P0598
PowertrainThermostat Heater Control Circuit Low
The ECU watches the voltage on the control circuit that feeds the little heater built into your electronic thermostat. When it sees the voltage drop too low, it logs P0598. Modern engines use a heated, map-controlled thermostat so the ECU can open and close it for better fuel economy and emissions, rather than relying purely on coolant temperature. If that heater circuit shorts out or loses voltage, the thermostat can't be commanded properly and the engine struggles to manage its temperature.
ⓘ Information only. This page provides general educational information about fault code P0598. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0598 mean?
P0598 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Thermostat Heater Control Circuit Low.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on the dash, often with no other obvious sign
- • Engine takes much longer than normal to warm up, especially on short trips in winter
- • Cabin heater feels weak or slow to get warm
- • Fuel economy drops, more noticeable on shorter runs around town
- • Temperature gauge sitting lower than usual, or occasionally creeping high if the thermostat sticks
- • Slightly rougher running from cold until the engine reaches temperature
Possible causes
Causes commonly associated with P0598, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed thermostat heater element with an internal short, the most common cause on these map-controlled units once they age
- 2. Corroded or water-damaged connector at the thermostat, very common because it sits low in the engine bay and gets wet
- 3. Chafed or shorted wiring between the thermostat and the ECU, often where the loom rubs against the housing
- 4. Blown fuse feeding the thermostat heater circuit
- 5. Poor or corroded earth in the circuit giving false low readings
- 6. Coolant getting into the electrical connector through a cracked thermostat housing
- 7. Faulty ECU driver output, rare but does happen once everything else checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read all stored codes and note any P0597 or P0599 alongside it, they point at the same circuit and help narrow things down
- 2. Unplug the thermostat connector and look hard for green corrosion, moisture or melted pins, this catches a good share of these faults
- 3. Check the relevant fuse for the thermostat heater circuit before you go chasing wiring
- 4. Back-probe the connector and check for supply voltage with the ignition on, then test the heater element resistance against the manufacturer spec
- 5. Wiggle-test the loom between the thermostat and ECU while watching for the reading to flicker, that flushes out chafed or broken wires
- 6. If the wiring, earth and connector all test sound, the thermostat assembly itself is the likely fix
Common questions about P0598
Should I just buy a cheap thermostat off eBay or pay for a proper one? +
On these electronic thermostats I'd steer clear of the bargain-basement parts. The heater element and integral sensor are exactly what fails, and the no-name units often use poorer components that pack up again within a year or two. On VW, Audi, BMW and Mini engines especially, the ECU is fussy and a cheap copy can throw the code straight back at you. Spend on a quality OEM-equivalent unit from a recognised brand. The whole housing usually comes as one assembly anyway, so doing the job twice is a false economy.
Can I keep driving with P0598 showing? +
For a short while, yes, the car will still drive. The risk is your engine never reaching proper operating temperature, which hammers your fuel economy and can let the thermostat stick. If it sticks closed you can overheat, and if it sticks open the engine runs cold and wears faster. I wouldn't leave it for weeks. Get it looked at before the next cold snap, because a heater that's already drawing wrong is a fault that tends to get worse, not better.
Is this going to be an MOT problem? +
The P0598 code on its own isn't a direct MOT failure point. What catches people out is the engine warning light. If the MIL is glowing on the dash when the tester looks at the dashboard, that's a fail on its own. Clear the fault, drive a few cycles to make sure the light stays off, then put it in. A failed cold-running engine can also nudge your emissions reading the wrong way if the cat hasn't lit off properly.
What sort of money am I looking at to sort it? +
If it's just a dirty connector or a blown fuse, you're into pocket-change territory, maybe £20 to £40 at an independent. A full electronic thermostat assembly is the usual repair, and on a typical German engine you're looking at roughly £150 to £350 fitted at a good independent garage. A main dealer will commonly be £350 to £600 once you add their labour rates and genuine parts. Get the wiring properly checked first, no point fitting a new thermostat if a chafed wire is the real culprit.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →