How is the A/C and Check engine light related?
 

If I drive my car with the a/c not engaged everything is fine. I could drive forever and not have the CEL come on. But everytime I turn the a/c and it runs for a few minutes the CEL light comes on and gives me a fault 11 code for air injecton system. I reset the code and drove with the a/c off.........for a few days as a test, then I turned the a/c and within minutes the code was back. I reset the code and then drove the car again with the a/c and the code was back in a few minutes. And just to make sure it wasn't coincidence I rest the code yet another time drove with the a/c off for about 45 minutes then I turned the a/c on the code was back. Anyone know what this is all about?

Perhaps the loading of the engine by the compressor causes the code to be set or a vacuum issue with the air injection system. I can't remember if the W201 HVAC is vacuum operated or not.

how can a vac issue affect air injection only when a/c is on? shouldn't it give a code regardless?

There might be a vacuum switchover valve for the the SAI which then doesn't have enough vacuum available to actuate when the (possibly leaking) HVAC system uses up the available vacuum.

-J

where is this switch over valve and hvac thing located