P1807: Transmission 4-Wheel Drive High Indicator Short Circuit To Ground

P1807 is a powertrain code that sets when the transmission or transfer case control module detects that the circuit feeding the 4-Wheel Drive High indicator has shorted to ground. This is a signal circuit fault, meaning the wire that should carry a switched voltage to illuminate the 4WD High indicator is instead reading a ground potential. The indicator light may be on permanently or behave erratically, and in some configurations the transfer case control logic may also be affected depending on how the indicator circuit is integrated.

The most likely cause is a short to ground in the wiring between the control module and the indicator circuit, either at the instrument cluster or in the harness routing between components. Chafed wiring against a body panel or bracket is a common culprit on 4WD vehicles, particularly in the areas around the transfer case and the cab floor where harnesses are often routed near moving or sharp-edged components. Check the connectors at the module and trace the wiring for any visible damage before going further.

Severity is medium. The vehicle may still operate in 4WD modes, but an unresolved short to ground can cause broader electrical issues over time if the fault loads up shared circuitry. It's a practical fix to get done, and the wiring inspection itself is straightforward once you locate the relevant harness run.