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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Jimmy - 4WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 DTC P0506

Circuit Description

The idle air control (IAC) valve is located in the throttle body of both TBI and the MFI systems. The IAC valve consists of a movable pintle that is driven by a gear attached to an electric motor called a stepper motor. The IAC valve motor is a 2-phase bi-polar permanent magnet stepper motor that is capable of highly accurate rotation, or movement, every time the polarity of a winding is changed. This change in polarity can be seen when observing a test lamp connected between ground or B+ and an IAC valve circuit while the powertrain control module (PCM) is attempting to change the engine RPM. The test lamp will flash ON or OFF each time the polarity is changed. The PCM does not use a physical sensor in order to determine the IAC pintle position. The PCM uses a predicted number of counts. One count represents 1 change in polarity which equals 1 step of the stepper motor. The PCM counts the steps the PCM has commanded in order to determine the IAC pintle position. The PCM uses the IAC valve in order to control the engine idle speed. The PCM controls the engine idle speed by changing the pintle position In the idle air passage of the throttle body. This process varies the air flow around the throttle plate when the throttle is closed. In order to determine the desired position of the IAC pintle at idle or during deceleration, the PCM refers to the following inputs:

    • The engine RPM
    • The battery voltage
    • The air temperature
    • The engine coolant temperature
    • The throttle position sensor angle
    • The engine load
    • The vehicle speed

When the ignition key is turned OFF after an ignition cycle, the PCM will first seat the IAC pintle in the air bypass bore and then retract the pintle a predetermined amount of counts in order to allow the proper amount of air to bypass the throttle plate for engine start-up. This procedure is known as an IAC reset.

Conditions For Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0171, P0172, P0174, P0175, P0200, P0300, P0401, P0404, P0405, P0440, P0442, P0443, P0446, P0449, P1111, P1112, P1114, P1115, P1121, P1122, P1380, P1381, P1404 or P1441 are not set.
    • The engine coolant temperature (ECT) is more than 60°C (140°F).
    • The barometric pressure (BARO) is more than 65 kPa.
    • The engine run time is more than 60 seconds.
    • The throttle position (TP) sensor is less than 0.7 percent.
    • The vehicle speed sensor (VSS) is less than 1.6 km/h (1 mph).
    • The ignition voltage is between 9-18 volts.
    • The intake air temperature (IAT) is more than 10°C (14°F).
    • The above conditions are present for 2 seconds.

Conditions for Setting the DTC

The actual engine speed is 50 RPM less than the desired engine speed.

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:

    •  A restricted air intake system
    • Incorrect operation and installation of all air intake components such as collapsed, clogged, or loose air intake ducts, clogged air filter, and incorrect operation of the mass air flow (MAF) sensor
    • A throttle stop screw that shows evidence of damage or tampering
    • A throttle plate, a throttle shaft, or throttle linkage that show evidence of damage or tampering
    • Objects blocking the intake air control (IAC) passage or the 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 can be caused by a non-IAC system condition that can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls .
    • If the condition is intermittent, refer to Intermittent Conditions .

Test Description

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

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

    2.

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

DTC P0506, P1508 Idle Speed Low Table

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 that engine speed stabilizes with each commanded RPM change in order to determine if engine speed stays within 50 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 1,800 RPM, then to 600 RPM, then to 1,800 RPM.
  6. Exit the RPM control function.

Does the engine speed stabilize within 50 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 as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

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 an J 37027-A IAC Motor Driver to the IAC valve.
  4. Start the engine.
  5. With the J 37027-A , command the IAC valve in until near 600 RPM is reached.
  6. With the J 37027-A , command the IAC valve out until near 1,800 RPM is reached.
  7. Return engine speed to the 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 1,800 RPM when the IAC valve was commanded in and out?

Go to Step 5

Go to Step 10

5

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

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

Go to Step 9

Go to Step 6

6

Did the test lamp remain OFF and never flashing while the IAC counts were incrementing at any of the IAC valve circuits during the above test?

Go to Step 8

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 theJ 35616 .
  2. With the J 37027-A , command low RPM while observing a scan tool until the IAC counts start to increment.
  3. With the J 37027-A , 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 the 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 Diagnostic Aids

Go to Step 14

8
  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connectors.
  3. With a DMM, test for an open or for a short to ground on the IAC valve circuit where the test lamp remained OFF. Refer to Circuit Testing , and refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 15

Go to Step 11

9
  1. Turn ON the ignition.
  2. With a DMM, test for a short to voltage on the IAC valve circuit where the test lamp remained illuminated. Refer to Circuit Testing , and refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 15

Go to Step 11

10
  1. Inspect for the following conditions:
    2. • Throttle body damage and tampering, or throttle lever screw tampering if the vehicle has this equipment
    • A restricted air intake system--Inspect for a possible collapsed or clogged air intake duct before and after the air filter element, a restricted air filter element, and a restriction at the throttle body intake screen, if the vehicle has this equipment.
  2. Remove the IAC valve. Refer to Idle Air Control Valve Replacement .
  3. Inspect for a clogged IAC passage.
  4. Inspect for excessive deposits on the throttle plate.
  5. Inspect for excessive deposits in the throttle bore.
  6. Inspect for excessive deposits on IAC valve pintle.
  7. Repair any of the above conditions as necessary. Refer to the appropriate repair procedure.

Did you find and correct the condition?

Go to Step 15

Go to Step 12

11

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 15

Go to Step 14

12

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 15

Go to Step 13

13

Replace the IAC valve. Refer to Idle Air Control Valve Replacement .

Did you complete the replacement?

Go to Step 15

--

14

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

Go to Step 15

--

15
  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 16

Go to Step 2

16

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