GLK350 Coolant Warning Mystery: Owner Replaces Sensors, Issue Persists

A 2014 GLK350 owner is dealing with a frustrating coolant temperature warning that keeps returning even after replacing multiple sensors. The warning appears on the dash without any other message, and when it comes on, the A/C shuts off and the radiator fan stops. The temperature gauge drops to 80 degrees in about a second, then climbs to 100 degrees after a minute before the fan kicks back in and the A/C resumes.

This cycle can repeat multiple times or not happen at all during short trips.

The owner first scanned the car and found a diagnostic trouble code pointing to an open circuit in a temperature sensor module. They replaced the sensor attached to the radiator, and the issue disappeared for a few days before returning. After researching online, they found another coolant temperature sensor at the back of the engine block on the driver's side.

Using a scope, they spotted green corrosion there and replaced that sensor as well, along with the manifold gaskets and a broken coolant line. The job took two days in the garage.

After that replacement, the temperature gauge stopped dropping to 80 or rising to 100, staying around 86 to 90 degrees. However, the warning still comes back. The owner also replaced the coolant. A forum responder suggests checking the coolant level with the engine cold, ensuring it covers the C-shaped insert in the reservoir.

If the level is fine, the coolant level sensor may be bad, unplugged, or have an open circuit.