ECT - Engine Coolant Temperature sensor

2.2) ECT - Engine Coolant Temperature (sensor)

What is it? This is small electrical device for measuring the coolant temperature in the engine

Where is it located? It is usually located on the engine near to the thermostat housing. The ECT is sited on the 'hot' side of the thermostat so that it senses the coolant/engine temperature before the thermostat opens and allows coolant to flow through the radiator.

How does it work? Modern temperature sensors consist of a thermistor in a sealed unit. As the temperature rises the electrical resistance varies proportionately; some thermistors increase their resistance with temperature (PTC - positive temperature correlation) whilst others decrease their resistance (NTC - negative temperature correlation). When the engine is cold at start up the coolant sensor sends an appropriate signal to the ECU. The ECU responds by increasing the length of the injection cycles to enrich the combustion mix. This is an electronic equivalent of pulling the 'choke' out on a carburetor. As the engine warms up the signals from the coolant sensor cause the ECU to shorten the injection cycles making the fuel mix progressively leaner. The process of coolant sensor and ECU interaction explains why engines have a slightly faster idle when starting cold than when running hot.

Symptoms of faulty coolant sensor

Associated OBD2 error codes DTCs: P0115 - P119; P0125, P0126, P0128

• Poor starting - If the coolant sensor reports in error that the engine is warm the ECU will not enrich the fuel mix at ignition. The engine will falter at idle if it is not given additional help by the driver by pressing on the accelerator pedal to maintain speed. Once the engine has warmed up the engine will behave correctly.
• Fast/erratic idle, Poor fuel economy - conversely if the coolant sensor reports in error that the engine is permanently 'cold' the ECU will keep the fuel mix rich. This is OK at start up but will become more noticeable when the engine is hot; idle will be fast and lumpy. Fuel consumption will be high due the permanently rich fuel mix set by the ECU.
• Excessive emissions - the enriched fuel mix delivered in response to ECT signal error causes the exhaust to be heavy in un-burnt hydrocarbons. This often results in 'emission test' failure.

How to check? Most often the coolant sensor is quite separate to the temperature sender, so a correct read-out on the dash board does not necessarily indicate correct sensor function. Using a voltmeter the resistance across the electrical terminals on the sensor can be measured. By removing the device from the car and putting the end of the sensor in a pan of hot water it should be possible to see an immediate change in resistance, it does not matter so much that the resistance goes up or down but that there is a discernable change with change in temperature. Generally high resistance equates to cold temperatures and vice versa. If there is no resistance change commensurate with temperature change then the sensor is at fault. If there is simply no resistance measurable (open circuit) then the sensor is at fault. If the sensor is working correctly check the connector, the wiring and the wiring insulation for faults and possible shorting.

How to fix? Replace if found faulty

NEXT 3.1) CKP - Crankshaft position sensor