1996 Ford Aerostar Owner's Manual - Page 79

Check Engine Warning Light The Powertrain On-Board Diagnostic II (OBD II) system consists of the hardware and software necessary to monitor the operation of the powertrain. The OBD II system is designed to check the function of the vehicle's powertrain control system during normal operation. If an emission problem is detected, the Check Engine Warning Light (in the cluster) is turned on. Modification or additions to the vehicle may cause incorrect operation of the OBD II system. Additions such as burglar alarms, cellular phones, and CB radios must be carefully installed. Do not install these devices by tapping into or running wires close to powertrain control system wires or components. The light comes on briefly when you turn the ignition key to the ON position, but it should turn off when the engine starts. If the light does not come on when you turn the ignition to ON or if it comes on and stays on when you are driving, have your vehicle serviced as soon as possible. This indicates a possible problem with one of the vehicle's emission control systems. You do not need to have your vehicle towed in. If the light turns on and off at one (1) second intervals while you are driving the vehicle, it means that the engine is misfiring. If this condition persists, damage could occur to the engine or catalytic convertor. Have your vehicle serviced at the first opportunity. You do not need to have your vehicle towed in. If the light turns on and off on rare occasions while you are driving, it means that a malfunction occurred and the condition corrected itself. 78