SAE J1979
Workshop glossary › Diagnostics & protocols · UpdatedThe SAE standard that defines OBD-II service modes (01–0A) and the list of standard PIDs every emissions-related ECU must support.
SAE J1979 (also published as ISO 15031-5) is the spec that defines what an OBD-II scan tool can ask the ECU and what the ECU has to answer. It enumerates ten service modes:
- Mode 01 — current powertrain data (live PIDs)
- Mode 02 — freeze frame data
- Mode 03 — confirmed DTCs
- Mode 04 — clear DTCs and reset readiness monitors
- Mode 05 — oxygen sensor test results (legacy, pre-CAN)
- Mode 06 — on-board test results for non-continuously monitored systems
- Mode 07 — pending DTCs
- Mode 08 — bidirectional control of on-board systems
- Mode 09 — vehicle info (VIN, CALID, ECU name)
- Mode 0A — permanent DTCs
Most scan tools surface modes 01, 02, 03, 04, 07, 09, and 0A. Mode 06 is where the gold is for diagnosing borderline emissions faults — catalyst efficiency raw values, EVAP leak test results — but the data is rarely human-readable without a manufacturer reference.