P0598 on Audi: How to Diagnose & Fix
Thermostat Heater Control Circuit Low
2 Audi models · 2 documented issues · 1 critical · $400-$5,000 repair
P0598 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)
P0598 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 P0598 mean on a Audi?
P0598 stands for "Thermostat Heater Control Circuit Low." This code applies to engines with an electronically controlled (heated) thermostat and means the Powertrain Control Module detected an abnormally low voltage in the thermostat heater control circuit. The heater element lets the computer fine-tune coolant temperature for efficiency and emissions by opening the thermostat sooner under certain conditions. P0598 sets when the control or feedback circuit reads lower than expected (often a short to ground or loss of supply). It is found on makes that use heated thermostats, including several European makes (BMW, Mercedes, VW/Audi) and GM small engines such as those in the Chevrolet Cruze, Sonic and Aveo, and it usually has little effect on driving. On Audi it is documented across 2 models: A7, Q7.
What are the most common causes of P0598?
The most common causes of P0598 are: Faulty electronically controlled (map-controlled) thermostat, Short to ground in the heater control wiring, Corroded or loose thermostat connector, Open or damaged wiring in the control circuit, Low coolant level overheating the thermostat heater, Blown fuse or poor power supply to the circuit, Faulty PCM control driver. The specific cause varies by vehicle.
How much does it cost to fix P0598 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.