The N55 thermostat is not your grandfather's wax-element thermostat. BMW engineers put a map-controlled electric thermostat in the N55 - a unit that does not simply open and close based on coolant temperature alone but responds to DME commands, adjusting coolant flow dynamically based on engine load, ambient temperature, and operating mode. In theory it is more efficient and more precise than a conventional thermostat. In practice, it is an electrical component in a hot, wet, chemically hostile environment, and it fails. When it does, it fails in ways that confuse owners who are used to traditional thermostat behavior.
I have replaced the N55 thermostat on F30 335i, F22 235i, F10 535i, and F15 X5 35i platforms, and the symptoms are remarkably consistent. The most common failure mode is the thermostat sticking open - this means the engine takes forever to reach operating temperature, fuel economy drops noticeably, heat output from the cabin vents is weak, and the temperature gauge sits below the normal mark. Less commonly, the thermostat fails closed or gets stuck partially closed, leading to overheating. Either failure mode will eventually store a fault code in the DME.
As with the water pump job, I always replace the thermostat and water pump together when either one comes in for service. They live in the same cooling circuit, they fail at similar intervals, and you are already draining and refilling the coolant. The incremental parts cost of doing both together is small compared to the labor of draining the system twice.
Electric map-controlled
N55 Thermostat Type
85-105 degrees C (load dependent)
Target Operating Temp
Sticking open
Common Failure Mode
$40 - $70
MAHLE/Behr Kit Cost
How the N55 Map Thermostat Works - and Why It Fails
Understanding how the map thermostat works helps you diagnose it accurately. The DME sends a variable current to the thermostat heating element. When the DME wants the engine to run cooler (heavy load, high boost, track conditions), it sends current to the heating element inside the thermostat, which effectively tells the wax element that the coolant is hotter than it actually is, causing the thermostat to open earlier. This allows more coolant flow and lower engine temps under load.
When the DME wants the engine to run warm (light load, short trips, emissions optimization), it reduces or eliminates the heating element current, allowing the wax element to operate at its natural opening temperature of approximately 88 to 92 degrees C. This is why on a gentle highway cruise you might see the temperature gauge sit at a slightly lower position than during a spirited drive on the same route.
The failure mechanism is usually the heating element or its associated wiring degrading over time. A failed heating element means the DME cannot command early thermostat opening, and you lose the load-based cooling adjustment. In many cases the thermostat then defaults to its mechanical wax element behavior, which on a cold or worn thermostat means it sticks open permanently.
| Operating Condition | Target Coolant Temp |
|---|---|
| Light load, warm ambient | 100 - 105 degrees C |
| Heavy load, high boost | 85 - 90 degrees C |
| Cold start warmup | Thermostat closed (max temp) |
| Fault / stuck open | Temperature does not reach normal |
Diagnosing N55 Thermostat Failure with a Scanner
Before ordering parts, confirm the thermostat with a proper scan. A basic ELM327-type scanner will not cut it here - you need one that reads BMW-specific DME data. The Vgate VLinker BM or equivalent Bluetooth scanner with a BMW-capable app like BimmerCode or BimmerLink gives you access to live coolant temperature data and fault codes.
Look for fault code 4530 (thermostat stuck open), 2E81 (coolant thermostat actuation), or similar N55 cooling system faults. With live data, monitor the coolant temperature during a cold start. On a healthy N55, the temperature should climb steadily and reach approximately 90 to 95 degrees C within 5 to 8 minutes of a cold start in moderate ambient temperatures. If it is still below 70 degrees C after 10 minutes of driving, the thermostat is likely stuck open.
Also check the coolant temperature with the live data tool versus what the gauge shows. BMW's temperature gauge is heavily damped and shows a steady reading across a wide temperature range. The actual DME coolant temperature reading shows the real story and can reveal subtle thermostat issues that the gauge never shows.
Choosing the Right Replacement Thermostat for N55
OEM quality is the only acceptable choice for the N55 electric thermostat. I have tried cheaper aftermarket units from generic suppliers and they consistently fail early or do not respond correctly to DME commands. MAHLE and Behr are the OEM suppliers for BMW thermostats and both offer replacement units through aftermarket channels at prices well below the dealer.
WGBAB Coolant Thermostat Housing Assembly with Sensors — N20 2.0L
$31.59
Get the complete housing assembly rather than just the thermostat element. The housing includes the temperature sensor, the housing body, and the thermostat unit pre-installed. This eliminates the risk of installing the element incorrectly or damaging the housing body during the process of extracting the old element. Complete housing assemblies cost a bit more but are worth the peace of mind.
Tools and Prep for N55 Thermostat Replacement
Tools - 10mm socket, T30 Torx, hose pliers for spring clamps, coolant drain pan, fresh coolant (1.5 to 2 gallons premixed 50/50), and a bleed tube or vacuum fill kit. Parts - replacement thermostat housing assembly, and if you are doing it right, a new water pump as well.
Park on a level surface. Engine stone cold. Disconnect the battery negative terminal - the thermostat has an electrical connector that you do not want energized during removal. Pop the hood and have everything on the bench before you start. Spontaneity is the enemy of cooling system work.
Step-by-Step N55 Thermostat Removal
Drain the coolant via the radiator petcock (lower driver corner of radiator, accessible from under the car). Remove the reservoir cap to speed draining. With approximately 7 liters drained (or when flow from the petcock slows to a drip), close the petcock.
