Faulty thermostat heater element (internal short circuit) in the electronically controlled thermostat; Damaged, chafed or corroded wiring to the thermostat heater, causing a short to ground; Corroded, loose or water-contaminated connector at the thermostat; Coolant leak at the thermostat housing seeping into the connector; Fault in the engine control unit output driver for the thermostat heater; Outdated or faulty control unit software, only where the manufacturer has issued a known fix for the model.

P0598

Thermostat Heater Control Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
5: AreaVehicle speed, idle control and auxiliary inputs
98: FaultThe specific fault in that area

What it means

P0598 is the generic code for a low signal in the thermostat heater control circuit. The engine control unit drives a heating element inside an electronically controlled thermostat so coolant temperature can be adjusted to suit load. It sets the code when the voltage it sees on that control circuit is lower than expected, which points to a short to ground, a wiring fault or a faulty heater element.

Symptoms

Engine management warning light on
Engine running cooler than normal, or slow to reach operating temperature
Engine running hotter than normal under load in some cases
Poor fuel economy
Cooling fan running more than usual
Temperature gauge reading unusually low or erratic

Possible causes

Faulty thermostat heater element (internal short circuit) in the electronically controlled thermostatlikely
Damaged, chafed or corroded wiring to the thermostat heater, causing a short to groundlikely
Corroded, loose or water-contaminated connector at the thermostatlikely
Coolant leak at the thermostat housing seeping into the connectorpossible
Fault in the engine control unit output driver for the thermostat heaterrare
Outdated or faulty control unit software, only where the manufacturer has issued a known fix for the modelrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting coolant temperature and load when the fault set, then check for related cooling system codes.
Check coolant level and look for leaks around the thermostat housing and connector.
Visually inspect the thermostat connector and wiring for chafing, heat damage, corrosion, moisture or pushed-back pins.
Check coolant temperature readings on live data against actual engine temperature to confirm the engine reaches and holds normal operating temperature.
With the connector unplugged, measure the heater element resistance and compare it with the manufacturer's specification, which varies by model and engine.
Check the control circuit wire for a short to ground and for continuity back to the control unit, with the battery disconnected as appropriate.
Check the supply voltage and ground feeds to the thermostat circuit, including relevant fuses where fitted.
Check for any manufacturer software updates or technical bulletins relating to this code on the specific model.

How to fix

Replace the electronically controlled thermostat if the heater element is shorted or out of specification.
Repair or replace damaged wiring between the thermostat and the control unit.
Clean, dry or replace the corroded or contaminated connector and its terminals.
Fix any coolant leak at the thermostat housing and top up the coolant to the correct specification.
Update the control unit software only if the manufacturer lists it as a fix for this fault, or repair or replace the control unit if the output driver is proven faulty.

Can I keep driving?

Medium severity

The car can usually be driven short distances, but book it in soon because poor temperature control can raise fuel use and wear, and may cause overheating under load. Stop and investigate if the temperature gauge rises or a coolant warning appears.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.