phooey

Hi, i'm new to the forum and looking for some help with a secondary air injection error code on my 2000 CLK200K. I believe my car uses the Kompressor to inject the air into the cat via a vacuum valve and a check valve. I have checked the two valves and they appear to be ok. However I have found an issue with the change over valve which controls the vacuum valve. I have checked the voltage on the two pin connector from cold and the voltage on both pins do not change state suggesting I have a fault prior to the change over valve. Can anyone tell me where to check next? is the change over valve driven by a temperature sensor or is it the ECU? Sorry for the long thread but I am now at a loss!

Thanks in advance.