P0599 Thermostat Heater Control Circuit High

P0599 indicates a high condition in the thermostat heater control circuit. The controller sees heater control voltage or feedback above the expected range.

Code details

Generic OBD-II · P0

System: Speed / Idle
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not condemn the control module before checking thermostat heater wiring, ground path, and coolant-contaminated connectors.

Symptoms of P0599

Unstable, high, or low idle, or stalling.
Erratic speedometer, cruise-control, or shift behaviour.
Check-engine light with a hesitation or surge.

Common causes of P0599

A dirty throttle body, idle-control valve, or throttle-position sensor.
A failing vehicle-speed or wheel-speed sensor, or its wiring.
Vacuum leaks or a related sensor forcing the idle/speed strategy.

Useful checks before replacing parts

Review thermostat heater command, control voltage, current draw, driver status, coolant temperature, warm-up behavior, and freeze-frame conditions.
Inspect for short-to-voltage faults, open grounds, connector coolant intrusion, damaged wiring, failed heater elements, and module driver issues.
Command the thermostat heater and confirm circuit voltage, current, and coolant temperature response match expectations.

Related OBD-II codes

These nearby speed / idle entries can help compare wording and pin down the fault area.