P0597 on Mercedes-Benz: How to Diagnose & Fix
Thermostat Heater Control Circuit Open
2 Mercedes-Benz models · 2 documented issues · $350-$3,600 repair
P0597 on Mercedes-Benz by model: symptoms, diagnosis and fix
The 1.3L M282 engine develops coolant leaks at the integrated thermostat housing/gasket and at the water pump and coolant hoses, commonly in the 55,000-70,000 mile range. A failing thermostat housing causes temperature gauge fluctuation, coolant loss, and slow cabin heating; if ignored it can lead to overheating. Coolant leaks are repeatedly cited as one of the most common cooling-system issues on A 250/A 200 owners.
Common Symptoms
- Coolant warning / low coolant level
- Temperature gauge fluctuation
- Overheating
- Slow cabin heat-up
- Sweet smell / coolant puddle under car
- Visible coolant residue at front of engine
How to Fix
Replace the complete OEM thermostat housing assembly (it is plastic and integrated, so the full unit is required), inspect/replace water pump and coolant hoses, then refill and bleed the cooling system with the correct MB-spec coolant. Address promptly to avoid overheating damage.
Sources for this issue (2)
CLA owners report a check engine light with P0128 (coolant temperature below thermostat regulating temperature): the gauge sits low, cabin heat is weak and fuel economy drops because the map-controlled thermostat fails open. On the M270 CLA250 the thermostat sits behind the water pump so labor is significant, and dealers have replaced the thermostat under warranty after a second visit. On the M133 CLA45 the job is worse: a 2018 CLA45 owner was quoted about $3,600 because the dealer booked 9 hours of labor. Related codes P0597 (thermostat heater control) appear alongside P0128 on some cars.
Common Symptoms
- Check engine light with P0128
- Temperature gauge stays low or takes very long to rise
- Weak cabin heat
- Reduced fuel economy
- P0597 thermostat heater fault on some cars
How to Fix
Confirm with live data that coolant temperature stalls below ~80 C. Replace the thermostat (and housing/seal) with the current Mercedes part and clear codes; check the coolant temperature sensor and coolant level if the code returns. Independent M270 replacement is typically $400-900; the M133 is far more labor-intensive. Warranty applies if in term.
Sources for this issue (4)
P0597 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 P0597 mean on a Mercedes-Benz?
P0597 stands for "Thermostat Heater Control Circuit Open." P0597 is a cooling-system code indicating an open electrical circuit in the control for the heated (electronically controlled) engine thermostat. Many modern engines use a thermostat with a built-in heater element that the computer can energize to open the thermostat sooner for better temperature management and emissions. When the PCM commands that heater but detects no electrical continuity (an open circuit), it sets this code. It generally means the thermostat heater isn't being controlled, which can affect coolant temperature regulation, fuel economy, and emissions. On Mercedes-Benz it is documented across 2 models: A-Class, CLA.
What are the most common causes of P0597?
The most common causes of P0597 are: Failed thermostat heater element or integrated control thermostat, Open or broken wiring to the thermostat heater circuit, Corroded, loose, or disconnected thermostat connector, Blown fuse or open in the heater power feed, Poor ground or control-circuit fault from the PCM, Damaged PCM driver for the heater (rare). The specific cause varies by vehicle.
How much does it cost to fix P0597 on a Mercedes-Benz?
Published repair estimates on Mercedes-Benz range from $350 to $3,600 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.
Looking for P0597 on a different make?
View P0597 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.