Cadillac Repair: service engine soon and emissions system message, barometric pressure sensor, many cars

Question
After climbing a pretty steep mountain and going down the other side, the SERVICE ENGINE SOON and EMISSIONS SYSTEM display came on at the first stoplight. The engine seemed a bit jumpy at the stop, but was ok again after accelerating. I cleared my onboard codes to see if I could replicate the problem today.So far, 20 miles driving and three restarts, nothing. Anything I missed?  

Answer
Chris,
Depends on the model and year of the car but I'd say that it's a pretty fair bet that the altitude got to it. Once back at a lower altitude, the system was okay again. The problem is that on many cars, the "baro" (barometric pressure) sensor only takes a sampling at engine start-up, so it was unable to handle the change of barometric pressure since it thought it was still at the lower altitude.