P0599 GMC Code - Thermostat Heater Control Circuit High

P0599 indicates a "Thermostat Heater Control Circuit High," meaning the Engine Control Module (ECM) has detected a higher-than-expected voltage in the thermostat heater control circuit.

Symptoms may include an illuminated check engine light, engine overheating, poor fuel economy, irregular engine temperature readings, or the heater not functioning properly.

Common causes include a faulty thermostat heater, damaged wiring or connectors in the thermostat control circuit, a malfunctioning ECM, or a short circuit in the electrical system.

Driving with a P0599 code is not recommended, as it may lead to engine overheating or inefficient performance, potentially causing further damage to the engine.

Diagnosis involves checking the thermostat heater's resistance, inspecting the wiring and connectors for damage, testing the voltage in the thermostat control circuit, and verifying the ECM's functionality.

Repairs may include replacing a faulty thermostat heater, repairing or replacing damaged wiring or connectors, or reprogramming or replacing the ECM if necessary.

You’ll need an OBDII scanner, a multimeter to test electrical connections, and potentially a repair manual specific to your GMC model.

Repair costs can vary widely, but replacing a thermostat heater typically ranges from $150 to $400, including parts and labor. Wiring repairs or ECM replacements may cost more.

While unlikely, a weak or failing battery can sometimes create electrical issues that may contribute to codes like P0599. It’s worth testing the battery and charging system during diagnosis.

Clearing the code may temporarily turn off the check engine light, but if the underlying issue isn’t resolved, the code will likely return. Proper diagnosis and repair are necessary to fix the problem.