If you have a spare AMM or another 240 with a known good one, I'd suggest just swapping it out. Clear the codes, and if they don't come back, you've identified the problem. There have been some posts on checking the performance of an AMM with a multimeter. You may want to check the FAQ. You can also hold AMMs up to the light to make sure the hot wires (four of 'em, I believe) are still intact.

Also check the vaccum system. Look for missing hoses, little cracks on the end of hoses, etc. Check the air induction hoses, esp. the one that runs from the AMM to the throttle body. Look for cracks in the "bellows."

A bad O2 sensor can also cause a "check engine" light and a too rich condition. My personal belief is that these sensors are more robust than the volume of posts on the topic would indicate.

Good luck. If it's any consolation, driving the car in this condition to get it home will do no harm other than burn a little more fuel and generate a bit more smog.