Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
System Overview
When trouble shooting OBD II vehicles, the only difference from the usual trouble shooting procedure is that you connect the vehicle to the OBD II scan tool complying with SAE J1978 or the hand-held tester, and read the various data output from the vehicle's engine control module (ECM).
The OBD II regulations require that the vehicle's onboard computer illuminates the malfunction indicator light (MIL) on the instrument panel when the computer detects a malfunction in the emission control system/components or in the powertrain control components which affect vehicle emissions, or a malfunction in the computer. In addition to the MIL illuminating when a malfunction is detected, the applicable diagnostic trouble codes (DTCs) prescribed by SAE J2012 are recorded in the ECM memory. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
If the malfunction does not reoccur in 3 consecutive trips, the MIL goes off automatically but the DTCs remain in the ECM memory. To check the DTCs, connect the OBD II scan tool or hand-held tester to the data link connector No. 3 (DLC3) on the vehicle. See Figure. The OBD II scan tool or hand-held tester also enables you to erase the DTCs and the check freeze frame data and various forms of engine data (for operating instructions, see the OBD II scan tool's instruction book). The DTCs include SAE controlled codes and manufacturer controlled codes. SAE controlled codes must be set as prescribed by the SAE, while manufacturer controlled codes can be set freely by the manufacturer within the prescribed limits. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.