Coolant Temperature Sensor

Replace the coolant temperature sensor on the M275 6.0L V12 BiTurbo engine to restore accurate engine temperature monitoring and prevent cooling system issues.

Warnings

Tools required

Parts

• Coolant temperature sensor × 1 — A0009050900 or equivalent OEM
• Sensor O-ring/seal × 1 — Usually included with sensor

Fluids

• Mercedes-Benz Coolant (Blue) — 2 qt

Preparation

1. Ensure engine is completely cold to touch before beginning work
2. Park vehicle on level surface and engage parking brake
3. Place coolant drain pan beneath vehicle under radiator drain location
4. Remove engine cover by pulling upward on front and rear mounting points
5. Locate coolant temperature sensor on driver's side (left) cylinder bank near firewall

Procedure

1. 1

   Depressurize cooling system
   Slowly turn the coolant expansion tank cap counterclockwise to release any residual pressure. Remove cap completely and set aside. Listen for any hissing indicating remaining pressure and wait until fully depressurized.
2. 2

   Partially drain coolant
   Open the radiator drain petcock at the bottom of the radiator (driver's side) and drain approximately 2 quarts of coolant into the drain pan. This lowers the coolant level below the sensor location. Close the drain petcock once coolant level drops below sensor mounting point.
   ⚠Coolant is toxic to animals and children. Collect and dispose of properly according to local regulations.
   Torque spec

   Drain Petcock9 Nm (7 lb-ft)
3. 3

   Disconnect sensor electrical connector
   Locate the electrical connector on the coolant temperature sensor. Press the locking tab and pull the connector straight away from the sensor. Move the wiring harness aside to provide clear access to the sensor body.
4. 4

   Remove old coolant temperature sensor
   Using a 19mm deep socket with extension, carefully loosen the coolant temperature sensor by turning counterclockwise. Once loose, unthread by hand and remove from the housing. Be prepared for minor coolant spillage. Inspect the sensor O-ring and sealing surface.
   ℹ️Some coolant will drain from the sensor port. Keep shop towels ready to catch drips.
5. 5

   Prepare new sensor
   Inspect the new coolant temperature sensor and verify the O-ring/seal is properly seated in the sensor groove. Lightly lubricate the O-ring with fresh coolant to ease installation. Verify the sensor threads are clean and undamaged.
6. 6

   Install new sensor
   Thread the new coolant temperature sensor into the housing by hand, turning clockwise until hand-tight. Ensure the sensor threads in smoothly without cross-threading. Use the 19mm socket and torque wrench to tighten to 20 Nm (15 lb-ft). Do not overtighten as this can damage the sensor or housing threads.
   ⚠While no specific torque spec is provided for this sensor, typical M275 coolant sensors require 20 Nm. Verify with dealer technical documentation if available.
7. 7

   Reconnect electrical connector
   Push the electrical connector onto the new coolant temperature sensor until the locking tab clicks into place. Tug gently on the connector to verify it is fully seated and locked.
8. 8

   Refill cooling system
   Using a funnel, pour fresh Mercedes-Benz Blue coolant into the expansion tank until the level reaches the MAX mark. Start the engine and let it idle while monitoring coolant level. Add coolant as needed as air is purged from the system. Run engine until thermostat opens (approximately 10 minutes).
   ⚠Monitor engine temperature gauge closely during warm-up. If temperature rises above normal, shut off engine immediately and check for leaks or air pockets.
9. 9

   Bleed cooling system
   With engine at operating temperature, locate the bleed screw on the cooling system (typically on upper radiator hose or thermostat housing). Open the bleed screw slightly until coolant flows without air bubbles, then close. Top off expansion tank to MAX mark. Allow engine to idle for additional 5 minutes, then recheck level.
   Torque spec

   Bleed Screw9 Nm (7 lb-ft)
10. 10

    Clear diagnostic codes
    Connect Mercedes-Benz Star Diagnostic tool or OBD-II scanner and clear any coolant temperature-related fault codes stored in the engine control module. This ensures the new sensor reading is recognized by the system.

Reassembly

1. Reinstall engine cover by aligning mounting points and pressing down firmly until it clicks into place
2. Verify coolant expansion tank is filled to MAX mark with cap securely tightened
3. Inspect area around sensor for any coolant leaks
4. Wipe down any spilled coolant from engine components

Verification

• Start engine and verify coolant temperature gauge rises smoothly to normal operating temperature within 10 minutes
• Scan for diagnostic trouble codes - there should be no temperature sensor faults present
• Check for coolant leaks around the sensor after engine reaches operating temperature
• Monitor coolant level in expansion tank after test drive - level should remain at MAX mark when cold
• Verify cooling fans activate when engine reaches normal operating temperature
• Confirm no warning lights related to cooling system appear on instrument cluster

Related trouble codes on this vehicle

Codes that commonly send this job to the bay — tap one for symptoms, causes, and diagnostic steps.

Chasing one of these codes and not sure which part is guilty? Ask a master mechanic about YOUR car →

More procedures for this vehicle