why does my BMW 3 Series E90/E91 have erratic temperature readings?

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the battery to prevent electrical shorts while working on the vehicle.

2. Replace Temperature Sensor

• Tools Required: Socket set, torque wrench, screwdriver, multimeter
• Locate the temperature sensor on the engine block.
• Disconnect the electrical connector from the temperature sensor.
• Remove the sensor using a socket and wrench.
• Install the new temperature sensor and torque it to the manufacturer's specification (usually around 10 Nm).
• Reconnect the electrical connector securely.

3. Replace Thermostat (if faulty)

• Tools Required: Socket set, torque wrench, gasket scraper
• Drain the engine coolant into a suitable container.
• Remove the coolant hose connected to the thermostat housing.
• Unscrew the thermostat housing and remove the old thermostat.
• Clean the mating surfaces of the housing and engine block thoroughly.
• Install the new thermostat and gasket, securing the housing with bolts and torquing to specification (usually around 20 Nm).
• Reconnect the coolant hose and refill the cooling system with the appropriate coolant mixture.

4. Test for Leaks

• Start the engine and allow it to reach operating temperature while monitoring for leaks around the thermostat and temperature sensor.
• Recheck the coolant level and top off if necessary.