Rick's Free Auto Repair Advice

P0597 Engine Coolant Thermostat Heater Control Circuit

Fix P0597 trouble code

What is a P0597 trouble code?

Late model engine use a heated thermostat. A P0597 Engine Coolant Thermostat Heater Control Circuit Trouble code indicates a problem with the thermostat, heater, wiring harness, electrical connector or the thermostat housing itself.

Why use a heated thermostat?

Car makers routinely install smaller turbocharged engines to lower vehicle weight and improve gas mileage and emissions. These smaller engines often run hotter when the turbo is in operation, but can fail to output enough heat to warm the passenger cabin when the vehicle is at idle in cold conditions. Car makers use various methods to improve cabin heating. Some rely on a radiator shutter mechanism to block airflow across the radiator to reduce heat loss on the engine coolant. But that can also impair engine cooling once the driver takes off from idle. So car makers also incorporate a heating element in the thermostat to speed up thermostat opening to provide optimum cooling.

GM uses a heated thermostat assembly as a single unit. The thermostat, housing, heater and sensor are a single unit. The thermostat begins to open at 221°F and is fully open at 248°F. The PCM monitors coolant temperature at the sensor and use a pulse width modulated power to regulate how much heat to apply to the wax pellet in the thermostat to speed up opening time. Unfortunately, these units have a high failure rate.

What causes a P0597?

On a GM vehicles, the heated thermostat is built in a plastic housing. Shops are reporting a very high failure rate for this item due to poor design in the plastic housing. Aftermarket suppliers have determined that the plastic housing suffers from weak construction, small tube diameter and an insufficient gasket groove. Dorman Products and Gates both offer an upgraded part to replace the factory design.


GM designed plastic heated thermostat housing










Gates 34710 redesign using a metal housing

















Dorman 902_033 redesigned heated thermostat and housing

Diagnose and fix P0597

The thermostat/heater assembly can generate three distinct trouble codes:

P0597 – Engine Coolant Thermostat Heater Control Circuit

P0598 – Engine Coolant Thermostat Heater Control Circuit Low Voltage

P0599 – Engine Coolant Thermostat Heater Control Circuit High Voltage

P0597 or P0598 can be set by a short to ground condition or an open or high resistance condition

P0559 can also be set by a short to voltage condition

P0597 Diagnostic steps:

  1. Remove the electrical connector the thermostat housing. Check for corrosion or coolant infiltration, bent pins.

2) Check the fuse to the thermostat heater.

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