P0599

Symptoms

• Check Engine light is on
• The engine runs at a reduced operating temperature
• Increased fuel consumption, especially in winter
• Poor interior heating
• Coolant temperature sensor shows low values

Causes

• Short circuit of the thermostat heater control wire to power (+12 V)
• Broken feedback wire or thermostat ground
• Internal thermostat fault (always open)
• Damage to wiring insulation
• ECU malfunction (rare)

How to Fix

1. Read error codes and check for additional temperature-related codes
2. Inspect the thermostat connector and wiring for damage and melting.
3. Check the voltage in the control circuit with the ignition on and the engine running.
4. Check the insulation of the control wire for short circuit with power (+12 V)
5. Measure the resistance of the thermostat heating element
6. If the malfunction is confirmed, replace the thermostat assembly
7. Reset error codes and perform a diagnostic test drive

Related codes

Error P0599 by Vehicle Brand