My "check engine" light came on & I was going to book my WH in to get it checked out, I have been putting off buying a code reader and it finally bit me in the a*#.
I have ordered one now.
Then I found info on the web about engine check light and the fuel cap. As I have had trouble with my fuel cap before I checked it out. Removed fuel cap, gave it a blast with my compressor and bingo, the check light is gone. So if your "engine check" light comes on check your fuel cap.
P/S
I believe my fuel cap issues are a result of me over filling the tank right till I can see fuel in the filler neck and diesel getting inside the inbuilt breather/venting mechanism in the cap.