When the vehicle's "Check engine" light comes on, the only thing you can be certain of is the engine is not functioning properly. To determine the problem, you must connect an engine analyzer to the car's computer. An engine analyzer, also know as a "trouble code reader," communicates with the vehicle's computer, reading the engine's diagnostics. The analyzer will display a code that, when referenced, will tell you what the problem is. Connecting an engine analyzer is as simple as plugging a cord into a socket.
Locate the vehicle's plug-in. On most post-1995 vehicles there is a "D"-shaped socket attached to the bottom of the dashboard, under the steering wheel.
Insert the engine analyzer's connector into the plug, and turn the analyzer on.
Place the key in the ignition and turn the ignition switch to the "on" position. Do not start the engine.
Wait approximate three minutes for the analyzer to display the trouble code. Look the code up in the manual or on the software that was supplied with the code reader.
On some pre-1995 vehicles, the connector is under the hood, on or near the firewall. On earlier model year vehicles, you may also need an adaptor to mate the socket and connector.