Circuit Description
The control module controls idle speed with an idle air control (IAC) valve. The desired RPM is based on sensor inputs and the actual engine RPM. Four circuits are used in order to control the IAC valve, allowing a precise amount of air into the intake manifold. In order to increase the idle RPM, the control module will command the valve pintle to move away from its seat, allowing more air to bypass the throttle plate. To decrease RPM, the control module commands the IAC pintle to move closer to its seat, causing a lower airflow to bypass the throttle plate. A scan tool will display the control module commands to the IAC valve in counts. The higher the counts, the more air is allowed to bypass the throttle plate. Lower counts indicate that less air is commanded by the control module.
This DTC is designed to detect low engine RPM that is not responding to increased IAC counts.
Conditions for Running the DTC
| • | No active MAP sensor DTCs |
| • | No active TP sensor DTCs |
| • | No active VS sensor DTCs |
| • | No active ECT sensor DTCs |
| • | The ECT is greater than 50°C(122°F). |
| • | The system voltage is between 10-16 volts. |
| • | The IAT is greater than -25°C(-13°F). |
| • | The engine has been running for more than 30 seconds. |
| • | The BARO is greater than 70 kPa. |
| • | The MAF is between 17.5-50 g/s. |
| • | The vehicle speed is between 25-85 mph(40-138 km/h). |
| • | Any change in throttle position is less than 1 %. |
| • | Any change in engine speed is less than 50 RPM. |
| • | The IAC valve was commanded to move a specified number of counts. |
| • | The above conditions are present for more than 2 seconds. |
Conditions For Setting DTC
The EVAP system develops a vacuum greater than a calibrated value when purge is not commanded.
Action Taken When the DTC Sets
The VCM will turn ON the MIL after 2 consecutive driving cycles with the fault active.
Conditions for Clearing the MIL or DTC
| • | The control module turns OFF the MIL after 3 consecutive drive trips when the test has run and passed. |
| • | A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle. |
| • | Use a scan tool in order to clear the DTCs. |
Diagnostic Aids
An IAC valve which is stopped and cannot respond to the VCM, a throttle stop screw which has been tampered with, or a damaged throttle or linkage could cause this DTC. A slow, unstable, or fast idle may be caused by a non-IAC valve system problem that cannot be overcome by the IAC valve. Out of range control IAC scan tool counts will be above 60 if idle is too low and zero if idle is too high. Check for the following conditions:
| • | Fuel system too lean or rich. |
| • | Throttle body--inspect bore for foreign material. |
| • | Refer to Rough, Unstable, or Incorrect Idle and Stalling . |
| • | IAC reset is done with scan tool misc functions mode. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The IAC tester is used to extend and retract the IAC valve. The valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be retested when removed from the throttle body.
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This step checks the quality of the IAC movement. Between 700 RPM and about 1200 RPM, the engine speed should change smoothly with each flash of the tester lamp in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed will begin to drop which is normal. On certain engines, fully extending IAC may cause engine stall.
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This step checks the IAC circuits. Each lamp on the node lamp should flash red and green while the IAC valve is cycled. While the sequence of color is not important if either lamp is OFF or does not flash red and green, check the circuits for faults beginning with poor terminal contacts
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and the Failure Records for reference. This information will be lost when the Clear DTC Information function is used. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||
Did the Engine RPM decrease and increase as the IAC is cycled? | -- | |||||||||||
Observe the RPM, it should change smoothly with each flash of the IAC Driver lamp. Does RPM change smoothly within the specified value? | 700-1500 RPM | |||||||||||
4 | Check the IAC passages. If OK, replace the IAC. Important: If a repair has been made refer to the IAC Reset Procedure Is the action complete? | -- | -- | |||||||||
Do both Green and Red lamps flash on and off? | -- | |||||||||||
6 | If circuits did not test green and red check for the following:
Was a problem found? | -- | ||||||||||
7 | Repair the connections and terminal contacts as necessary. Refer to Wiring Repairs in Repair Instructions. Is the action complete? | -- | -- | |||||||||
8 |
Is the resistance within the specified value? | 40-80 ohms | ||||||||||
9 |
Is the resistance at the specified value? | ∞ | Go to The Diagnostic Aids | |||||||||
10 | Replace the IAC valve. If a repair has been made refer to IAC Reset Procedure before resetting. Is the action complete? | -- | -- | |||||||||
11 |
Is the action complete? | -- | -- | |||||||||
12 |
Does the scan tool indicate that this diagnostic ran and passed? | -- | ||||||||||
13 | Use the scan tool in order to display the Capture Info and the Review Capture Info functions. Are there any DTCs displayed that have not been diagnosed? | -- | Go to applicable DTC table | System OK |
