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.
-
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.
-
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 | |
2 |
Does this DTC reset? | Go to Step 4 | Go to Step 3 |
3 | Is there an idle quality concern? | Go to Diagnostic Aids | |
4 |
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 |
Does engine speed increase as the terminal 1 jumper is rapidly grounded ON and OFF? | Go to Step 7 | Go to Step 10 | |
7 |
Was a circuit repair made? | Go to Step 12 | Go to Step 11 |
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 |
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? | System OK | |
