I have a 2003 BMW 330xi the yellow indicator light stands on even when the radiator is full. I have changed the sensor twice, but the light stays on.
If it’s the low coolant level light the sensor or its wiring is bad. The sensor resides in the bottom of the expansion tank. It’s basically a reed switch. That is, the switch contacts are made up of 2 pieces on thin metal. They are normally not touching. The expansion tank float has a magnet at its bottom. When the coolant level is low the magnet surrounds the reeds and the magnetic field causes them to close. This turns on the low coolant level light in the gauge cluster.
The first step is to take off black aerotray that sits under the engine. This will give you access to the bottom of the expansion tank. Then check the wiring going to the reed switch to ensure that the wires are intact. If they are, remove the connector from the reed switch. Check the low coolant light in the gauge cluster. It should be off. If it is the reed switch is likely bad. Luckily the switch is fairly inexpensive and can be removed without draining the coolant. The switch resides in a tube in the bottom of the expansion tank. With the connector off merely rotate the switch counterclockwise. Then insert the new switch and rotate it clockwise. Connect the connector tor.
Now, there’s a very remove possibility that the magnet in the float has dropped into the bottom of the tank. The only solution is to replace the tank.
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