P0599: How to Diagnose & Fix — Thermostat Heater Control Circuit High
14 vehicles · 9 makes · $60-$6,000 repair
How to diagnose and fix P0599: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
MINI Cooper · 2007–2016
Both R56 and early F56 Coopers use an electric thermostat housing and electric water pump that are prone to premature failure. The thermostat housing cracks causing coolant leaks, and the water pump impeller can fail leading to overheating.
Common Symptoms
- Coolant warning light
- Overheating
- Coolant puddle under car
- Heater blowing cold air
- Temperature gauge fluctuation
How to Fix
Replace thermostat housing assembly and/or electric water pump. Use updated MINI parts. The thermostat housing is on the front of the engine and relatively accessible. Water pump is belt-driven on N12/N16, electric on newer B38/B48.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Saleri MINI water pump 11517648827OEMSaleri PA1425 / MINI 11517648827
Confirm VIN, engine and current interchange before purchase; this is not an el
- Mahle MINI thermostat assembly 11538699290OEMMahle TM57105 / MINI 11538699290
R56 N12/N14 and VIN-confirmed N16 thermostat-housing branch Fitment/availability: The inspected direct U.S. Confirm VIN, engine, production date and connector; do not use for an N18 or
- Rein MINI thermostat housing 11538674895OEMRein CTA0051 / MINI 11538674895
R56 N18 and VIN-confirmed later N16 thermostat-housing branch Fitment/availability: The inspected direct U.S. Use only after VIN, engine and connector verification confirms the N18-style branch; do not infer it from
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Audi A7 · 2012–2018
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.
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 (1)
Mercedes-Benz GLA · 2015–2020
The electric water pump on the M270 engine fails, causing loss of coolant circulation and rapid overheating. The pump motor or impeller breaks, and failure is often sudden with no warning. Common between 50,000-90,000 miles.
Common Symptoms
- Engine temperature spikes rapidly
- No heat from cabin heater
How to Fix
Replace the electric water pump and thermostat together. Use OEM or Pierburg (OEM supplier) parts. Flush the cooling system with fresh Mercedes-approved coolant. Monitor the temperature gauge closely on high-mileage GLA models.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Hepu 2702000800KT M270 water-pump and thermostat replacement kitOEMHepu/FCP 2702000800KT
Confirmed X156 GLA250 M270 main mechanical-pump replacement with thermostat/coolant service parts Fitment/availability: The inspected direct U.S.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Owner tips & cautions
- WarningIf the temperature gauge moves above the halfway mark, pull over immediately. The electric water pump gives zero warning before complete failure — overheating for even 2 minutes can warp the head.
Sources for this issue (4)
Cadillac STS · 1998–2011
Jeep Patriot · 2007–2017
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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?
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.
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?
Repair costs for P0599 range from $60 to $6,000, depending on the vehicle and root cause.
Which vehicles are affected by P0599?
Au7o has documented P0599 across 14 vehicle models from 9 manufacturers: Audi, BMW, Buick, Cadillac, Chevrolet, Dodge, Jeep, MINI, Mercedes-Benz.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.