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by Bob O’Neill and Bob Lincoln
Code 25 is thrown when the Automatic Idle Speed (AIS) motor circuit is shorted, or the target idle speed is not achieved.
The ECU (engine computer) monitors the voltage on the idle speed motor circuit, which should be near zero with the motor off and near 12 volts with the motor on. If it does not detect the appropriate voltage, it will register fault code 25. Likewise, if the computer repeatedly tries to achieve a desired idle speed and cannot, it will also register code 25. When it does this, it does not turn the Power Limited light or go into limp mode; presumably the programmers figured that the driver would know something was wrong.
Vacuum leak - Check all vacuum lines for cracks or breaks. Check the plastic tubes as they get brittle as they age due to the heat of the engine.
Poor wiring or connection - Read the page ‘How to troubleshoot drivability issues’. Check the wiring and connections between the sensor and the logic module. Check the harness from the sensor to the computer. Inspect every connector. Clean and re-grease the connectors with dielectric grease.
Shorted or seized AIS motor - The AIS motor is attached to the throttle body. Locate the AIS and note its position while mounted. Remove the AIS bolts and slide out the AIS from the throttle body.
The AIS has a pintle which moved in and out of the orifice in the throttle body to regulate the amount of air which enters the engine at idle. By connecting the AIS to a 12vdc supply, you should see the pintle should move in or out. If not, replace the AIS.
If the AIS is seized, spray all the gears and parts with silicone spray lubricant and insert the motor. Don’t use WD40 as it is not suitable for this. Give the engine some time to settle once the AIS is installed as the engine may race. If it doesn’t settle, it is possible that the AIS was installed backwards.
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