P0598 Thermostat heater control circuit low: the computer sees too little voltage in the circuit of an electrically heated thermostat

Thermostat heater control circuit low

Severity not verified — if the car runs badly, have it inspected

We have not sourced a severity ruling for P0598. A thermostat that is not controlled correctly could affect engine temperature, so have a shop check it if the engine runs hot or cold.

Source: how we rate severity

P0598 in short

P0598 means the computer sees abnormally low voltage in the control circuit for a thermostat with a built-in electric heater. It is listed for BMW, Mercedes, Jaguar, MINI, Audi and VW. Damaged wiring, a faulty thermostat heater or low coolant are possible causes, but treat the list as leads for testing.

Some engines use a thermostat that has an electric heating element, which the computer uses to open it early and run the engine cooler under load. P0598 sets when the voltage in that heater circuit reads too low.

TroubleCodes.net lists the code for BMW, Mercedes, Jaguar, MINI, Audi and VW. It names damaged or corroded wiring and connectors, a defective thermostat or heater element, low coolant, a faulty coolant sensor or fan and, rarely, the computer.

We did not find a service bulletin for P0598. Two Chevrolet Volt owners report it in a long code list with a blown underhood fuse and an EGR valve short, which suggests a shared power supply fault in that case. Treat that as a lead only.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Damaged or corroded wiring or connector in the thermostat circuit

    Common general cause
    Why it ranks here
    TroubleCodes.net lists damaged, shorted or corroded wiring and connectors first.
    What would change this
    Rises if the connector is wet, corroded or the harness is damaged. Falls if wiring tests good.

  2. Defective thermostat or thermostat heater element

    Common general cause
    Why it ranks here
    TroubleCodes.net lists a defective thermostat or heater element; it ranks near the top because the circuit ends there.
    What would change this
    Rises if the heater resistance is out of specification or the circuit tests good up to the thermostat. Falls if the heater tests normal.
    How to test the thermostat

  3. Blown fuse or shared power fault (reported on Chevrolet Volt)

    Possible — needs testing
    Why it ranks here
    Two Volt owners say dealers found P0598 with a blown underhood fuse and an EGR valve short, but those are single repair reports with many other codes.
    What would change this
    Rises if other codes point to the same fuse or power supply. Falls if the fuse is intact and no other circuits are affected.
    How to test the underhood fuse block (engine compartment fuse box)

  4. Low coolant, coolant sensor or fan fault

    Possible — needs testing
    Why it ranks here
    TroubleCodes.net lists low coolant, a faulty coolant sensor or cooling fan as other possible causes.
    What would change this
    Rises if coolant is low or other cooling codes are stored. Falls if the cooling system is in good order.
    How to test the engine coolant temperature (ect) sensor

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all stored codes and freeze-frame data. Note any other cooling, fuse or power-supply codes.

    Low riskTool: OBD-II scan tool

    • IfSeveral codes share a power or fuse problem

      It meansCheck the fuse and shared power supply first.

      Go to step 2

    • IfOnly P0598 is stored

      It meansInspect the thermostat connector and wiring.

      Go to step 2

  2. Check coolant level when the engine is cold, the related fuses, and the thermostat connector and wiring for damage or corrosion.

    Low riskTool: flashlight, test light

  3. Use a bidirectional scan tool to command the thermostat heater and measure voltage and resistance at the connector.

    Best left to a shopTool: bidirectional scan tool, multimeter

    Warning:This needs specialist tools and vehicle service information. Leave it to a shop.

Symptoms

  • Check engine light on
  • Engine running hotter or colder than normal
  • Reduced power (reported on Volt with other codes)

How to fix

  1. Check the coolant level only when the engine is cold.

Parts and cost context

  • Thermostat:The thermostat housing assembly is the likely part on engines with an electrically heated thermostat, but test the circuit first. Get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Chevrolet

  • 2017–2018 · Volt

    Possible — needs testing

    Two Volt owners report P0598 in a long code list with a blown underhood fuse for the EGR valve and an EGR valve short.

  • P0597Thermostat heater circuit open

    Same system Supported by vehicle data

    P0597 is the thermostat heater control circuit open code; two Volt owners report P0597 and P0598 together.

All Engine cooling (thermostat) codes

P0598 questions

What is a thermostat heater?

Some engines use a thermostat with an electric heating element so the computer can open it early. P0598 is a low-voltage fault in that circuit.

Sources and review

  1. P0598 trouble code overview — TroubleCodes.net
  2. NHTSA complaint ODI 11757544: Chevrolet Volt 2018, P0598 with P0597 and EGR valve short — NHTSA Office of Defects Investigation
  3. NHTSA complaint ODI 11632222: Chevrolet Volt 2017, P0598 with blown EGR fuse — NHTSA Office of Defects Investigation
Last reviewed
Written by
Saurabh K.
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