Mercedes Engine Coolant Temperature Sensor - Genuine Mercedes 0115425117

Product Information

The engine coolant temperature (ECT) sensor, or simply coolant temp sensor, plays a vital role in monitoring the engine's operating temperature and communicating this information to various vehicle systems. This sensor is located in the engine's coolant passage typically in the cylinder head or block. It works by varying its resistance based on the temperature: as the engine heats up, the resistance decreases, and as it cools down, the resistance increases. The ECT sensor sends this data to the ECU, which uses it for several critical functions, such as the air-fuel mixture, ignition timing, and control of the cooling fans and dashboard temperature gauge. There are many European applications where there is more than one coolant temp sensor, separating the signals used for engine ECU and other functions such as gauges and fans.

A faulty sensor might send incorrect temperature readings to the ECU, resulting in poor engine performance. The most common example is if the sensor incorrectly signals that the engine is colder than it is, the ECU will enrich the air-fuel mixture unnecessarily, leading to rough idling, poor fuel economy, or even black smoke from the exhaust due to unburnt fuel.

Conversely, if the sensor falsely indicates an engine overheating condition, the ECU might trigger warning lights, prematurely activate the cooling fans, or even cause the car to go into "limp mode," limiting engine power to prevent damage (however this is a rarer failure mode).

• Wrong Temp Readings: If the coolant temperature gauge is acting unpredictably, or reading consistently too high or too low, it may be either a coolant temperature sensor or a thermostat. Because of this, it’s common to replace both of these parts at the same time for the best probability of success while the cooling system is opened.
• Running too rich: If you have noticed the temperature gauge is low and the exhaust is black and sooty, the engine might be stuck in the warm-up loop due to a bad coolant temperature sensor even though the engine is at the proper operating temperature controlled by the thermostat.
• Non-linear or suspicious live data output: For vehicles equipped with OBDII, use a scan tool to monitor the exact temperatures being output from the sensor. As the car warms up the output should be smooth and linear. If there are dead spots or it acts suspiciously, you may need to replace the sensor.

About Genuine Mercedes

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Be sure to keep an eye out for OE tags on our product pages to quickly identify parts coming direct from OE manufacturers. That extra bit of insight could save you some serious money while shopping! Refer to our OE Academy for further information regarding the OE/OEM tags throughout the site.