Mercedes-Benz 300D 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 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.

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.

Engine coolant flushes are one of the most common services needlessly recommended, raising repair costs. However, there is a schedule for when these services should be performed. The factory scheduled maintenance section in the owner’s manual provides information for the customer concerning which maintenance items should be performed, and when they are needed. We always recommend following the factory scheduled maintenance intervals for preventive maintenance. If a leak is being repaired, and the coolant must be drained and replaced, flushing the coolant system should be performed if excessive corrosion is present, or if the factory scheduled service interval has been surpassed.

Without the appropriate scan tools or diagnostic experience, symptoms of a failed coolant temperature sensor may lead you in the wrong direction. Rough running, poor acceleration, decreased fuel economy, and even overheating can result from a faulty temperature sensor. It is important that the vehicle is properly diagnosed before replacing any parts.

The average DIYer can maintain the engine cooling system without much trouble. However, diagnosing the system when problems arise can be a daunting task if the function of each individual component is not fully understood. If the temperature sensor is confirmed to be faulty, replacement of the sensor is typically easy for the moderately experienced DIYer. If knowledge of the engine cooling system and diagnosing cooling issues are lacking, this repair should be handed to a certified technician.