GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1999 🢒 Chevy C Pickup - 2WD (CNG) 🢒 DTC 🢒 DTC P0112 Intake Air Temperature (IAT) Sensor Circuit Low Voltage

Object Number: 357768 Size: LF
Engine Controls Components
Engine Controls Schematics
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The Intake Air Temperature (IAT) sensor is a thermistor. The IAT thermistor has high resistance when cold and low resistance when hot. The gasoline Control Module (VCM / PCM or ECM) supplies about 5 volts on the IAT signal circuit. The gasoline Control Module also supplies a ground circuit to the IAT sensor. When IAT resistance is high (cold sensor) the IAT signal voltage remains near the supplied 5 volts. As the IAT sensor warms and resistance drops, more signal circuit voltage is pulled lower to the sensor ground. The gasoline Control Module and the AF ECU monitor the IAT signal circuit voltage in order to determine the temperature of the air entering the engine.

This DTC is used to indicate an IAT signal circuit voltage that is below the normal operating range of the sensor.

Conditions for Running the DTC
    • The engine is operating on alternative fuel.
    • The engine has been operating for greater than 1.6 minutes.
    • The vehicle speed is greater than 1 MPH.

Conditions for Setting the DTC

The AF ECU monitors an IAT sensor signal voltage of less than 0.019 volts for at least 5 seconds continuously.

Action Taken When the DTC Sets

    • A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.
    • The AF ECU will illuminate the MIL and store the DTC as History after TWO consecutive drive trips that the diagnostic runs and fails.
    • The AF ECU will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

    • The Last Test Failed status will clear when the DTC runs and passes.
    • The AF ECU will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.
    • The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.
    • All DTC Information can be cleared using a scan tool.
    • Interruption of Control Module (AF ECU / VCM / PCM or ECM) power or ground MAY clear DTC Information. Clearing DTC Information with this method is inconsistent and should not be performed.

Diagnostic Aids

Fault Not Present

Fault Not Present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to Intermittent Conditions Symptom diagnosis for additional information. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in the OBD System Check.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. This step determines if the failure condition is present.

    2.

  2. This step determines if the IAT sensor is shorted.

    3.

  3. This step determines if a short is present in the wiring harness.

    4.

  4. This step determines if the gasoline control module or the AF ECU is the cause of the shorted condition.

    5.

  5. This step determines if an open circuit condition is present between the gasoline control module and the signal circuit splice.

    If an open condition exists in this circuit the gasoline control module should indicate HIGH signal voltage (a high signal DTC should be set) while the AF ECU would indicate LOW signal voltage (this DTC).

Step

Action

Value(s)

Yes

No

1

Did you perform the AF Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check

2

  1. Using the scan tool establish communication with the AF ECU.
  2. Observe the IAT Sensor data parameter.

Does the IAT Sensor data parameter indicate IAT voltage less than the specified value?

0.019 volts

Go to Step 3

Go to Step 4

3
  1. Using the scan tool establish communication with the gasoline control module (VCM/PCM or ECM).
  2. Observe the IAT Sensor data parameter.

Does the IAT Sensor data parameter indicate IAT voltage less than the specified value?

0.019 volts

Go to Step 5

Go to Step 8

4

The fault is not present. Refer to Diagnostic Aids.

Are any DTC's stored that have not been diagnosed?

--

Go to the Diagnostic Trouble Code (DTC) List

Go to Intermittent Conditions

5

Disconnect the IAT sensor.

Does the IAT Sensor data parameter indicate IAT voltage less than the specified value?

0.019 volts

Go to Step 6

Go to Step 9

6

  1. Turn the ignition OFF.
  2. Disconnect the gasoline control module connector that contains the IAT Signal circuit.
  3. Disconnect the AF ECU connector that contains the IAT Signal circuit.
  4. With the IAT sensor connector disconnected test the IAT Signal circuit for continuity to engine ground and also to the Sensor Ground circuit.
  5. If a shorted to ground IAT signal circuit is indicated repair the circuit as necessary.

Was a IAT signal circuit repair made?

--

Go to Step 12

Go to Step 7

7

  1. Connect the gasoline control module connector that contains the IAT Signal circuit.

    2. Leave the AF ECU connector disconnected.

  2. Turn the ignition ON with the engine OFF.
  3. With the IAT sensor connector disconnected observe the gasoline control module (VCM/PCM or ECM) IAT Sensor data parameter.

Does the IAT Sensor data parameter indicate IAT voltage less than the specified value?

0.019 volts

Go to Step 10

Go to Step 11

8

  1. Inspect the signal circuit between the circuit splice and the gasoline control module (VCM/PCM or ECM) connector for the following conditions:
    2. • an open
    • high resistance
    • poor terminal contact
  2. Repair wiring or terminals as necessary.

Was a wiring or terminal repair made?

--

Go to Step 12

Go to Step 11

9

Replace the IAT sensor. Refer to Intake Air Temperature Sensor Replacement in the Engine Controls section of the gasoline service manual.

Is the replacement complete?

--

Go to Step 12

--

10

Important: The following control module replacement procedures must all be completed before attempting to operate the vehicle.

  1. Replace the gasoline control module (VCM/PCM or ECM). Refer to VCM Replacement/Programming in the Engine Controls section.
  2. Program (flash) the replacement control module with the correct vehicle software and calibration.
  3. Perform the PASSLOCK Reprogramming Seed and Key in the Body and Accessories, Theft Deterrent section.

    4. Important: In order to perform the variation learn procedure a switch-over to gasoline operation is necessary. Remove the CNG 20 amp fuse in order to perform the switch-over.

  4. Perform the Crankshaft Position System Variation Learn in the Engine Controls section.

    5. All of the above procedures are located in the gasoline service manual.

Is the replacement complete?

--

Go to Step 12

--

11
  1. Replace the AF ECU. Refer to Engine Control Unit Replacement .
  2. Program (flash) the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

--

Go to Step 12

--

12
  1. Using a scan tool clear the DTC information from BOTH control modules.
  2. Operate the vehicle under the Conditions for Running the DTC.
  3. Using the scan tool parameter DTC STATUS, ensure that this DTC runs.

Did this DTC RUN and PASS?

--

System OK

Go to Step 2