Thermostat Heater Control Circuit Low

What it means

The engine computer has detected lower than expected voltage in the electronic thermostat heater control circuit. This usually points to a wire shorted to ground or a heating element fault, and the computer loses its ability to lower engine temperature under load.

Related codes: P0597P0599P0128

Symptoms4

• Check engine light on.
• Engine temperature slightly higher than usual under load.
• The circuit fuse may blow repeatedly.
• Often no other clear symptoms.

Possible causes4

• Control wire shorted to ground from chafing or heat.
• Internal fault in the heating element.
• Coolant entering the connector.
• Fault in the driver circuit inside the computer (rare).

How to diagnose5

1. Inspect the connector for coolant or corrosion.
2. Inspect the wiring between the thermostat and the computer for chafing near hot engine parts.
3. Measure the heating element resistance and compare it with the manufacturer specification.
4. Check the control wire for a short to ground with the connector unplugged at both ends.
5. Command the heater on with a scan tool and measure voltage while it runs.

How to fix5

1. Repair the shorted wire and route it away from heat.
2. Clean or replace the connector and fix the coolant leak.
3. Replace the electronic thermostat if the heater is proven faulty.
4. Test the computer only after ruling out the wiring and thermostat.
5. Clear the codes and confirm the code does not return after a full drive cycle.

A wire or connector repair keeps the cost low; it rises when the electronic thermostat is replaced and the coolant drained and refilled.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions