P0599

Thermostat Heater Control Circuit High

Q: 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.

Q: Can I just clear P0599?

It returns until the open or shorted heater circuit is repaired.

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.

Real-world scan data

Seen in

130

scans in the past year

Found in

<0.1%

of scans

<0.1%

of scans that found faults

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear P0599 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX