P0785 Diagnostic and Fix for 1999 GMC K1500 Suburban: Electrical Issues with PRNDL Fuse and Limp Mode
I'm experiencing a P0785 fault code on my 1999 GMC K1500 Suburban, which has resulted in the transmission entering 'limp mode'. I've already replaced all internal solenoids in the transmission, but the issue persists. The dashboard lights do not activate with daytime running lights, although all gauges function normally. Upon inspection, there is no power at the PRNDL fuse located in the instrument panel. This has led me to suspect an electrical fault within the starter switch assembly or related wiring. I'm seeking advice on how to properly diagnose and resolve this issue. The P0785 code indicates that the Powertrain Control Module (PCM) is not detecting proper voltage on circuit #687, which connects to the 3-2 solenoid in the transmission. This circuit runs from the fuse box through the transmission harness back to the PCM. If fuse #20 is open or faulty, it could trigger P0785 and potentially affect all solenoids. I've verified that there is no 12V at fuse #20 under any key position, and a fused jumper across the fuse terminals did not resolve the issue—code remains present and limp mode persists. I’ve also confirmed that no additional electronics (such as GPS, alarm, or remote start) are installed. The vehicle is bone stock with no aftermarket modifications. I’m now considering whether to inspect wiring harness connections, verify circuit diagrams, or replace the ignition switch. Any insights or step-by-step troubleshooting tips would be greatly appreciated—especially for a 1999 K1500 Suburban with a gasoline engine.
The P0785 code typically indicates a circuit fault. Check if any aftermarket electronics (remote start, GPS, alarm) are installed—these can interfere with the signal path. Confirm whether the vehicle is stock or modified.