Those are not good codes. 920 and 921 are both related to the speed signal to the dash from the PCM dropping out the 999 is a code saying that there are other error codes. The last time I saw these and the vehicle had those same symptoms it was a bad PCM that would overheat internally and shut down, then cool off and run again. Solution was a new PCM and flashing the system to accept it and the dash had to be re-flashed as well as it reads the PCM information for the security system. Unless you have a DRB scan tool and a dealer account to do the programming it isn't a DIY job as the way it is programmed is a strange process.
The repair itself is simple, disconnect the battery, remove the three connectors from the PCM and the bolts and remove the PCM. Reverse to install the replacement. Then program the module itself and then the dash.
A test to confirm it's the PCM would be to use some canned air like is sold to dust off items, if you turn the can upside down it will spray out the refrigerant in it which will freeze most things on contact, the test is simple, run the vehicle until it shuts off, take the canned air, invert it and spray the case of the PCM a few times to cool it fast. Does the truck start back up right after the cooling and without waiting, if yes you have the culprit picked. It's a somewhat a common thing.
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Monday, December 13th, 2021 AT 10:37 PM
