Check engine light

1991 Caddy Coup DeVille, 155k miles. When I turn on the heater in winter , or, turn on the airconditioner, the"Check engine " warning light comes on almost immediately.

We shut off the air/heater and the light dissapears. A serious problem? How to fix?

thanks - georgeanddave

It almost sounds like it has to be due to the added current draw from the fan, although slight. Which points to the alternator as a possibility. Start by having the charging system load checked. Post back with the results.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code not just their translation into English and post it back here. 

While you are there have them check the battery and charging circuit as well. Likely the whole thing will be free. Now if you do this, remember to come back to the same place for windshield wipers, oil etc.

The auto parts stores WON’T (can’t) do scans on cars older than 1996. On older cars, if the check engine goes out, there’s nothing to read.

thx to all. I cleaned the battery connections and incidentaly, by the disconnect, the warning light re-set. So, no more problem. /great responses.