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, only difference from usual trouble shooting procedure is that you need to connect vehicle to OBD II scan tool complying with SAE J1978 or hand-held tester, and read off various data output from vehicle's engine control module (ECM).
OBD II regulations require that vehicle's on-board computer lights up malfunction indicator light (MIL) on instrument panel when computer detects a malfunction in emission control system/components or in powertrain control components which affect vehicle emissions, or a malfunction in computer. In addition to MIL lighting up when a malfunction is detected, applicable diagnostic trouble code (DTC) prescribed by SAE J2012 is recorded in ECM memory. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If malfunction does not reoccur in 3 consecutive trips, MIL goes off automatically but DTCs remain in ECM memory.
To check DTCs, connect OBD II scan tool or hand-held tester to data link connector No. 3 (DLC3) on vehicle. OBD II scan tool or hand-held tester also enables you to erase DTCs and check freeze frame data and various forms of engine data. For operating instructions, see OBD II scan tool's instruction book. DTCs include SAE controlled codes and manufacturer controlled codes. SAE controlled codes must be set as prescribed by SAE, while manufacturer controlled codes can be set freely by manufacturer within 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.
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