P0599 Code: Thermostat Heater Control Circuit High

What the P0599 code means

Map-controlled thermostats (BMW, Volkswagen, Audi, Opel, Mercedes) have a heating resistor inside the wax. The computer turns it on so the thermostat opens sooner and the engine runs cooler under high load.

The computer watches the control circuit from its own driver transistor. The P0599 code means high voltage in the control circuit (an open circuit or a short to voltage). Before you replace the part, check the fuse, relay, connector and wiring, which are the most usual cause.

If the heater does not work, the thermostat still opens based on temperature, but the engine runs hotter than intended under load. It is not urgent, but it should be repaired. In many cases the whole thermostat is replaced.

To confirm it, measure the continuity of each wire between the component connector and the computer connector with the key off. It should be less than 1 ohm. An infinite reading means the wire is open. If the wiring is good, measure the component itself: infinite resistance confirms it is open inside.

P0599 code symptoms

• Check engine light or another warning light on.
• Engine temperature that is less precise.
• Engine temperature somewhat higher under load.

Likely causes of the P0599 code

1. Loose or broken connector. The most common cause, especially after another repair.
2. Open coil or motor with infinite resistance.
3. Cut control wire between the component and the computer.
4. Short to 12 V. The control wire touches another wire that carries voltage.

How to diagnose the P0599 code step by step

1. Activate the component with the scan tool and check whether it responds.
2. Check the fuse and relay. Measure voltage on both sides of the fuse and swap the relay with an identical one.
3. Measure the supply at the connector. You should see battery voltage with the key on or when you command it.
4. Measure the component resistance and compare it with the manual, and also check the insulation to ground.
5. Check the control wire for continuity to the computer and for the absence of shorts.
6. Measure the heater resistance. As a guide, 10 to 20 ohms.

How to fix the P0599 code

• Repair the connector, the wiring or the power supply.
• Replace the thermostat heater if it does not respond with correct power and command.
• Clear the codes and verify.

Technical reference data

• 12 V heating resistor inside the thermostat.
• Reference values only: check the manufacturer data for your vehicle.

Brand-specific notes

This is very frequent on BMW (electronic thermostat) and on Volkswagen and Audi vehicles with a map-controlled thermostat.

P0599 on BMW

Very frequent on BMW vehicles with the electronic thermostat.

P0599 on Volkswagen and Audi

Very frequent on Volkswagen and Audi vehicles with a map-controlled thermostat.

Frequently asked questions about P0599