1990 gmc p/u 5.7 wont idle showing code 12
Try the idle air control motor
possibly a vacuum leak, low fuel pressure, or the idle air control has failed,
Check the idle sensor if you have one.
Check the throttle position sensor or the idle control motor
modual
stopped up fuel filter or poor fuel delivery
Likely heater core. I had same problem with 1996 S-10 with 4.3L.
To answer your question I need to know the year model of your Chevy Celebrity as well as the engine that it has in it.1996 chevy calavlier 2.2
Could be you need a new fuel idle motor. Or you need new spark plugs
Bad temperature sending unit? Bad gauge?
Does it turn over? No Fuel? No Spark? No compression? Weak or bad battery?