What does it mean?
We already know that an engine needs fuel, air, and spark to run, but to run efficiently and most effectively, the engine control unit powertrain control module (ECU or PCM) needs to control these elements precisely. Depending on engine speed, engine load, coolant temperature, and air temperature, the ECU fine tunes fuel volume, ignition timing, variable valve timing (if applicable), and some emissions controls.
Intake air temperature (IAT) is a measurement of the ambient air coming into the engine at any given moment. This is important, since warmer air is less dense and vaporizes fuel faster, so less fuel needs to be injected. Conversely, colder air is denser and vaporizes fuel slower, so more fuel needs to be injected to get the same amount of power. The IAT sensor is therefore located in the air stream, usually after the air filter.
Sometimes, the IAT sensor is integrated in the MAF (mass air flow) meter, otherwise it will be a separate sensor. °F °C kΩ V 50 10 58.75 3.52 104 40 16.15 2.16 176 80 3.84 0.88 248 120 1.18 0.28 The IAT is a thermistor, which works by modifying a 5 V reference signal from the ECU. As temperature goes up, the IAT sensor’s resistance goes down, and vice versa. The voltage that the ECU reads on the other side is calculated as the temperature of the intake air.
Different vehicles will have different values, but one typical Ford IAT sensor reads as in the table above. Many manufacturers use a similar scheme, resistance dropping as temperature rises, while some use the opposite scheme, resistance increasing as temperature rises. The ECU’s internal memory table will recognize the voltage, not only to measure intake air temperature, but also to identify any fault.
If the IAT sensor sends back a voltage far outside the acceptable range, the ECU will set a diagnostic trouble code (DTC). In case the IAT sensor reading returns an unusually high temperature, typically over 300 °F (149 °C) or less than 0.1 V at the ECU, the ECU will ignore the reading, set DTC P0112 – Intake Air Temperature Low Input, and illuminate the MIL or CEL (malfunction indicator lamp or check engine light).
DTC P0112 may be accompanied by other IAT- and MAF-related DTCs, such as P0110, P0101, or P0102. Note: Circuit Low Input codes are often the result of low battery voltages (that can have many possible causes), bad connections across electrical connectors or previously repaired wiring, as well as corrosion in electrical connectors.
Other possible causes of low input voltages include poor installation of aftermarket components, poor quality aftermarket components like fuses, relays, and switches, and modification of the electrical system that could include the use of conductors that are not rated for use in a particular application. However, poor connections often result in high resistances in some parts of the circuit, which is why it is important to perform resistance and continuity checks during the diagnostic procedure.