To read engine trouble codes for newer cars, find the 16-pin OBD II diagnostic plug and plug in the code reader or scan tool. Turn the car on but do not start the engine. The process varies depending on the code reader, but press the Read Code option for most devices. For older cars, find the vehicle diagnostic plug and bridge the wires. Watch the flashing light and diagnose the code using car-specific engine light code lists.
The owner's manual lists the location of the 16-pin OBD II diagnostic plug. Some scanner tools do not automatically recognize the make, model and year of the vehicle. If that is the case, enter the information prior to pressing Read Code. Write down engine trouble codes to look up the meanings if the tool does not display them.
To find the vehicle diagnostic plug on older cars, look for an unplugged connector inside the passenger dashboard. For further assistance, search online or in the owner's manual to find the connector location. Using a paper clip or a wire with the ends stripped off, connect the two holes of the diagnostic plug to make a circuit. Put the car into the "run" position and watch the Check Engine light for flashes. Record the number of flashes and whether the flashes are short or long. Refer to the vehicle's Check Engine light error code reference to diagnose the trouble code.