Where is the IAT sensor for a 99 5.9 diesel ?

The two-wire Intake Manifold Air Temperature (IAT) sensor is installed in the intake manifold with the sensor element extending into the air stream:

1. This is a Negative Thermal Coefficient (NTC), meaning that as the manifold air temperature increases, the resistance (voltage) in the sensor decreases, and vice-versa.
2. The IAT sensor provides an input voltage to the PCM, indicating the density of the air entering the intake manifold based on intake manifold air temperature. When the ignition is turned ON , a 5-volt power circuit is supplied to the sensor from the PCM. The sensor is grounded at the PCM through a low-noise, sensor-return circuit.
3. The PCM uses this input to determine:
   Injector pulse-width (air: fuel ratio) Ignition timing (to prevent spark knock)