P0598 Thermostat heater: signal or voltage too low

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

P0598 means: Thermostat heater: signal or voltage too low. The cooling system keeps the engine at the right temperature. OK to drive, but have it checked soon. The most common repair costs about €45–90 in Estonia, depending on make and model. The most common cause: connector loose, damp or corroded.

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 low. Usually a wire is broken, a connector is loose or a wire is shorted to ground.

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 · Wiring or connector repair €45–90
2. 2Wire broken or shorted to ground · Wiring or connector repair €45–90
3. 3Faulty thermostat or sensor · Thermostat replacement €100–200
4. 4Engine computer fault (rare) · Control unit check and software €150–300

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 Low