P0599 Thermostat heater: signal or voltage too high

By: Diagno editorial team · Updated: 2026-10-15

P0599 means: Thermostat heater: signal or voltage too high. The cooling system keeps the engine at the right temperature. OK to drive, but have it checked soon. The repair usually costs about €30–200 in Estonia. The most common cause: connector loose, damp or corroded. Other possible causes: wire shorted to power or ground broken, faulty thermostat or sensor.

What it means

The cooling system keeps the engine at the right temperature. The computer found the engine not warming up enough, or a temperature reading that isn't plausible. The signal voltage is too high. Usually a wire is shorted to power, the ground is broken or the part itself has failed.

What you may notice

• The engine warms up slowly and the cabin stays cool
• Higher fuel consumption
• Engine light on

What could be wrong

1. 1Connector loose, damp or corroded
2. 2Wire shorted to power or ground broken
3. 3Faulty thermostat or sensor
4. 4Engine computer fault (rare)

Repair difficulty: Easy

What you can check yourself

First look at the connector and wiring: anything loose, melted or green with corrosion? Check the coolant level with the engine cold. If the temperature gauge rises into the red, stop at once. You can do the simpler checks yourself.

How we fix it

1. We read the fault data on the diagnostic tool and check whether the fault is constant
2. We measure the circuit's supply, ground and signal
3. We repair the wire or replace the connector
4. If the wiring is fine, we replace the thermostat or sensor

Standard SAE description: Thermostat Heater Control Circuit High