For 1990-2009 cars

### Circuit Description

The engine idle speed is controlled by the idle air control (IAC) valve. The IAC valve is on the throttle body. The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The valve consists of a movable pintle, driven by a gear attached to a two phase bi-polar permanent magnet electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has two separate windings that are called coils. Each coil is fed by two circuits from the powertrain control module (PCM). When the PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed diagnostic trouble code (DTC) may set.

### Conditions For Running the DTC

• | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0300, P0401, P0403, P0404, P0405, P0443, P1121, P1404, and P1441 are not set.

• | The engine run time is more than 2 minutes.

• | The engine coolant temperature (ECT) sensor is more than 70°C (158°F).

• | The intake air temperature (IAT) sensor is more than -18°C (-0.4°F).

• | The barometric pressure (BARO) sensor is more than 70 kPa.

• | The ignition voltage is between 9-18 volts.

• | The throttle position (TP) sensor angle is less than 1.5 percent.

• | The vehicle speed sensor (VSS) sensor is less than 5 km/h (3 mph).

• | The above conditions are met for more than 8 seconds.

### Conditions for Setting the DTC

• | The actual engine speed is 100 RPM less than the desired engine speed for more than 8 seconds for each test.

• | The PCM detects the IAC test failed 4 consecutive times per ignition cycle.

• | The vehicle must enter the above criteria and leave the criteria for 1 second in order to complete one test.

### Action Taken When the DTC Sets

• | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

• | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

### Conditions for Clearing the MIL/DTC

• | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• | A current DTC, Last Test Failed, clears when the diagnostic runs and passes.

• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.

• | Clear the MIL and the DTC with a scan tool.

### Diagnostic Aids

Inspect for the following conditions:

• | High resistance in an IAC circuit

• | Restricted air intake system

• | Proper operation and installation of all air intake components

• | Collapsed, clogged, or loose air intake ducts

• | A clogged air filter

• | Proper operation of the mass air flow (MAF) sensor, if equipped

• | A tampered with or damaged throttle stop screw

• | A tampered with or damaged throttle plate, throttle shaft, or throttle linkage

• | Objects blocking the IAC passage or throttle bore

• | Excessive deposits in the IAC passage or on the IAC pintle

• | Excessive deposits in the throttle bore or on the throttle plate

• | Vacuum leaks

• | A low or unstable idle condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve--Refer to Symptoms - Engine Controls .

If the problem is determined to be intermittent, refer to Intermittent Conditions .

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. This test will determine the ability of the engine controller and IAC valve circuits to control the IAC valve.

2. This test will determine the ability of the PCM to provide the IAC circuits with a ground. On a normal operating system, the test lamp should not flash while the IAC counts are incrementing.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 | Important: Ensure engine speed stabilizes with each commanded RPM change to determine if engine speed stays within 100 RPM of the commanded RPM.

1. Set the parking brake and block the drive wheels.
2. Install a scan tool.
3. Start the engine.
4. Turn OFF all accessories.
5. With the scan tool RPM control function slowly increment engine speed to 1,700 RPM, then to 600 RPM, then to 1,700 RPM.
6. Exit the RPM control function.

Does the engine speed stabilize within 100 RPM of the commanded RPM during the above test?

Go to Step 3

Go to Step 4

3 |

1. Observe the Freeze Frame/Failure Records for this DTC.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC, and within the parameters observed in the Freeze Frame/Failure Records.

Does the DTC fail this ignition?

Go to Step 4

Go to Diagnostic Aids

4 |

1. Turn OFF the ignition.
2. Disconnect the IAC valve.
3. Connect the J 37027-1A IAC Motor Driver to the IAC valve.
4. Start the engine.
5. With the J 37027-1A , command the IAC valve in until near 600 RPM is reached.
6. With the J 37027-1A , command the IAC valve out until near 1,700 RPM is reached.
7. Return engine speed to desired idle as indicated on the scan tool data list.

Did the engine speed steadily decrease to near 600 RPM and steadily increase to near 1,700 RPM when the IAC valve was commanded in and out?

Go to Step 5

Go to Step 11

5 |

1. With a test lamp connected to ground, probe one of the IAC valve circuits at the IAC valve harness connector using the J 35616 Connector Test Adaptor Kit .
2. Start the engine.
3. With the J 37027-1A , command low RPM while observing a scan tool until the IAC counts start to increment.
4. With the J 37027-1A , command high RPM while observing a scan tool until the IAC counts start to increment.
5. While the IAC counts are incrementing, observe the test lamp.
6. Return engine idle speed to desired idle as indicated on the scan tool data list.
7. Repeat the above procedure for the other three IAC valve circuits.

Did the test lamp remain ON, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the above test?

Go to Step 10

Go to Step 6

6 | Did the test lamp remain OFF, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the above test? | Go to Step 9 | Go to Step 7
7 |

1. Connect a test lamp between the IAC coil A low circuit and the IAC coil A high circuit at the IAC valve harness connector using the J 35616 .
2. With the J 37027-1A , command low RPM while observing a scan tool until the IAC counts start to increment.
3. With the J 37027-1A , command high RPM while observing a scan tool until the IAC counts start to increment.
4. While the IAC counts are incrementing, observe the test lamp.
5. Return engine idle speed to desired idle as indicated on the scan tool data list.
6. Repeat the above procedure with the test lamp connected between the IAC coil B low circuit and the IAC coil B high circuit at the IAC valve harness connector.
7. Return engine speed to the desired idle as indicated on the scan tool data list.

Did the test lamp stay illuminated and never flashing while the IAC counts where incrementing during the above test?

Go to Step 8

Go to Step 16

8 | Inspect for a poor connection at the IAC valve harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 17 | Go to Step 12
9 |

1. Turn OFF the ignition.
2. Disconnect the PCM harness connectors.
3. With a DMM, test the circuit where the test lamp remained OFF for the following conditions:

• | An open circuit

• | A short to ground

• | A short to another IAC valve circuit--Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 12

10 |

1. Turn ON the ignition.
2. With a DMM, test the circuit where the test lamp remained ON for the following conditions:

• | A short to voltage

• | A short to another IAC valve circuit

3. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 12

11 |

1. Inspect for the following conditions:

• | Throttle body damage and tampering

• | Throttle lever screw tampering, if equipped

• | Restricted air intake system--Inspect for a possible collapsed/clogged air intake duct, before and after the air filter element, restricted air filter element, restriction at the throttle body intake screen, if equipped.

2. Remove the IAC valve. Refer to Idle Air Control Valve Replacement .
3. Inspect for the following conditions:

• | A clogged IAC passage

• | Excessive deposits on the throttle plate

• | Excessive deposits in the throttle bore

• | Excessive deposits on IAC valve pintle

4. Repair any of the above conditions as necessary. Refer to the appropriate repair procedure.

Did you find and correct the condition?

Go to Step 17

Go to Step 14

12 | Inspect for a poor connection at the PCM harness connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 17 | Go to Step 13
13 |

1. Disconnect the PCM harness connectors.
2. With a DMM, test all IAC valve circuits for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 17

Go to Step 16

14 | Inspect for a poor connection at the IAC valve harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 17 | Go to Step 15
15 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Did you complete the replacement? | Go to Step 17 | --
16 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | Go to Step 17 | --
17 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

Go to Step 18

Go to Step 2

18 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | Go to Diagnostic Trouble Code (DTC) List | System OK