Thermostat Heater Control Circuit High

What it means

The engine computer has detected higher than expected voltage in the electronic thermostat heater control circuit. This usually points to the control wire shorted to a voltage source or a heating element fault, so the computer cannot control the heater properly.

Related codes: P0597P0598P0128

Symptoms3

• Check engine light on.
• Engine temperature may run slightly higher or lower than usual depending on the heater state.
• Often no other clear symptoms.

Possible causes4

• Control wire shorted to a supply wire or other voltage source.
• Internal fault in the heating element.
• Damaged connector or coolant inside it.
• Fault in the driver circuit inside the computer (rare).

How to diagnose5

1. Inspect the connector and wiring for damage or melted insulation.
2. Measure voltage on the control wire with the heater off; unexpected voltage points to a short to a voltage source.
3. Measure the heating element resistance and compare it with the manufacturer specification.
4. Command the heater on and off with a scan tool and watch the voltage change.
5. Test the computer only after ruling out the wiring and thermostat.

How to fix5

1. Repair the short between the control wire and the voltage source.
2. Clean or replace the connector.
3. Replace the electronic thermostat if the heater is proven faulty.
4. Repair or replace the computer if its driver circuit is proven faulty.
5. Clear the codes and confirm the code does not return and temperature is stable.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost stays low for a wiring fault, and rises when the electronic thermostat and coolant are replaced.

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