1991 Mercedes-Benz 300SEL Engine Coolant Temperature Sensor Replacement Cost

As the engine runs, combustion events occur thousands of times per minute; heat is a byproduct of engine operation. To prevent that heat from destroying the engine, a liquid cooling system is used. Beginning with the engine, the cooling system uses coolant passages inside the engine block, cylinder head, and possibly the intake manifold. These passages allow coolant to flow through these components to absorb heat. The engine coolant temperature sensor reads the temperature of the coolant in one or more of these passages, and relays that information to the vehicle computer. The computer will send another signal to the engine temperature gauge, and alert the driver if the temperature exceeds the specified range for that vehicle.

When the engine coolant temperature sensor is defective, the vehicle may be hard to start and/or get poor fuel economy. Also, a defective coolant sensor will not signal the power train control module (PCM) to operate the cooling fans, which may cause the vehicle to overheat.

The Check Engine Light may illuminate with a fault code set for the engine coolant temperature sensor.

When the engine coolant temperature sensor fails, it will send inaccurate information to the vehicle’s computer, causing the computer to react to false operating conditions. This will cause the engine to consume more fuel than normal, depleting fuel mileage, and causing black, sooty smoke from the engine under moderate to heavy acceleration. The check engine light will illuminate, and on-board diagnostic trouble codes may be stored for exhaust and emission system failure, fuel delivery system failure, and engine cooling system failure. Overheating may also occur, as the coolant sensor may be leaking fluid out of the engine, creating a an air pocket in the system.

Driving a vehicle with a faulty engine cooling system can result in overheating, cylinder head gasket failure, engine block failure, or cylinder head warping. It is never recommended to drive a vehicle with engine cooling issues, especially with modern engine casting materials. Additionally, the operating conditions for the engine may cause fouling of the catalyst, and oxygen sensors.

Engine temperature sensor failure is not uncommon. Failing to maintain the engine cooling systems per factory scheduled maintenance intervals leads to corrosion of the sensor where it enters the engine coolant passages. Failure due to corrosion will generally be after the 100,000 mile mark, but may occur earlier if the cooling system is not properly maintained.

When diagnosing a check engine light related to the engine coolant temperature sensor, the servicing technician will connect a professional scan tool to the vehicle via the on-board diagnostics port under the dashboard. The technician will read the information stored in the vehicle's computer, and compare that data to actual conditions. If the temperature readings from the computer are incorrect, the technician will suspect the coolant temperature sensor. The sensor circuit and sensor will be tested to verify the diagnosis.

An engine coolant temperature sensor code is not always caused by a defective sensor. It can be triggered by a defective thermostat, defective wiring, rusty coolant, or by a plugged or restricted radiator.

After the diagnosis has been confirmed, the technician will unplug the electrical connector from the sensor prior to removal. The sensor may be bench tested for further confirmation. If required, sealant will be applied to the new sensors threads before being inserted into its place in the thermostat housing, engine block, or other. The cooling system will then be refilled, bled of any air, and the vehicle will be checked for normal operation. If there is significant corrosion noted in the cooling system, the technician may recommend a coolant flush.