Remove the engine cover. The N55 F30 engine cover lifts straight up. Remove the intake duct from the airbox to the turbo inlet (one spring clamp). This gives you working room in the engine bay.
Locate the thermostat housing - it is on the front of the engine on the driver side, adjacent to the water pump. You will see the electrical connector going into the top of the housing, and two coolant hoses connecting to it.
Unplug the electrical connector (press the tab, pull straight off). Loosen the hose clamps and pull the hoses. Residual coolant will drip - have your rag positioned. Remove the two or three mounting bolts. The housing lifts straight up and off. Note the orientation of the housing and any positioning tabs before removal.
Installing the New N55 Thermostat Housing
Inspect the mating surface on the block. Clean any old O-ring material with brake cleaner. Lightly coat the new housing O-ring with fresh coolant. Position the new housing in the correct orientation (the temperature sensor should point in the same direction as the original) and press it down onto the block. Thread in the mounting bolts finger-tight, confirm the housing is fully seated, then torque to spec - typically 10 Nm.
Reconnect the coolant hoses and clamps. Reconnect the electrical connector. Reconnect the battery.
Refilling and Bleeding After N55 Thermostat Replacement
The bleed procedure after a thermostat replacement is identical to after a water pump replacement. Set heater to maximum heat, fan on high, open the bleed nipple on the upper radiator hose, fill reservoir until coolant flows from bleed nipple, close bleed nipple, continue filling to MAX.
Start the engine and idle for 10 minutes. Monitor temperature gauge throughout - it should rise steadily to normal and hold. After 10 minutes of idling, increase to 2,000 RPM for 30 seconds to help circulate and purge any residual air. Shut off, let cool, top off reservoir. Repeat after 50 miles of driving.
For more on the N55 cooling system including water pump replacement, see /cooling/water-pumps. Thermostat buying guides and comparisons are at /cooling/thermostats. Full cooling system hub at /cooling. N55 common problems overview at /articles/bmw-n55-common-problems. Related B58 architecture information at /articles/what-bmws-have-the-b58.
N55 Thermostat Failure and Fuel Economy Impact
A stuck-open N55 thermostat does more than produce weak heat from the cabin vents. When the engine runs below its intended operating temperature for extended periods, several connected problems develop. First, fuel economy drops noticeably - often 1.5 to 2 MPG on city routes where the thermostat failure effect is most pronounced. The DME stays in cold-start enrichment mode longer than intended, injecting additional fuel to compensate for the cold-engine combustion inefficiency it detects through the coolant temperature sensor.
Second, engine oil does not reach its proper operating viscosity as quickly or as consistently as on a healthy thermostat. Cold oil is thicker and provides less effective lubrication at the tightest engine clearances - VANOS actuator passages, turbo shaft bearings, and timing chain guides are all dependent on properly warmed and thinned oil. On an N55 that regularly runs below operating temperature due to a stuck-open thermostat, accelerated wear at these points is a real risk over tens of thousands of miles.
Third, catalytic converter efficiency drops at below-normal exhaust temperatures. Over time this can affect emissions test results and trigger secondary O2 sensor fault codes that are actually downstream of the thermostat problem. If your N55 has multiple apparently unrelated fault codes spanning fuel trim, emissions, and cooling categories, check the thermostat first - a single root-cause fix often resolves the cascade of secondary codes.
Preventing N55 Thermostat Failure - Coolant Quality Matters
The N55 map thermostat does not have a mandatory replacement interval - BMW treats it as a replace-on-failure component. But maintenance habits that protect the overall cooling system also extend thermostat life. The most important habit is maintaining coolant quality. Old coolant loses its inhibitor package, becomes slightly acidic, and attacks aluminum and rubber components throughout the cooling circuit - including the thermostat housing O-ring, the housing body itself, and the electrical connector terminals on the thermostat element.
Use only BMW-spec coolant or an equivalent low-silicate or OAT-formulation. Change it every four years regardless of the service indicator. Never use conventional green coolant in these engines - the silicate inhibitor is chemically incompatible with BMW aluminum alloys and will cause corrosion rather than prevent it.
Resources: /cooling/water-pumps, /cooling/thermostats, /engine, /articles/what-bmws-have-the-b58.
N55 Thermostat Housing Sensor - Testing and Replacement
The N55 thermostat housing contains an integrated coolant temperature sensor that the DME uses to determine operating temperature and manage the thermostat's heating element. This sensor is often overlooked when diagnosing thermostat issues. A sensor that reads incorrectly can cause the DME to believe the engine is hotter or cooler than it actually is, leading to abnormal thermostat commands and what appears to be thermostat failure when the thermostat itself is actually functional.
Testing the coolant temperature sensor is straightforward with a scanner and live data. Compare the DME coolant temperature reading to an infrared thermometer aimed at the thermostat housing while the engine is warming up. They should track closely within 5 to 10 degrees C. A sensor reading significantly hotter or cooler than actual surface temperature while the car is cold and warming up indicates sensor fault. The integrated sensor in the complete MAHLE or Behr housing assembly is pre-tested from the manufacturer - buying the full housing assembly rather than just the thermostat element eliminates sensor replacement as a variable entirely, which is another reason I always recommend the complete housing kit over individual components.
For a full index of N55 engine maintenance resources see /engine. Cooling-specific resources at /cooling and /cooling/water-pumps.
