I wanted to create this video as I struggled with this issue for a couple days... basically, i plugged in the engine block heater on my Sierra 3.0 Diesel Duramax and it caused the ECU to flip a code, or made the ECU think the truck was in danger of overheating and basically the radiator fan went on full blast, every time i started the truck and wouldn't stop. So the fan was stuck on high and wouldn't turn off unless I turned off the truck... frustrating. My truck didn't pop an actual code or pop the check engine light, but some reported that it did, so in your case, if you did pop a code, be sure to clear it prior to disconnecting battery. Or you can try clearing it and then let the truck sit overnight, though I don't know if that will work.

This bug caused my truck not to get up to temperature and I had no heat in the truck so it wasn't fun. If you want to avoid this issue, don't use your engine block heater unless it is below zero while plugged in and started. I figured out how to fix it after trying a few things (disconnect the battery for a little while) but here is the permanent fix for those who are going to get service from GM soon:

https://gm-techlink.com/?p=12389

Here is what GMC has to say about it and the permanent fix:

Some 2020 Silverado 1500 and Sierra 1500 models equipped with the 3.0L diesel engine (RPO LM2) (Fig. 12) may have the engine cooling fans running continuously for an extended period of time. The Check Engine MIL also may be illuminated.

Currently, an engine calibration in the Engine Control Module (ECM) will falsely set DTC P0111 (Intake Air Temperature (IAT) Sensor 1 Performance), which occurs when the engine block heater is used in ambient temperatures above 0°F/–18°C. As a result, the cooling system will go into remedial action and turn on the cooling fans at 100%. The remedial action will continue until the engine goes through a six hour cold soak.

If this condition is found, clear the DTC and return the vehicle to the customer. Do not perform any further diagnosis or replace any related components. An updated calibration will be available shortly.

Also inform the customer to avoid engine block heater use in temperatures above 0°F/–18°C. While the owner’s manual states an engine block heater should be used in temperatures less than 0°F/–18°C, it isn’t vital until much lower temperatures of less than –13°F/–25°C.