Hello, everyone,I introduce myself. I'm Almir from Germany and I'm 39 years old. I drive a 2017 GMC Sierra 1500 Crewcab with the 5.3 V8 engine and the 6L80 automatic transmission. Unfortunately I have had a problem with my truck for about 5 days. Suddenly 5 days ago he just didn't want to start. Put the key in and then turned it and it won't start. No noise from the starter.So the first thing I checked was the battery and it'* fine. Then the starter removed and checked and OK.then measured the continuous current at the starter and it is there too. then while my son was turning the key to start it, i measured the control cable and there was no electricity. One step further. So I checked the relays and fuses and all fine. Then I controlled the starter via the relay slot with terminals 30 and 70 and the starter did its job. so I have ruled out that a defective cable disturbs the flow from the relay to the starter. So I assumed it was a faulty ignition switch. I finally replaced it and hoped that it would start, but unfortunately without success. I also checked that the immobilizer is on but it isn't. Then I started the car with the help of the relay. Key on contact in the interior and in the fuse box the relay on plug 87 was directly powered and it started. I then tried the key and it worked. Then the next day the same problem. So I removed the fuse box and opened it up. it had slightly formed oxidation on the board and the relays sitting on the board. cleaned it up and put it back together. the problem has not disappeared. The only thing I found out is that the relays suddenly get no power, which triggers the ignition and fuel pump.My question could be that the circuit board in the fuse box is defective or, better said, the relays that are soldered are defective and the circuit board is sporadically not supplied with power. But if I give electricity directly to it, it works like a redirect.I hope you can help me because we don't have a capable GM service here.

Okay so here'* what has to happen to cause the starter to run in normal conditions:

1. Ignition switch verifies key
2. Ignition switch turned to [Start]
3. Start signal to PCM
4. PCM signal to low side of relay
5. Relay closes
6. Power sent to starter solenoid
7. Starter solenoid cycles
8. Starter turns

From what I can gather, you have cleared all except 1, 3, and 4. Is this accurate?

If so, test those next. I'd start with 4.

Hello,thanks i will check that today.I also noticed acoustically when I turn the contact to start in the ignition lock with the key, i.e. if I start it, the relay in the interior of the fuse box works. I think that step 1 has also been checked.What I noticed is that I have no power on pin 87 on the circuit board of the fuse box in the engine compartment. only when I apply current to pin 87 directly from the battery does something happen. It doesn't matter whether I do it on relay pin 87 of the starter or on the fuel pump relay pin 87.I also built the fuse box in the engine compartment outside and saw that there was oxidation. also on the relays that are soldered on. Then tried to clean it. Which of course can also be that the oxidation has progressed so far that the relays on the board are also oxidized on the inside.But today I ordered a new fuse box including all relays, fuses and circuit board to rule out sources of error for the future. The only steps left to me are the 3rd and 4th you mentioned.

Have you replaced that relay? How did you test your battery?