P0598 Thermostat Heater Control Circuit Low

Investigate soon

The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.

Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

The definition

P0598: Thermostat Heater Control Circuit Low.

P0598 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

Unplug the sensor. If the reading jumps to full scale, the wiring is fine and the sensor has failed low.
Check the sensor's 5V supply and its earth before condemning the sensor itself.

What actually causes this on a VAG

Same circuit as P0597, but this time the ECU sees the heater circuit sitting low - in VAG's own wording for this fault, a short to earth rather than a break. Either the heater inside the map-controlled thermostat has shorted internally, or its wiring is touching earth somewhere along the way. As with P0597 the thermostat still does its normal job on coolant temperature, so the engine is protected, though it will work to its higher base opening temperature until this is fixed; the fault itself is purely electrical.

Most likely causes, in order

Chafed or pinched wire shorting to earth. The most likely cause, and the cheapest to put right. Follow the two wires from the thermostat plug and look for insulation rubbed through against the engine, a bracket or a heat shield, particularly if anything has been off in that area recently.
Water or corrosion inside the connector. Coolant residue, road spray or a slow seep from the housing can bridge the pins inside the plug. Open it up, look for green deposits, and dry and clean it properly before condemning anything.
Heater element shorted inside the thermostat. The element can fail short as well as open. On most VAG engines that means replacing the thermostat as a complete housing assembly, with a coolant drain and a proper bleed afterwards.
ECU output driver. Last on the list, and only after both the harness and the component have been proved good.

Worth knowing: Unplug the thermostat and clear the code, then give it a key cycle and a short drive so the ECU gets a chance to re-test the circuit. If the low/short fault comes straight back with the thermostat disconnected, it's in the wiring; if it changes to an open-circuit fault or stays away, the short is inside the thermostat. That one test decides between a wiring repair and a new thermostat housing - and plug it back in before you finish.

Reading the code is the easy part

A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.

VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.

$249 once. No subscription, unlimited cars. Windows 10/11.

See VAGPULSE — $249 Look up another code

Software downloads instantly · cable ships September 30 · cable guarantee — refund if it won't connect

Prefer to work on your phone? The VAGPULSE Android app is a free download and its demo mode runs against a sample car with no hardware at all, so you can see exactly what it reads before spending anything. Reading your car with it needs the VAGPULSE cable, which comes with the licence.

Common questions

What does P0598 mean?

P0598 means Thermostat Heater Control Circuit Low. The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.

Can I drive with P0598?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.