Well, unfortunately, you're gonna have to go back to the dealer, or an independent with the means of reading and clearing the climate control codes. Since it only happens with the AC on, and neither you nor your car are running rich, opt for the outside temperature sensor, particularly if you have an ambient temperature reading in your gauges that doesn't seem to be giving you an accurate reading. These go bad all the time since a) they are in a vulnerable spot and b) they tend to come loose from there home and bang around next to your radiator. Should you still continue to have the problem, then replace the ECT and thermostat (which, really, if it is still original and hasn't failed yet, it's about to).

Once the problem is solved with either one or two of the components, visit the dealer or independent to clear your codes.

Oh...off the subject...I love that laffy taffy color on the 850!