Just wondering if anyone knows what can be the cause of a moody check engine light in an 88 635. The only thing I can think of is the O2 sensor. Besides for that and the purge valve for the charcoal canister (which have been capped/removed) I cannot think of anything else that would cause the light to come on. It will come on sometimes, and if I kill the motor and restart sometimes it turns off and sometimes it doesn't. Any ideas? Good place to get the correct sensor?