circuits might be broken or plugged.
(2) Control valve might be broken.
(3) Air pump cannot work as usual.
I checked that the circuits were ok. And I found that the hose of air pump also can work. I activated the secondary air pump. It worked intermittently. I checked the combination valve. I used compressed air to blow the pressure hose. Gas outputs were less than the ordinary valve. So I checked the combination valve. There were so many carbon depositions.
The combination valve had less ventilation because carbon depositions clogged it. The engine electric control system judged that secondary air system had troubles. It cannot confirm the specific trouble components. The trouble code showed that secondary air system had incorrect outputs. And the check engine light was on.
Cleaned out the combination valve. The trouble was erased.