Live data OBD2: PID, fuel trim и что записывать в лог

OBD2 live data streams Parameter IDs (PIDs) from the ECU at a rate limited by the adapter and vehicle. High-frequency PIDs like RPM update quickly; some manufacturer-specific items may be unavailable on generic mode.

Short-term and long-term fuel trim show how the ECU compensates for lean or rich conditions—large positive LTFT with a P0171 family code often points to unmetered air or sensor bias, while large negative values may suggest fuel pressure or leaking injectors (context matters).

Oxygen sensor voltage or wideband equivalence ratio helps judge closed-loop operation; post-cat sensors should be relatively steady if the catalyst is healthy. MAF grams per second should track load; compare to expected values for your engine size.

Logging beats snapshots for intermittent issues: graph trim, O2, and misfire counters over the same drive cycle where symptoms appear.