What it means
The thermostat heater control circuit voltage is below 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 the thermostat's opening behavior. P0598 sets when the heater control circuit voltage is below its expected range, often because the control line is shorted to ground; coolant temperature or warm-up behavior may be affected.
Common causes
Most likely first.
- Control line shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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 P0598 and review freeze-frame data, coolant temperature, and thermostat-heater command to see when the circuit went low.
- With the engine cool, inspect the thermostat heater wiring and connector for chafing to ground, moisture, corrosion, or loose or pushed-back pins.
- Using the wiring diagram, test the control line for a short to ground and check terminal contact.
- Wiggle the harness while monitoring the circuit or scan-tool data to find an intermittent short or poor connection.
- If the wiring is sound, test the thermostat heater element and control circuit against the manufacturer's specification.
Common fixes
- Repair chafed heater wiring that contacts ground.
- Clean or replace corroded connector terminals and secure loose pins.
- Repair a control line shorted to ground.
- Replace the thermostat assembly if its heater element fails testing.
Can I keep driving?
You can usually drive if engine temperature remains normal, but monitor the gauge closely because thermostat control may be affected. Stop if the engine overheats to avoid 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.