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P codepowertrainMedium severity

P0599Thermostat Heater Control Circuit High

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

Definition

The PCM drives the heater element in a map-controlled thermostat and watches the circuit voltage for feedback. P0599 sets when the control circuit voltage stays above the expected range, usually from a short to battery voltage or an open that lets the line float high. The thermostat heater cannot be commanded correctly, so engine temperature may not follow the PCM's target map, the MIL illuminates, and the vehicle can fail an emissions test; day-to-day driveability is usually unaffected.

Common causes

  • Control wire shorted to battery voltage or ignition feed
  • Open circuit in the heater control wire allowing the line to float high
  • Corroded or spread terminals at the thermostat connector creating high resistance
  • Failed heater element with an internal open
  • Faulty PCM driver circuit (least common)

Common misdiagnoses

  • Replacing the ECT sensor instead of testing the heater control circuit
  • Replacing the thermostat when the fault is a chafed harness shorted to power
  • Condemning the PCM without checking for a voltage short in the harness
  • Replacing cooling fans or the fan control module

Troubleshooting steps

  1. 1.Verify the code and review temperature data

    Watch live coolant temperature during warm-up; temperature that plateaus below the specified regulating point often accompanies thermostat heater faults.

  2. 2.Check the control circuit voltage

    Backprobe the control wire key-on; steady battery voltage when the PCM is not commanding the heater indicates a short to power.

  3. 3.Measure heater element and wiring continuity

    Confirm the element reads its nominal resistance (typically 10-20 ohms) and that the control wire has under 1 ohm of resistance end to end.

  4. 4.Inspect harness routing for shorts to power

    Look for the thermostat harness rubbing against other powered circuits, especially where looms cross near the front of the engine.

  5. 5.Repair the fault and confirm the fix

    Repair wiring or replace the heated thermostat as indicated, clear codes, and verify the monitor completes without resetting P0599.

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