1. Oxygen Sensor(s): These sensors monitor the amount of oxygen in the exhaust. A faulty sensor provides inaccurate data to the engine control module (ECM), leading to a rich or lean fuel mixture. This directly impacts emissions and triggers the light. The Windstar likely has multiple oxygen sensors (one or more upstream and downstream of the catalytic converter). A failing sensor is a very common cause of both the light and failing emissions.
2. Mass Airflow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor provides incorrect information to the ECM, again resulting in an improper air/fuel mixture and poor emissions.
3. Catalytic Converter: This is the component that converts harmful emissions into less harmful ones. A failing catalytic converter is a major emissions problem and will almost certainly trigger the "Service Engine Soon" light. They are expensive to replace. *Note:* A bad oxygen sensor or MAF sensor can *damage* the catalytic converter over time by causing it to overheat due to incorrect fuel mixtures.
4. Evaporative Emission System (EVAP) Leak: This system prevents fuel vapors from escaping into the atmosphere. A leak in the system (e.g., in hoses, the gas cap, or the charcoal canister) can cause a failure in the emissions test and illuminate the SES light.
5. PCV Valve (Positive Crankcase Ventilation): A faulty PCV valve can cause excessive crankcase pressure, leading to blow-by and affecting the air/fuel mixture and emissions.
6. Ignition System Problems: Issues like worn spark plugs, failing ignition wires, or a malfunctioning distributor (if equipped) can lead to incomplete combustion, resulting in higher emissions and triggering the light.
7. Engine Vacuum Leaks: Leaks in the intake manifold or vacuum hoses can disrupt the proper functioning of the engine, negatively affecting fuel mixture and emissions.
8. Throttle Position Sensor (TPS): An inaccurate reading from the TPS can lead to poor fuel delivery and emissions issues.
Important Note: The "Service Engine Soon" light itself doesn't pinpoint the exact problem. You need to use an OBD-II scanner to retrieve the diagnostic trouble codes (DTCs) stored by the ECM. These codes will provide a much more specific indication of what's wrong. A local auto parts store often offers free code reading services. This will save you time and money in diagnosing the problem. Fixing the underlying issue indicated by the codes will be necessary to pass the emissions test.
