My "check engine" light flashed on briefly the other night. Can anybody tell me what the ECU is sensing to cause it to illuminate this output?
(I assume it is a direct output from the engine management ECU...)

 

There are many sensors on the car, that informs the ECU of a error.
Your ECU will now have stored an or some error codes. You can go to a dealer and get them to put a select monitor on the car to see what the error(s) are or you can get a reading from the check engine light by connecting the two black wires under the steering column and turning the ignition on.
Have a look at these web sites for more info.
subman
MRT
Cheers,
Ian.

 

Surfed some previous posts about this subject yesterday. I tried the specified green-to-green and black-to-black connections, but didn't get any joy.
Looks like it should have just been black-to-black!

And...Ta-ta! Oxygen sensor. Nuts. That looks expensive...

 

I had the error come from the lambda sensor. Subaru charged me £150 ish to put a new one in, for the light to still come on.
It turned out to be the wires round by the battery and the earth straps that just needed cleaning, try this first it may save you some money.
Cheers.
Ian.