Coolant Temp Sensor

by Admin on August 7, 2011

Daimler SP250 - V8 engine
Creative Commons License photo credit: dave_7

Faulty coolant temperature sensor  symptoms:

•    Stalling engine
•    Hard starting
•    Hesitation
•    Running rich or lean
•    Reduced fuel economy
•    Emission control devices not functioning properly
•    Improper transmission shifting

If the ECT sensor is suspected to be faulty, the first step in testing it should be to inspect the sensor’s wires and connections.  Many times sensor problems are caused by faulty or even loose connections and not by the sensor itself.  Repair any defective wiring or loose connections.  If the ECT sensor is suspected to be bad, it should be tested.


Remove the sensor and place in a container of water along with a thermometer.  When the water is heated, the resistance measured across the terminals should be a specific number at a specific temperature.  The specifications  can be requested from the sensor’s manufacturer.  Never apply a torch or any kind of open flame to an ECT during testing.  This will damage the sensor.

The wiring between the sensor and the computer may also be the problem.  These wires can be tested  by disconnecting the wires at the sensor and the corresponding wires at the computer.  Next, measure the resistance of a wire from the sensor to the computer with an ohmmeter and compare the result to specifications from the vehicle manufacturer.  If the reading is above specifications, replace the wire.  Before disconnecting any computer system component, make sure that the ignition switch is turned to the off position as not to cause any damage to the computer.


The replacement process is simple if the sensor is in a fairly accessible location, just follow these steps:

•    Disconnect the battery
•    Drain the coolant (partially)
•    Unplug old sensor
•    Remove sensor
•    Install new sensor by reversing procedure
•    Refill coolant
•    Reconnect the battery
•    Clear any ECT codes that may have been set
•    Road test vehicle to insure that the problem is resolved

Coolant temperature sensors are relatively inexpensive and available at any auto part store.  Proper diagnosis and testing can result in substantial savings.

Leave a Comment

Previous post:

Next post: