What does it mean?
OBD II fault code P0127 is a generic code that is defined as “Intake air temperature too high”, and is set when the PCM (Powertrain Control Module) detects an abnormally high signal voltage from the IAT (Intake Air Temperature) sensor. Note that this code refers to a high voltage in the control circuit of the IAT sensor, and NOT to the actual temperature of the intake air.
In a fully functional system, the PCM uses input data from both the MAF (Mass Airflow) sensor and the IAT sensor to calculate fuel delivery strategies. While the MAF sensor monitors the total volume of air that enters the engine, the IAT sensor monitors the actual temperature of the intake air.
Intake air temperature plays a critical role in fuel management, since the hotter the intake air is, the less dense it becomes, meaning that in order to maintain a stoichiometric (14.7: 1) air fuel mixture, the PCM needs to continually adapt the volume of fuel delivered by the injectors to the volume of available air, whose density changes as its temperature changes.
On most applications the IAT sensor is a simple thermistor whose resistance changes as the temperature of the air that flows across it changes. In practice, the PCM delivers a 5-volt reference voltage to the sensor, which voltage changes in direct relation to the changes in the resistance of the sensing element. The changes in resistance are signalled to the PCM via a dedicated signal wire, to be interpreted by the PCM as changes in the temperature of the intake air.
Note that it is common for abnormally high signal voltages to be interpreted as impossibly high intake air temperatures. In some cases, a scanner might indicate an intake air temperature of hundreds of degrees, which is clearly impossible. Thus, when the PCM detects a signal voltage that exceeds the maximum allowable voltage, the PCM will set code P0127, and may also illuminate a warning light. The image below shows a typical Intake Air Temperature sensor.
Note though that the appearance and location of these sensors vary widely between applications and manufactures, so refer to the manual for the application to identify/ locate the IAT sensor correctly to avoid a misdiagnosis.