P0599 on Audi
Thermostat Heater Control Circuit High
P0599 on Audi vehicles indicates thermostat heater control circuit high. Au7o has documented this code across 6 Audi models — most commonly on A4, A7, Q5. 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. Typical repair costs on Audi range from $150 to $5,000, depending on the specific model and root cause.
Common Causes of P0599
- •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)
P0599 on Audi by Model
Audi A4(1 issue)
- Water Pump/Thermostat Failure2009-2020
The plastic water pump housing and thermostat are known failure points on Audi 2.0T engines. Leaks and failures typically occur between 60,000-100,000 miles. The plastic components crack from heat cycling.
Audi A7(1 issue)
- Thermostat Housing Coolant Leak2012-2018
The 3.0T V6 thermostat housing in the C7 A7 develops coolant leaks from the plastic housing and its O-ring seals. The housing is located at the front of the engine 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.
Audi Q5(1 issue)
- Coolant Leaks (Thermostat Housing and Hoses)2009-2017
Audi Q5 (2009-2017) experiences coolant leaks from thermostat housing, coolant flanges, and hoses. Plastic components crack with age/heat cycles, causing leaks. Symptoms include coolant smell, visible leaks under vehicle, low coolant warning, and overheating. If leaks ignored, engine overheats causing severe damage. Common leak points: thermostat housing, upper coolant flange, lower coolant flange, heater core hoses.
Audi Q7(1 issue)
- Water Pump and Thermostat Failure (Overheating)2007-2015
The 2007-2015 Audi Q7 (3.0T, 3.6L, TDI) experiences water pump and thermostat failures causing engine overheating. Plastic impeller water pumps fail from debris clogging or electronic failures. When 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.
Audi S3(1 issue)
- Water Pump and Thermostat Housing Failure (EA888 Gen 3)2015-2024
The Audi S3 (2015-2024) with the EA888 Gen 3 2.0T engine suffers from chronic water pump and thermostat housing failures. The integrated water pump/thermostat assembly uses a plastic housing that cracks or warps from heat cycling, causing coolant leaks. The internal seal between the water pump and thermostat housing degrades, allowing coolant to leak externally or internally into the vacuum system. VW/Audi settled a class action lawsuit and extended the water pump warranty to 8 years/80,000 miles for 2014-2021 models. Failures typically occur between 40,000-80,000 miles. Audizine and BobIsTheOilGuy forums report this as the single most common EA888 Gen 3 failure. Dealer repair costs approach $2,000 due to 3.5 hours of labor.
Audi TTS(1 issue)
- Water Pump and Thermostat Housing Failure (2.0 TFSI)2009-2024
The Audi TTS across Mk2 (2009-2015) and Mk3 (2016-2024) generations suffers from water pump and thermostat housing failures. The plastic impeller water pump has a notorious lifespan of approximately 60,000 miles. On Mk2 models, the timing belt-driven water pump should be replaced during timing belt service. On Mk3 models with the EA888 Gen 3, the integrated water pump/thermostat housing uses plastic construction that cracks and leaks coolant. The Mk3 is additionally covered by the VW/Audi water pump class action settlement (8 years/80,000 miles warranty extension for 2014-2021 models). TTForum.co.uk reports the water pump as one of the most common TTS failures, with many owners replacing preemptively at 60,000 miles. Ignoring the leak leads to overheating, warped heads, and blown head gaskets.
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View P0599 across all makes →Frequently Asked Questions
What does P0599 mean on 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 specifically, this code is documented across 6 models.
What causes P0599 on Audi vehicles?▼
Common causes on Audi: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0599 on a Audi?▼
Repair costs on Audi range from $150 to $5,000, depending on the specific model and root cause.
Which Audi models have P0599 documented?▼
Au7o has documented P0599 on 6 Audi models: A4, A7, Q5, Q7, S3, TTS.