Yesterday my 2020 C8 would not start in Costco parking lot. Starter continued to turn but car would not start. Of course it would not go into neutral either. I tried to start off an on about 8-10 times. Tried key fob start but no luck! I called for roadside service to tow into dealership. Ater being told it would be at least 2.5 hour wait I called the tow company we use at work. Driver arrived in 40 minutes. I told him the condition of vehicle and he said not to worry he could let all air out of his airbags and get low enough no damage would occur. I told him I would check to see if maybe it would go into neutral. Pushed starter button, It fires up!
I take it to the dealer to see what happened. I told them the exact time of issue and exact mileage. They found no codes and cannot get car to replicate issue. They told me to pick up the car and I said NO. I went back with a couple of GM Bulletins I found from 2020 that said to replace the ECU. Another from 2021 said to reprogram the ECU. The dealership tells me that the update they did today will fix any issues.
Does this sound right?

 

I meant to say the ECM not ECU

 

There if very little else the dealer can do if there is no code, and the problem cannot be duplicated.

 

Would assume your out of warranty being that your C8 is a 2020.

Have the battery checked out at any retailer.

Being that your battery is 3+ years old it's not uncommon that a replacement is in your near future.

 

Doing a quick search there's dozens of threads on this forum from a couple years ago. Looking through them quickly it looks like most that the ECU replaced. I see GM apparently had a problem with all models the bulletin shows every GM model affected from cars to trucks in 2020.

 

Well, in the old days when mechanics were actually mechanics they would trace all the wiring in the ignition system to look for any issues (loose connections, burned wires), and then the fuel system.. But today they're "technicians," and most of them have no idea how to troubleshoot anything if it involves more than replacing parts.

 

Absolutely, we actually solved problems.

Zero motivation today, sad watching the addicted with their little screens.

 

That little screen tells them everything they need to know in life........

 

Yeah. In the old days there was a wire from the ignition switch to the starter solenoid. On an automatic, that wire might go through neutral/park switch to prevent activating the starter if the car is in gear. Another to the ignition coil. Perhaps another to the bypass the ballast resistor during the crank event. Maybe another to the electric choke heater. The mechanical fuel pump needed no wire.

But that's all over now.

Look for a serial digital signal from the ignition switch to the BCM via the internal network. Then the BCM communicates with the ECM via the network. The ECM will send signals to the fuel pump control module, the ignition control circuit, starter relay via the network. The ECM receives signals from the transmission control module, and electronic brake control module to verify that the transmission is neutral or park, and the brakes are applied. There are also signals from the radio that have verified that the key fob is in the car. Probably a dozen other signals to verify that all conditions are ready for a crank event. Try and find a list of all the communications and conditions necessary for successful crank event in the 7000 page service manual. The technican's only insight into this, and only hope for trouble shooting it is a scan tool that will provide specific status information about all of the communications and status'. But there are pages and pages and pages of scan tool parameters that indicate the system status' and communication events. It helps enormously to have diagnostic trouble code that leads to specific diagnostic sequence - without that, where would would one start?

There was a recent example in another thread that illustrates how difficult it is (or can be) to troubleshoot a problem. A forum member had trouble with his car loosing power and top speed, after sustained high speed driving. Eventually GM got involved (Harlan). They determined that the extra 2 liters of fluid in the transmission raised the fluid level enough that 6th gear (which he was using a lot) was spraying a sensor that gave a false impression of a high temp condition, when the actual fluid temp in the sump was fine. That example of "troubleshooting" shows that a pretty significant level of knowledge about how the car works is needed. I don't know that the solution offered (remove the 2 liters) did actually solve his problem - he could not test it yet - but that's not the point. Its just shows how complex the car can be when the different systems are all interconnected by a network of computers.

The schematic makes its look simple doesn't it? I could not find one that shows more detail - but its probably in there somewhere.