* OBD-II Scanner: The most reliable method is using an OBD-II scanner. These are inexpensive and readily available at auto parts stores. The scanner will read the Diagnostic Trouble Codes (DTCs) stored in the vehicle's computer. These codes give you a specific indication of the problem.
* Auto Parts Store: Many auto parts stores will scan your vehicle's computer for free. They'll give you the DTCs, and may even offer advice on the potential causes and repairs.
* Mechanic: A qualified mechanic can diagnose and repair the issue. This is the most expensive option, but it ensures a professional diagnosis and repair.
Without knowing the trouble codes, any guess at the cause is purely speculative. However, common issues in older vehicles like the 1998 Mercury Villager that can trigger the check engine light include:
* Oxygen Sensor: A faulty oxygen sensor can lead to poor fuel efficiency and emissions problems.
* Mass Airflow Sensor (MAF): This sensor measures the amount of air entering the engine. A faulty MAF sensor can cause poor performance and fuel economy issues.
* Catalytic Converter: A failing catalytic converter can cause reduced engine performance and emissions problems.
* Ignition System: Problems with spark plugs, ignition wires, or the distributor (if equipped) can trigger the light.
* Engine Coolant Temperature Sensor: This sensor monitors the engine's temperature. A faulty sensor can lead to inaccurate readings and affect the engine's performance.
Once you have the trouble codes, you can search online (e.g., using Google) for information on the specific code and its likely causes. Remember to always consult a repair manual or a qualified mechanic if you're unsure about any repair procedures. Attempting repairs without proper knowledge can potentially damage your vehicle further.
