What it means
The thermostat heater control circuit voltage is above the expected range. This code concerns an electrically heated thermostat, not an oxygen sensor heater.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The thermostat heater is an electrical element the control module uses to adjust when the thermostat opens. P0599 sets when the heater control circuit voltage is above its expected range, commonly because the control line is shorted to supply voltage; engine temperature behavior may change.
Common causes
Most likely first.
- Control line shorted to supply
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
Symptoms
- Check-engine light
- Engine may take longer to warm up or run hotter than usual
- Temperature gauge may behave unusually
How to diagnose it
- Use a scan tool to confirm P0599 and review freeze-frame data, coolant temperature, and thermostat-heater command to determine when the circuit went high.
- With the engine cool, inspect the heater wiring and connector for a pinched wire touching B+, chafing, moisture, corrosion, or pushed-back pins.
- Using the wiring diagram, test the control line for a short to supply voltage and check connector contact.
- Wiggle the harness while monitoring the circuit or scan-tool data to locate an intermittent short.
- If the wiring is intact, test the thermostat heater and control circuit against the manufacturer's specification.
Common fixes
- Repair or reroute a pinched heater control wire that contacts a supply wire.
- Repair chafed insulation and secure the thermostat harness.
- Clean or replace corroded or damaged connector terminals.
- Replace the thermostat assembly if its heater element fails testing.
Can I keep driving?
You can usually drive if the temperature gauge remains normal, but monitor it closely because thermostat control may be affected. Stop if the engine overheats to prevent engine damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.