I drive in the mountains here about once a week at an altitude of approximatley 8,600 feet. After about an hour or so of driving/climbing/descending my CEL comes on (has happened 2 out of about 15 drives). The next day I was driving to get the CEL downloaded but it turned off on my way. I didn't think CEL or DTC's cleared without being downloaded.

Will the CEL info still be stored with the light off?

My car lives at 4000' in the valley and at that altitude and all lower altitudes it never comes on. I figured it may be because of my TP install recently or my fuel cap.
(05 w/40K)

 

You need a DTC 1st to determine the cause

 

The code will be stored for (I think about) 40 run cycles.

Ill bet you had a p0171. Temporary lean condition from the altitude change.

 

I concur with B-Man. A few more details. The emission system is controlled by a system known as Obd-II. While the cel has cleared there will still be freeze frame data available in memory....but no need to bother. Obd II uses two fuel trim references, one short term (STFT) and the other long term (LTFT). Short term data is cleared each time the car is shut off. Engine management uses it to fine tune fuel/air ratio at 14.7:1 (referred to as stoichometry). Long term is used on a more macro scale and stores the long term environment of the car. The car constantly compares STFT with LTFT to verify all is well. Both trims can compensate for rich or lean conditions. But if either trim hits what is called the "adaptive limit" then a cel will be initiated. The system has pegged out if you will. When the condition is removed the cel will extinguish but data of the event will remain as "freeze frame" data. This type of cel is caused by a Type B run monitor. If the monitor sees a second occurance in 80 trips it will request a mil on.

The problem should correct itself over time (The 40 trips B-man is referring to) as the LTFT learns it's new environment.

Utah

P.S. See "Getting to know Obd-II" by Ralph Birnbaum if you would like more information on how this key system works.

 

Wow, very great info guys. Thanks a bunch.