My 2002 GMC Envoy will not start. When I put the key in the ignition, the interior lights, radio, etc turn on but the engine will not turn over. I'm noticing this happen when the temps dip below 30.

what computer of many?
to get them all pull battery neg lug for 15min
make sure radio license codes are learn first.
he radio manual (glove box) tells how.

Does it light up ? When you turn the key on do the warning lights come on ? Check engine , oil , abs , air bag etc... just gauges not working ? Does the vehicle start an run ? With vehicle running is check engine light on ? GM had a problem with the stepper motors that control the gauges . How to repair GM Instrument Cluster Speedometer Gauge Stepper

The solenoid on the starter motor is not engaging. Time for a new starter

when ever you replace ECM computer, you must take out your CHIP , to put it into your new ECM. did you do this ?????? if not your engine will not run ,even with GOD turning the key .

Key fell out of the ignition. to fix put key back in and start!!!

We need more info to solve the issue. Is the battery dead too? lights work? Check engine light on before it died? just a few for starters.

check fuses... get a 12v test light & see if getting voltage to starter. did you charge battery? check battery connections. did you spill water one cup? 500 gallons?in console? on engine? on ignition switch?

try car to car boost-battery charger may not be working correctly or set on low amp charge -interior lights and dash may all light but battery may not be strong enough to energize starter have battery tested if battery good than check for power to the starter if power-- starter may be shot can remove also and have tested at most auto stores free if no power battery -wiring-or connection issue would need to test fuse panel-ignition switch and starter solenoid

check battery and replace if required.

It could be a few different things. Do you hear a clicking during turn over? Its most likely your starter motor. After time the magnets wear down, and the bendix gear will not extend out to your fly wheel. Its actually a very common problem. I hope thats it, becouse it gets more expensive after that. If you have the knowledge to pull the starter out yourself, which is pretty ez, you can take it to a auto parts store, and they will test it for free.

If your 2002 gmc envoy or sierra will not even turn over with a kown good battery try starting it in nuetral then go back to park and try starting again. There is a park nuetral safety switch that can malfunction. Turn the key on then shift into nuetral, then turn fully to start.