P0599: Thermostat Heater Control Circuit High
P0599 means Thermostat Heater Control Circuit High. Some engines use an electrically heated (map-controlled) thermostat that lets the engine run cooler under load. The signal voltage is higher than the maximum the module expects. Typical causes are an open ground circuit, a signal wire shorted to voltage, an unplugged connector, or a failed sensor.
Can I keep driving? Safe to drive short-term, but have it checked to avoid failed emissions or worse fuel economy.
Common symptoms
- Check engine light on
- Engine runs warmer or cooler than normal
- Possible cooling fan running more often
Possible causes
- Failed thermostat heater element
- Wiring or connector fault
- Blown fuse
How to diagnose P0599
- Inspect the thermostat connector and wiring.
- Measure heater resistance against specification.
- Command the heater on with a scan tool where supported.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.