Overview
Up until 1996, most vehicles had a diagnostic program installed in the engine control module or ECM. This program helps automotive repair technicians diagnose vehicles with the check engine light on. When the ECM is checked for codes, it will communicate any stored trouble codes by flashing the service engine light.
Things you will need
Paper clip
Vehicle ignition key
Pen and paper
Vehicle manufacturer OBD code list (available online)
Step 1:
Look for the ECM test plug in the area under the steering wheel; it’s usually attached to the driver’s side kick panel or behind the driver’s side kick panel.
Step 2:
Visually inspect the ECM test plug and locate terminal A and terminal B. These two terminals will always be right next to each other and are commonly located on the top right of the ECM test plug.
Step 3:
Unbend the paper clip with your fingers until it’s straight and then bend the paper clip in half. Jump terminal A to terminal B by placing one end of the paper clip into terminal A and the other end into terminal B. Make sure that all accessories are turned off, including the radio and heater controls.
Step 4:
Place the ignition key into the vehicle ignition and turn the key to the “On” position, being careful not to start the vehicle. Watch the check engine light as it will begin to flash. It will initially flash code 12, this means the ECM is in diagnostic mode. Write down the codes that follow code 12.
Step 5:
Turn the ignition key off and remove it from the ignition. Remove the paper clip. Compare to the codes in the vehicle manufacturer’s code list.
Quick Tips:
If you receive a code for a faulty sensor, always check the electrical or vacuum circuit that controls that sensor for faults before replacing the sensor.