P0599 Thermostat Heater Control Circuit High

Investigate soon

The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.

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

P0599: Thermostat Heater Control Circuit High.

P0599 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 and check whether the signal falls. If it stays high, suspect the wiring rather than the sensor.
Check the earth path - a missing earth makes a sensor read high.

What actually causes this on a VAG

Many VAG engines - petrol ones especially - don't use a plain wax thermostat. They use a map-controlled one with a small electric heater built into it: left alone it opens at a higher, more economical temperature, and when the ECU feeds current to that heater the wax is warmed artificially so the thermostat opens earlier and the engine runs cooler under load. P0599 means the ECU is seeing the voltage on that heater control circuit higher than it expects. In practice that usually means an open circuit - a broken wire, a bad connection, or a burnt-out element - though a short to battery voltage on the same wire can read the same way. The engine still warms up and still cools; it just loses the ability to force the cooler setting, so there may be nothing at all to feel from the driver's seat beyond the warning light.

Most likely causes, in order

Connector or wiring at the thermostat housing. This is the free thing to rule out first. The plug sits on the coolant housing in a hot, damp corner of the bay, and these plastic housings are known for weeping coolant that corrodes pins and wicks into the loom. Unplug it and look for green or white corrosion, spread or pushed-back pins, and chafed insulation where the loom passes anything hard.
Heater element inside the thermostat has gone open circuit. The element is a small resistive heater inside the thermostat itself, and when it burns out the circuit reads open - which is exactly what 'high' describes. With the plug off, a meter across the thermostat's two terminals should show continuity and a low resistance rather than no reading at all. If you want a proper pass/fail figure, take it from workshop data for that exact part rather than a number quoted on a forum. On most of these engines the element isn't sold separately, so the thermostat, or the whole housing it sits in, is replaced as an assembly.
ECU output driver has failed. The ECU switches this heater - on these systems typically on the earth side - and that output can fail internally. It's genuinely rare and should only be considered once the element and the wiring back to the ECU have both been proved good with a meter.

Worth knowing: Don't judge this one by the temperature gauge. VAG dash gauges are heavily damped and sit in the same place across a wide band, so the needle will look perfectly normal with this fault present. And don't let it push you into a full coolant-housing replacement before someone has actually put a meter on the element and the connector, because the connector is the cheap failure and costs nothing to check.

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.

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Common questions

What does P0599 mean?

P0599 means Thermostat Heater Control Circuit High. The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.

Can I drive with P0599?

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.