P0599
Thermostat Heater Control Circuit High
Cooling / thermostat
Generic OBD-II
Medium
What it means
P0599 means the control circuit for the thermostat heater — used on electronically managed "map-controlled" thermostats — is reading a high voltage, typically an open circuit or a short to power. The ECM uses this heater to fine-tune coolant temperature, so the engine may run at a non-ideal temperature, but it still starts and runs.
Medium
Can I keep driving? Usually safe to drive — the engine still runs, but a map-controlled thermostat that can't be regulated may run cooler than ideal and hurt fuel economy; keep an eye on the temperature gauge.
Common symptoms
What you'll notice
- Check-engine light on
- Engine running cooler than normal or slow to warm up
- Slightly reduced fuel economy
Common causes
Most likely first
- Open circuit or short to power in the thermostat heater control wiringLikely
- Damaged wiring or a corroded connector at the thermostat heaterCommon
- Failed thermostat heater element (map-controlled thermostat)Common
- Failed ECM driver (rare)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and confirm how coolant temperature is behaving.
- Inspect the thermostat heater connector and wiring for opens and shorts to power.
- Measure the heater element's resistance against spec.
- Replace the map-controlled thermostat if the heater circuit is open.
- Repair wiring; replace the ECM only if its driver has failed.
FAQ
Quick answers
Is it safe to drive with P0599?+
Generally yes — the engine runs, but fix it to restore proper warm-up and fuel economy and to keep the cooling system regulating correctly.
Can I just clear P0599?+
It returns until the open or shorted heater circuit is repaired.
Related codes
Often seen with P0599
Read & clear P0599 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX