P0598 Thermostat Heater Control Circuit Low

P0598 points to a low condition in the thermostat heater control circuit. The controller sees heater control voltage, current, or feedback below the expected range.

Code details

Generic OBD-II · P0

System: Speed / Idle
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the thermostat housing until the low heater-control circuit fault is isolated.

Symptoms of P0598

Unstable, high, or low idle, or stalling.
Erratic speedometer, cruise-control, or shift behaviour.
Check-engine light with a hesitation or surge.

Common causes of P0598

A dirty throttle body, idle-control valve, or throttle-position sensor.
A failing vehicle-speed or wheel-speed sensor, or its wiring.
Vacuum leaks or a related sensor forcing the idle/speed strategy.

Useful checks before replacing parts

Check thermostat heater command, control voltage, current draw, fuse feed, ground or driver control, coolant temperature, and freeze-frame data.
Inspect for short-to-ground faults, open power feed, damaged thermostat wiring, connector coolant intrusion, failed heater element, and weak grounds.
Measure circuit response while the heater is commanded to identify whether the low reading comes from wiring, driver, or heater load.

Related OBD-II codes

These nearby speed / idle entries can help compare wording and pin down the fault area.