Safety Tip: Always wear safety glasses when working on your vehicle. Wear other personal protective equipment (PPE) when necessary, for example latex gloves or closed toe shoes.

1
Start your vehicle.
2
Locate your check engine light and monitor its status.
3
Verify that the check engine light is on while the engine is running. Turn the e is running. Turn the engine / key off.
4
For 1996 and later vehicles, connect the scanner to the data link connector(DLC) under the driver side instrument panel. Turn the ignition key to the battery ‘ON’ position – do not start the engine at this time.
5
Follow the Diagnostic Scanner’s instructions to retrieve diagnostic trouble code(s).
6
Troubleshoot all possible symptoms that relate to the diagnostic trouble code(s). Example: P0300 – Random/Multiple Cylinder Misfire Detected. This code indicates that the vehicle’s onboard computer has detected that not all of the engine’s cylinders are firing properly. The last 2 digits of the code represent the detected cylinder. In this case 00 is a random cylinder misfire code. A code P0301 would be a misfire detected on cylinder 1. It will be much easier to diagnose if a specific cylinder is detected. There are numerous components that could set a misfire code. Below is a list of possible components that could be at fault: Faulty Spark Plug Wire(s) and/or Plug(s) Faulty Coil Pack(s) Faulty Fuel Injector(s) Faulty Oxygen Sensor(s) Faulty Catalytic Converter(s), Burned Exhaust Valve, Poor Compression.
7
Once you have diagnosed the problem that set the check engine light on, you must now repair and/or replace the faulty component(s).
8
Once the new component(s) has been installed and the repairs are completed, use the OBD Scanner to erase the diagnostic trouble code(s).
9
Start your vehicle and monitor the status of the check engine light.
10
If further diagnosing is required, these steps must be repeated until the check engine light is no longer illuminated.
Source:https://www.automd.com/5/how-to-troubleshoot-a-check-engine-light/