Circuit Description
The Intake Air Temperature (IAT) sensor is a thermistor located in the fresh air duct to the throttle body. It monitors the temperature of the air entering the throttle body. The VCM applies 5 volts to the sensor on a 5 volt reference circuit. Resistance in the sensor measures high when the air is cool, and the ECU senses a high voltage signal. The sensor resistance measures low if the air is warm, and the ECU senses a low voltage signal.
This is a type B DTC.
Conditions for Setting the DTC
The following conditions will set the DTC (all of the conditions below must be met for at least 10 seconds):
| • | The vehicle is in the CNG operation mode. |
| • | The vehicle speed must be less than 1 mph. |
| • | The air flow must be less than 250 grams per second. |
| • | The IAT voltage is greater than 4.39 volts. |
| • | If the ECT at start is 0°C (32°F) or greater, the DTC will test at 100 seconds of run time. |
| OR |
| • | If ECT at start is less than 0°C (32°F), the DTC will test at 600 seconds of run time. |
Action Taken When the DTC Sets
The ECU sets the DTC 25 after 2 consecutive driving trips with the fault active.
Conditions for Clearing the MIL/DTC
The ECU turns the MIL OFF after 3 consecutive driving trips without a fault conditions present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.
Diagnostic Aids
Always diagnose ANY VCM codes that are present before beginning this diagnostic procedure.
A scan tool indicates the temperature of the ambient air entering the throttle body. The air temperature should read very close to the temperature of the outside air. The air temperature should rise gradually as the engine warms up and the underhood temperature increases. Check for poor connections at the ECU and IAT sensor by wiggling the connector gently and monitoring the IAT voltage on a DVOM connected to the ECU connector. Repair the connector if the voltage varies.
A sensor may also become skewed or mis-scaled. The Temperature vs. Resistance value table will help to detect a skewed sensor. Refer to Temperature vs. Resistance in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
Important: Use the same diagnostic test equipment for all of the measurements.
-
If the IAT sensor circuit voltage measures greater than 4.39 volts, the conditions for the DTC are still present and the problem is not intermittent.
-
With the ECU connector disconnected and the voltage still present, this eliminates the ECU as the possible cause of the fault.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check |
2 |
Is the IAT sensor voltage greater than the specified value? | 4.9 V | Go to DTC P0113 IAT Sensor Circuit High Voltage in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual | Go to Step 3 |
Using a DVOM, backprobe the IAT sensor signal in the ECU connector. Does the DVOM display the IAT sensor signal greater than the specified value? | 4.39 V | Go to Step 4 | Go to Step 5 | |
Does the DVOM display the IAT sensor signal greater than the specified value? | 4.39 V | Go to Step 7 | Go to Step 6 | |
5 |
Is there continuity? | -- | Go to Step 10 | Go to Step 14 |
6 | Replace the ECU. Refer to Engine Control Unit Replacement . Is the action complete? | -- | Go to Step 17 | -- |
7 |
Does the scan tool display an IAT sensor voltage less than the specified value? | 0.82 V | Go to Step 11 | Go to Step 8 |
8 | Jumper the IAT sensor signal circuit to a known good ground. Does the scan tool display an IAT sensor voltage less than the specified value? | 0.82 V | Go to Step 12 | Go to Step 9 |
9 | Is DTC P0123 also set? | -- | Go to DTC P0123 TP Sensor Circuit High Voltage in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual | Go to Step 13 |
10 | The DTC is intermittent. If no additional DTCs are stored, refer to the Diagnostic Aids. If additional DTCs are set, refer to those tables. Are any additional DTCs set? | -- | Go to the applicable DTC table | -- |
11 | Inspect the sensor connector and the VCM connector for the proper connection. Was a problem found? | -- | Go to Step 14 | Go to Step 15 |
12 | Check the IAT sensor ground circuit for an open between the IAT sensor and the VCM. Was a problem found? | -- | Go to Step 14 | Go to Step 16 |
13 | Check the sensor signal circuit for an open between the IAT sensor and the VCM. Was a problem found? | -- | Go to Step 14 | Go to Step 16 |
14 | Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 17 | -- |
15 | Replace the IAT sensor. Refer to IAT Sensor Replacement in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 17 | -- |
16 | Replace the VCM. Important: Reprogram the VCM if it is faulty. Refer to VCM Replacement/Programming in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 17 | -- |
17 |
Does the diagnostic run and pass? | -- | Go to Step 18 | Go to Step 2 |
18 | Are any ECU or VCM DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
