For 1990-2009 cars

### Circuit Description

The DTC P0505 Idle Air Control (IAC) System diagnostic observes engine speed in order to monitor the operation of the idle air control (IAC) valve. The powertrain control module (PCM) controls engine idle speed with the idle air control valve. To increase the idle speed, the PCM pulse width modulates the OPEN solenoid within the IAC valve. This action turns a cylinder within the IAC valve, allowing air to bypass the throttle plate. The PCM regulates fast idle speed via the IAC valve by increasing the opening of the air bypass port. The PCM regulates slow idle speed via the IAC valve by decreasing the opening of the air bypass port.

### Conditions for Running the DTC

The engine is running.

### Conditions for Setting the DTC

The PCM determines that the actual idle speed varies greatly from the desired idle speed.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) the first time the diagnostic runs with the fault active.

• | The PCM stores the conditions which were present when the DTC set as Freeze Frame data.

### Conditions for Clearing the MIL/DTC

• | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.

• | The DTC clears after 40 consecutive warm-up cycles have occurred without a fault.

• | A DTC can be cleared by using the scan tool Clear DTC Information function.

### Diagnostic Aids

With the engine idling at a normal operating temperature and all accessories OFF, monitor the following parameters on the scan tool for correct operation:

• | The IAC Motor Command--27-43 percent

• | The electrical load--No

• | The commanded A/C--OFF

The scan tool can be used to isolate the cause of an intermittent DTC P0505. Engine idle speed can be controlled with the scan tool. Operate the IAC valve with the scan tool while checking for a smooth increase and decrease in engine speed. If engine speed can be controlled, the IAC system is functioning normally.

Check for the following conditions:

• | Any blockage of the air intake system

• | Any blockage of the IAC passage

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

• | Any large vacuum leaks

An intermittent malfunction may be caused by a fault in the IAC sensor electrical circuit. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .

Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

If the DTC P0505 does not reset, but a driveability symptom still exists, refer to Symptoms - Engine Controls .

### Test Description

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

1. This step simulates the pulse width modulated (PWM) signal supplied to the IAC valve by the PCM by rapidly grounding and un-grounding terminal 1 of the IAC valve.

2. The most likely cause of no power is an open circuit condition between the IAC valve connector terminal 2 and the splice. A shorted or inoperative EFI relay circuit will cause a no-start condition.

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 |

1. Start and idle the engine until the engine reaches a normal operating temperature.
2. Using a scan tool, store the Freeze Frame and DTC information.
3. Turn OFF the ignition and wait 30 seconds.
4. Turn ON the ignition, with the engine OFF.
5. Perform the scan tool Clear DTC Information function.
6. Start and idle the engine.
7. Wait 60 seconds, then check for stored DTCs.

Does this DTC reset?

Go to Step 4

Go to Step 3

3 | Is there an idle quality concern? | Go to Rough, Unstable, or Incorrect Idle and Stalling | Go to Diagnostic Aids
4 |

1. Turn OFF the ignition.
2. Disconnect the IAC valve electrical connector.
3. Turn ON the ignition, with the engine OFF.
4. Using a test lamp connected to ground, probe terminal 2 of the IAC valve connector on the harness side.

Does the test lamp illuminate?

Go to Step 5

Go to Step 8

5 | Using a test lamp connected to B+, probe terminal 3 of the IAC valve connector . Does the test lamp illuminate? | Go to Step 6 | Go to Step 9
6 |

1. Turn OFF the ignition.
2. Jump terminal 2 of the IAC valve connector on the harness side to terminal 2 of the IAC valve connector with a fused jumper wire.
3. Jump terminal 3 of the IAC valve connector on the harness side to terminal 3 of the IAC valve connector with a fused jumper wire.
4. Connect a fused jumper wire to terminal 1 of the IAC valve--not the harness.
5. Insulate the terminal 1 jumper from contact to ground.
6. Start and idle the engine.
7. With the engine idling, repeatedly touch the terminal 1 jumper to battery ground with a rapid ON-OFF motion.

Does engine speed increase as the terminal 1 jumper is rapidly grounded ON and OFF?

Go to Step 7

Go to Step 10

7 |

1. Turn OFF the ignition.
2. Remove all jumpers.
3. Disconnect the PCM connector containing the IAC valve open control circuit.
4. Check terminal 1 of the IAC valve open control circuit for a grounded, open, or shorted to voltage circuit.
5. Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a circuit repair made?

Go to Step 12

Go to Step 11

8 |

1. Locate the cause of no power to the IAC valve connector. Refer to the Test Description.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

Go to Step 12

--

9 | Repair the open IAC valve ground circuit. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | Go to Step 12 | --
10 | Important: The IAC valve seals the coolant passage of the throttle body. Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Is the replacement complete? | Go to Step 12 | --
11 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the replacement complete? | Go to Step 12 | --
12 |

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.

Are any DTCs displayed on the scan tool?

Go to Step 13

Go to Step 2

13 | 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