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P codepowertrainMedium severity

P0598Thermostat Heater Control Circuit Low

The control circuit for the electrically heated thermostat is stuck at a lower voltage than the engine computer expects.

Definition

The PCM pulses the heater element in a map-controlled thermostat to fine-tune engine operating temperature, and it monitors the circuit voltage for plausibility. P0598 sets when that control circuit voltage remains below the expected range, typically indicating a short to ground in the wiring, connector, or heater element. Engine temperature regulation may be off-target, the MIL illuminates, and fuel economy or emissions performance can degrade even though the engine usually drives normally.

Common causes

  • Thermostat heater control wire shorted to ground (chafed harness or melted insulation near the engine)
  • Internally shorted thermostat heater element
  • Coolant or corrosion intrusion in the thermostat connector shorting the circuit low
  • Damaged harness section where it routes near hot or moving components
  • Faulty PCM low-side driver (rare)

Common misdiagnoses

  • Replacing the ECT sensor for what is a heater control circuit fault
  • Replacing the thermostat repeatedly without finding a harness short to ground
  • Condemning the PCM before isolating the wiring
  • Replacing the radiator fan or fan module for an unrelated perceived overheating issue

Troubleshooting steps

  1. 1.Confirm the code and check for companions

    Scan all modules and note whether P0597 or P0599 has also set, which points to an intermittent circuit rather than a hard short.

  2. 2.Measure the heater element resistance

    Disconnect the thermostat and measure the element; a reading well below the nominal 10-20 ohm range indicates an internally shorted element.

  3. 3.Test the control wire for a short to ground

    With the thermostat and PCM connectors disconnected, measure resistance from the control wire to chassis ground; anything under about 10 kilohms indicates a shorted harness.

  4. 4.Inspect the connector for coolant intrusion

    Look for green or white corrosion and wet terminals at the thermostat connector, a common failure when the thermostat housing seeps.

  5. 5.Repair or replace and re-verify

    Repair the harness or install an OE heated thermostat, clear the code, and confirm the PCM's circuit test passes on a warm-up drive cycle.

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