P0599
PowertrainThermostat Heater Control Circuit High
Most of the time this comes down to corrosion or moisture getting into the thermostat connector, throwing the voltage on the heater circuit too high. Modern cars use a map-controlled electric thermostat with a small heater element inside, and the ECU watches the voltage on that circuit. When it sees a reading that's higher than it should be, it flags P0599 and assumes a short, a wiring fault, or a dead heater element. It's an electrical complaint about the thermostat, not the thermostat's basic job of opening and closing.
ⓘ Information only. This page provides general educational information about fault code P0599. 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 P0599 mean?
P0599 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Thermostat Heater Control Circuit High.
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:
- • Check Engine light on, sometimes the only thing you'll notice
- • Engine takes longer than normal to warm up from cold
- • Cabin heater blows lukewarm or is slow to get hot, most noticeable on frosty mornings
- • Coolant temperature gauge creeps up more slowly than you're used to
- • Slightly worse fuel economy on short runs, because the engine sits in warm-up enrichment longer
- • On plenty of cars there's no drivability issue at all, just the warning light
Possible causes
Causes commonly associated with P0599, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Water and corrosion in the thermostat connector. This is the usual offender, especially on cars that have done a few winters
- 2. Internal heater element in the electronic thermostat has shorted or failed
- 3. Damaged or chafed wiring between the thermostat and the ECU, look for cracked insulation near heat sources
- 4. Poor earth connection on the circuit, a dirty or loose ground pushes the measured voltage up
- 5. Coolant seeping past a cracked thermostat housing and getting into the electrical contacts
- 6. Failed output driver inside the ECU itself, rare but it 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. Unplug the thermostat connector and have a proper look inside it. Green corrosion, white crust, or water sitting in the pins explains most P0599s before you've touched a multimeter
- 2. Read all stored codes, not just this one. A P0599 alongside coolant temp codes like P0125 or P0116 points more towards the thermostat unit than the wiring
- 3. Measure resistance across the heater element terminals. Under about 5 ohms suggests an internal short, over 100 ohms or open circuit means the element is dead
- 4. Back-probe the circuit voltage with the ignition on and confirm the high reading is real and not a scanner glitch
- 5. Check the earth point is clean and tight. A bad ground will lift the voltage and mimic a circuit fault
- 6. Inspect the thermostat housing for coolant weeping. If it's wet around the connector, that's your route for the moisture
Common questions about P0599
What am I likely to pay to get this sorted? +
If it turns out to be a dirty connector, a decent independent garage might charge you an hour of labour to clean it up and seal it, so under £80. If the electronic thermostat itself has failed, you're looking at the part plus housing, which on something like a VAG 1.4 TSI or a BMW N20 typically lands the bill somewhere around £200 to £450 fitted at an independent. A main dealer will want noticeably more for the same job, often £400 plus, mostly down to their labour rates and using the genuine part.
How do I know if it's the wiring or the thermostat on my car? +
Start at the connector, because moisture and corrosion there is the cheapest and most common cause. If the pins are clean and dry, measure across the heater element. A reading near zero or sky-high tells you the element is gone and the thermostat needs replacing. If the element reads fine and the wiring looks good, check your earth, and only suspect the ECU after all of that comes back clean. Don't go straight to a new thermostat without checking the plug first, a lot of people waste money that way.
Can I have a go at this myself? +
Cleaning the connector is a fair weekend job if you've got a multimeter and an electrical contact cleaner. Replacing the thermostat is doable on some engines but a pain on others, particularly tight V6 layouts where it's buried at the back. You'll be draining coolant either way, and on some cars the ECU wants the thermostat adaptation reset afterwards, which needs a capable scan tool. If yours falls into that camp, factor in a garage visit even if you fit the part yourself.
If I just clear the code, will it stay gone? +
If the fault is still live, no. Clear it and it'll come back within a drive cycle or two once the ECU sees the high voltage again. The only time clearing it sticks is when you've actually fixed the cause, say after cleaning out a corroded plug. Leaving it unfixed won't strand you, but the engine sits in warm-up longer than it should, which nibbles at fuel economy and emissions over the winter months.
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 →