FixItFor.Us
P codepowertrainMedium severity

P0597Thermostat Heater Control Circuit/Open

The engine computer has detected an open or fault in the control circuit for the electrically heated (map-controlled) thermostat.

Definition

Many modern engines use a map-controlled thermostat with a built-in heating element that the PCM energizes to lower the thermostat's opening temperature under high load. P0597 sets when the PCM's driver detects an open circuit or an implausible feedback voltage on the thermostat heater control line. The engine may run slightly hotter or colder than commanded, and coolant temperature may fail to track the target map. The MIL illuminates, fuel economy and emissions can suffer, and the vehicle will typically fail an emissions inspection with the code present.

Common causes

  • Failed thermostat heater element (open resistance) inside the map-controlled thermostat
  • Broken or chafed wiring between the PCM and the thermostat heater connector
  • Corroded, backed-out, or damaged thermostat connector terminals
  • Blown fuse or open feed supplying the thermostat heater circuit
  • Faulty PCM driver for the thermostat heater (least common)

Common misdiagnoses

  • Replacing the engine coolant temperature (ECT) sensor when the code is a heater circuit fault
  • Replacing a conventional thermostat with a non-heated aftermarket unit that keeps the circuit open
  • Replacing the water pump for a perceived cooling problem
  • Condemning the PCM before checking wiring and the heater element resistance

Troubleshooting steps

  1. 1.Verify the code and freeze frame

    Scan for P0597-P0599 and note coolant temperature data; a companion P0128 suggests the thermostat itself is also stuck open.

  2. 2.Measure the thermostat heater element resistance

    Disconnect the thermostat connector and measure across the heater pins; most elements read roughly 10-20 ohms, and an open (OL) reading confirms a failed thermostat.

  3. 3.Check power supply to the heater circuit

    With the key on, verify battery voltage on the feed side of the connector and inspect the fuse that supplies the circuit.

  4. 4.Test the control wire to the PCM

    Check continuity from the thermostat connector control pin to the PCM connector and inspect the harness for chafing near the thermostat housing, where heat damage is common.

  5. 5.Replace the map-controlled thermostat if the element is open

    Use an OE-specification heated thermostat, refill and bleed the cooling system, then clear the code and confirm the circuit passes its next self-test.

Related codes