Powertrain Controls - ECM/PCM

DTC 23 Chart:

IAT Sensor Circuit:

CIRCUIT DESCRIPTION
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage at the Powertrain Control Module (PCM). The PCM supplies a 5 Volt reference and a ground to the sensor. When the air is cold the sensor (thermistor) resistance is high, therefore the PCM terminal "PC9" voltage will be high. If the air is warm the sensor resistance is low, therefore the PCM terminal "PC9" voltage will be low.

DIAGNOSTIC TROUBLE CODE (DTC) 23 WILL SET WHEN
- IAT signal voltage indicates intake air temperature is below -40°C (-40°F).
- Engine run time is greater then 320 seconds.
- Vehicle speed is less than 15 mph.

ACTION TAKEN (PCM WILL DEFAULT TO)
The Malfunction Indicator Lamp (MIL) will illuminate, the PCM will default to 60°C for intake air temperature (Tech 1 scan tool will not show defaulted value), and the cooling fan will enable.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step checks if DTC 14 was set as a result of a hard or intermittent condition.
2. A DTC 23 will set due to an open sensor wire, or connection. This test will determine if the wiring and PCM are OK.
3. This will determine if the signal CKT 472 or the sensor ground CKT 452 is open.

DIAGNOSTIC AIDS
- A Tech 1 scan tool displays temperature of the air entering the engine, which should be close to ambient air temperature when engine is cold, and rise as underhood temperature increases.
- If engine is at ambient temperature compare IAT sensor to ECT sensor. They should be within a couple degrees of each other, or compare against a known good sensor.
- If DTCs 23, 33 and 66 are stored at the same time, it could be the result of an open CKT 452.
- The "Temperature vs. Resistance Values" scale in the image may be used to test the IAT sensor at various temperature levels to evaluate the possibility of a skewed (mis-scaled) sensor.
- Refer to Intermittents in Diagnosis by Symptom. - Intermittent Malfunctions