The Automatic Engine Stop/Start system (RPO KL9) may be inoperative on some 2019-2020 Silverado 1500 and Sierra 1500 models. DTCs P3055 (DC/DC Converter Output Voltage 1 Performance) and P3056 (XXX) may be set in the Engine Control Module.
The inoperative Stop/Start functionality may be caused by fuses F51DR and/or F52DR in the X51R right instrument panel fuse block (Fig. 36) when they should not be installed. With these fuses installed, it creates additional load for the Power Supply Transformer, which pulls down the output voltage.
Fig. 36
Inspect for a fuse in the F51DR and the F52DR locations of the X51R right instrument panel fuse block. Vehicles equipped with Automatic Engine Stop/Start system should not have any fuses in these two locations. If both locations are empty, then follow normal diagnostics in the appropriate Service Information.
– Thanks to Paul Radzwilowicz
