What is an engine-air temperature sensor (IAT), you ask? How does the intake temperature sensor work? What are the causes and symptoms of an air temperature sensor malfunction?
The Intake Air Temperature Sensor (IAT), measures the temperature of the air entering an engine. This data is used by the engine computer (PCM), in order to determine air density. It can then balance the fuel/air mixture. For the mixture to be smooth, cold air is more dense than warm air. The PCM adjusts fuel/air ratio by extending injector opening times.
This sensor was used on pre-OBD II vehicles (1995 or earlier) as Air Charge Temperature, Vane Temperature, Manifold Charging Temperature and Manifold Air Temperature sensors (CTS).
How does the Air Temperature Sensor work?
An Air Temperature sensor is attached to the intake manifold. It measures the amount of air entering the engine. Some engines use the Absolute Pressure Sensor, (MAP) or the Mass Air Flow Sensor (MAF). The internal air temperature sensor in the MAP sensor can be used to measure engine performance. Some engines might have multiple air temperature sensors. (If the engine has more than one intake manifold, like V6 and V8 engines.
The resistance of the air temperature sensor changes when it is heated. It functions in the same manner as the Coolant Temperature Sensor. The PCM sends a voltage to the sensor (usually 5 volts), and in response calculates the temperature of the air. As the temperature changes, the returning voltage signal will also change. The majority of air temperature sensors are Negative Temperature Coefficient thermistors (NTC) with high electrical resistance when it is cold. The resistance decreases with increasing temperature. Some work in the opposite direction, but this is rare. These are known as Positive Temperature Coefficient (PTC), thermistors. This means that the resistance will increase proportionally with temperature. Each time the PCM receives the sensor’s varied resistance, it is transmitted to them.
Older OBD II vehicles (1995 or earlier) used the signal from an air temperature sensor to turn on the cold starter injector, if it was equipped. This signal was used to delay opening the EGR valve until the engine warmed.
Automatic climate control systems also use air temperature sensors. The temperature of the outside and air inside the passenger compartment is monitored by one or more sensors. A separate outside temperature sensor is usually installed outside of the engine compartment. This ensures that it does not get affected by engine heat. The outside temperature sensor is typically located in the grille near the bottom of your fascia or under the hood. These sensors work exactly in the same manner as the engine air temperature sensor. Some models include an infrared sensor that measures the body temperature of passengers.
Air Temperature Sensor Failure
Backlash in the intake manifold can cause an air temperature sensor to fail. Carbon and oil contamination can cause the tip of an air temperature sensor to become less sensitive to sudden changes in temperature. It may also deteriorate over time. This can cause it to not respond quickly or cease working at all.
Other problems could also be caused by loose or damaged electrical connections. There are several factors that can cause problems between the sensor’s connection and the PCM connection, including corrosion, disconnection or looseness.
Sensor Failure Symptoms
The PCM will not know the engine’s temperature if the intake temperature sensor isn’t working correctly. It will incorrectly determine the temperature of the air/fuel mixture. The engine will not work properly if the fuel mixture is too rich or too lean. This will lead to irregularities and other problems.
The vehicle might not start if it is equipped with a cold-start injector.
An incorrect air temperature sensor could also impact the operation of an EGR valve. The PCM uses the temperature of the air to determine the opening time of the EGR (usually the engine cooler temperature sensor).
An incorrect air temperature sensor could prevent the Evaporative (EVAP Emission System) from being completed on 1996 or newer OBD II vehicles.
Low fuel conditions can be caused by an air temperature sensor reading higher than the normal. It increases the likelihood of a vehicle exploding in the combustion chamber because it will be lean. It increases exhaust emissions and fuel consumption.
If the air temperature sensor reads lower than normal, it causes the vehicle’s engine to run with a rich mixture. Both fuel consumption and exhaust emissions increase.
Although imbalances in the fuel/air mixture may indicate a problem with the sensor, it could also be due to something else. The sensor may read hotter than usual due to engine air leaks, or a blocked catalytic converter. This will cause the intake vacuum and airflow to be reduced.
Diagnose of Air Temperature Sensor Malfunction
The fault code is usually generated by the sensor and illuminates the engine fault lamps. It won’t give a code if the sensor only reads low or high values, or if it is dirty or malfunctioning.
It is easiest to determine the temperature sensor by comparing the temperature readings of the air and the coolant temperature readings while the engine is hot. This can be done with a scan tool. The air temperature sensor should be in good condition so that it is a few degrees cooler then the coolant temperature sensor.
Connect an ohmmeter to the ends of the sensor to measure its resistance. When the sensor is cold, measure its resistance. Use a blow dryer to blow hot air onto the sensor tip. Never use a flamethrower. Then measure the resistance again. You should observe a change of resistance as the sensor warms. If the resistance of the heated sensors does not change, it may be defective and should be replaced. If the sensor has a negative temperature, it should gradually decrease in resistance, while a positive one should gradually increase. You have a defective sensor if the values suddenly rise or are read quickly (with little resistance or none),
You should refer to the technical specifications of the air temperature sensors resistance. Next, measure the resistance at low and medium temperatures. High temperatures can cause a sensor to read outside of its range when it is cold. It can also read the opposite. In these cases, the sensor needs to be replaced.
Replacement or cleaning of an air temperature sensor
It is possible to clean and re-use a contaminated sensor. Clean the sensor’s tip and take it out of the intake manifold. Use an electronic cleaner spray to clean the sensor. To clean the sensors inside the MAF sensor, you can use aerosol electronic cleaners. Other cleaners may leave chemical residues which can damage or lead to other problems.
You should not overtighten the air temperature sensor to prevent damage to the sensor body or threads on the plastic intake tube. The replacement sensor does not require any adjustments.
