P0599 on Audi: How to Diagnose & Fix
Thermostat Heater Control Circuit High
2 Audi models · 2 documented issues · 1 critical · $400-$5,000 repair
P0599 on Audi by model: symptoms, diagnosis and fix
The 3.0T V6 thermostat housing in the C7 A7 develops coolant leaks from the plastic housing and its O-ring seals. The housing sits in the engine valley beneath the supercharger and cracks or warps over thermal cycling. Coolant leaks onto the serpentine belt area and can cause belt slippage or overheating if coolant loss is significant.
Common Symptoms
- Coolant puddle under front of vehicle
- Sweet coolant smell from engine bay
- Low coolant warning light
- Coolant visible on serpentine belt
- Temperature gauge reading higher than normal
- Steam from engine bay
- Coolant residue around thermostat housing area
How to Fix
Replace the thermostat housing assembly with an updated part. The O-ring seals should always be replaced. Some owners upgrade to aluminum aftermarket housings for improved durability. The water pump should be inspected during this repair as it shares the same coolant circuit.
Sources for this issue (2)
The 2007-2015 Audi Q7 (3.0T, 3.6L, TDI) experiences water pump and thermostat failures causing engine overheating. On some 2009-2011 vehicles with 3.0L TFSI or 3.0L TDI engines, Audi found that a seal inside the coolant pump can wear out prematurely and leak coolant, turning on the Coolant System Malfunction light (Audi service action 19H1). When the pump fails, coolant flow drops causing rapid overheating and severe engine damage (warped cylinder heads, blown gaskets). Thermostat housings crack and leak coolant. Failure often occurs around 60k-80k miles. Preventive replacement recommended.
Common Symptoms
- Engine temperature gauge rising into red
- Coolant warning light
- Coolant leaking under vehicle
- Steam or smoke from engine bay
- Check engine light with cooling system codes
- Heater not blowing hot air
- Overheating
How to Fix
IMMEDIATE if overheating: Pull over, turn off engine, DO NOT continue - engine damage occurs in minutes. Check with an Audi dealer whether service action 19H1 (free coolant pump replacement for affected 2009-2011 3.0L TFSI/TDI vehicles) applies to the VIN. Preventive: Replace water pump/thermostat assembly at 60k-80k miles ($800-1,500) to avoid catastrophic failure. Failed pump: Replace water pump/thermostat ($800-1,500). If overheated: Cylinder head inspection, possible head gasket replacement ($2,500-5,000). Use OEM Audi or premium Geba/Meyle parts.
Sources for this issue (1)
P0599 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0599 mean on a Audi?
P0599 stands for "Thermostat Heater Control Circuit High." This code relates to the engine cooling system's thermostat heater control circuit, used on engines with an electrically heated (map-controlled) thermostat that the computer can open early to fine-tune coolant temperature. The 'high' designation means the computer detected the control circuit voltage was higher than expected, indicating an electrical fault such as a short to voltage or an open circuit. It is generally an electrical/wiring problem rather than a mechanical cooling failure. It can affect how precisely the engine manages its operating temperature and fuel economy. On Audi it is documented across 2 models: A7, Q7.
What are the most common causes of P0599?
The most common causes of P0599 are: Faulty electronically heated thermostat element/heater, Short to voltage in the heater control circuit, Open or damaged wiring/connector, Corroded or loose connector at the thermostat, Poor electrical ground, Faulty PCM/ECM driver (less common). The specific cause varies by vehicle.
How much does it cost to fix P0599 on a Audi?
Published repair estimates on Audi range from $400 to $5,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.