Checking the engine coolant temperature sensor with and without a multimeter

In a car with an internal combustion engine, the engine will typically run hot. This is a natural consequence of using rapidly occurring explosions of fuel mixed with air to create the locomotion that drives the wheels. This in turn creates the need for cooling the engine, and this is where the engine cooling system comes into play. The most critical part of this system is the coolant. In olden times, this consisted of water, occasionally mixed with antifreeze in winter to keep the engine block from becoming an engine block of ice. With advances in technology, not only was plain water abandoned as the primary cooling liquid, but sensors for determining the coolant’s temperature were developed as well.

What is a coolant temperature sensor?

Coolant temperature sensors were developed to determine the operating temperature of the engine. In a nutshell, the sensor delivers a small charge which the engine control unit can read. This charge is the result of voltage passing the sensor’s internal resistance, the latter of which is lowered as temperatures increase and vice versa. Measuring the effect of the lowered or raised resistance allows the ECU to determine the temperature measured. It basically has the same function as an old thermometer, but uses resistance instead of mercury to determine the temperature.

Symptoms of a faulty engine coolant temperature sensor

There are several issues that point towards the coolant temperature sensor being damaged. Among the most common are an increase to fuel consumption, difficult engine starts, and higher idling speeds. Other indicators can include black smoke emanating from the exhaust pipe and cooling fans not activating. Often, the “Check Engine” light will turn on. These problems are caused by faulty readings, leading to the fuel system injecting inappropriate amounts of fuel and the engine running too hot on account of the cooling system not being told to engage by the control unit. In these cases, a faulty coolant temperature sensor can cause a car not to start at all.

Troubleshooting a defective sensor

There are a few ways how one can test for a faulty coolant temperature sensor. Generally, OBD II error codes from the ECU will be the first conclusive evidence pointing towards a defective coolant temperature sensor. Codes associated with this range from P0115 to P0119. If you see one of these codes or otherwise suspect the coolant sensor is the source of your woes, proceed along the following steps:

In the event that you cannot start your motor, you can simulate hot and cold readings by immersing the sensor in hot and cold water. Be sure that the sensor is sufficiently waterproof and avoid submerging the terminals. Use cold and barely boiling water to acquire your readings. Make sure your vehicle is turned off and the key is not in the ignition when removing the sensor. This will help you avoid suffering electrical shock.

More accurate numbers for what voltages and resistances should be expected from your coolant temperature sensor can be found in the technical documentation or online. Refer to these numbers if you are unsure about the received values.

Replacing a defective sensor

Unfortunately, a faulty coolant temperature sensor can’t really be repaired and you have to buy a new one. Fortunately, they are not particularly expensive. The replacement process is fairly simple and can be done at home: