Engine light on

You only replace parts that were diagnosed to be defective or worn out.

There are well over 2,000 potential defects the Engine Computer can detect. Each of those will set its own diagnostic fault code specific to that problem. About half of those codes refer to something that could adversely affect emissions. Those are the codes that turn on the Check Engine light.

You did not list any symptoms or problems, so I cannot offer any advice on what to test. The place to start is by having the diagnostic fault codes read and recorded. Those will indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. Fault codes never say to replace parts or that one is defective. When a part is referenced in a fault code, it is actually the cause of that code only about half of the time. First we have to rule out wiring and connector terminal problems, and mechanical problems associated with that part.

The people at many auto parts stores will read fault codes in the Engine Computer for you for free.