Ignition modules are pretty common on the 4.3 liters. Some things I would check are spark at the coil, power to the coil, pulse to the coil, and injector pulse. If you have injector pulse but no pulse at the coil chances are the module is the issue.
I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa
Check that the cam shaft is turning ( broken belt/ chain. check that the immobiliser is operational
SOURCE: 99 GMC sonoma will start with starting fluid then
Will the engine start on its own after the engine is warmed up?
Let me know.
Regards,
SOURCE: gmc safari van cranks over, but will not fire up
I had the same problem with my 2000 GMC Safari two years ago....drove me CRAZY!
Sometimes it would start right up...other times it just would crank and not start.
It turned out to be a cracked coil....I found it by using a spray bottle with water in it, and "misting" the engine, at night....with the engine running. (On a night when it DID start!)
It took a while, but finally the hairline crack let in some water and the electrical short showed by a spark coming out of the coil.
I replaced the coil with a new one, and it has never "not started" since.
Evidently the coil cover gets brittle and develops small, hairline cracks due to heat from the engine.
Oh...by the way....I tested the coil it showed that it was fine!....but it took the "misting" at night...in the dark...to find the spark "leaking" out.
SOURCE: 99 GMC Sierra Pickup: Crank but no start. starts
The theft light issue is a good observation. Have you replaced the key tumbler or an ignition component lately? Has there been a lightning storm close very recently?
What the flashing security light is indicating is an attempt to relearn the key you are using.
Turn the key to run position, allow light to flash for 10 minutes, when light stops flashing turn ignition to OFF position for 5 seconds, turn ignition to run position and start over. Do this for 3 consecutive cycles, the the theft system will relearn the key cylinder code and store it once again.
Try that first before you go any further.
Let me know what you find.
Regards,
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