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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P0599

P0599Thermostat Heater Control Circuit High

P0599 indicates that the engine control module (ECM) has detected an abnormally high voltage on the control circuit for the thermostat heater, which is a small heating element built into the thermostat housing. The ECM commands the heater on or off to regulate coolant temperature, and a high voltage reading means the circuit is likely open, shorted to power, or the heater element itself has failed.

SystemPowertrain
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Prolonged engine warm-up time or the temperature gauge staying lower than normal
  • Poor heater performance inside the cabin during cold weather
  • Slightly increased fuel consumption due to the engine running cooler than intended
  • In rare cases, the engine may run hotter than normal if the thermostat remains partially closed
  • A stored code P0599 is often the only sign, especially on vehicles where the thermostat heater is used mainly for emissions reduction

Common causes, most likely first

  1. Open or damaged wiring in the thermostat heater control circuit, such as a broken wire or corroded connector
  2. Failed thermostat heater element (open circuit inside the thermostat housing)
  3. Short to voltage on the control circuit, often caused by chafed wiring rubbing against a metal component
  4. Faulty engine control module (ECM) driver for the thermostat heater, though this is less common
  5. Poor electrical connection at the thermostat heater connector due to water intrusion or loose pins
  6. Aftermarket or incorrect thermostat installed that does not match the original heater specifications

How to diagnose and fix it

  1. Connect a scan tool and verify P0599 is current, then command the thermostat heater on and off while watching the circuit voltage to confirm the fault
  2. Visually inspect the thermostat heater connector and wiring for corrosion, loose pins, or obvious damage, and repair as needed
  3. Disconnect the thermostat heater and measure the resistance of the heater element with a multimeter, comparing to the manufacturer's specification
  4. Check for a short to voltage on the control wire by measuring voltage with the heater disconnected and the key on; repair any chafed or shorted wiring
  5. Test the control circuit for continuity from the ECM connector to the thermostat heater connector to rule out an open wire
  6. If the wiring and heater element test good, suspect the ECM; perform a driver test or substitute a known-good ECM before replacement
  7. Clear the code and perform a road test to confirm the repair, ensuring the thermostat heater activates and the code does not return

What a high voltage code really means

  • A high voltage reading on the thermostat heater control circuit often indicates an open circuit rather than a short, so technicians who only look for shorts to power may miss a broken wire or failed heater element.
  • Many vehicles use the thermostat heater primarily to speed engine warm-up for emissions purposes, so a failed heater may not cause any drivability symptoms and the code can be the only clue.
  • Replacing the thermostat without testing the heater circuit first is a common misdiagnosis; the heater element can fail independently of the thermostat's mechanical function.
  • Corrosion at the thermostat heater connector is a frequent cause on vehicles driven in wet or salty conditions, and simply cleaning the terminals can resolve the code.
  • Some aftermarket thermostats come without a heater or with a heater of different resistance, which can set P0599 even though the part appears to fit.

Can I keep driving with P0599?

The vehicle is safe to drive with P0599 because the fault typically affects only emissions-related warm-up and does not cause immediate engine damage. Avoid prolonged idling in cold weather and have the circuit inspected soon to prevent potential overheating if the thermostat remains closed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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