P0599 on Mercedes-Benz: How to Diagnose & Fix
Thermostat Heater Control Circuit High
1 Mercedes-Benz model · 1 documented issue · 1 critical · $400-$1,200 repair
P0599 on Mercedes-Benz by model: symptoms, diagnosis and fix
The water pump on the M270 engine fails, causing loss of coolant circulation and rapid overheating. The pump fails internally or leaks, 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 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)
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 Mercedes-Benz?
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 Mercedes-Benz it is documented across 1 model: GLA.
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 Mercedes-Benz?
Published repair estimates on Mercedes-Benz range from $400 to $1,200 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.