Live Data Basics
You don’t need 200 PIDs. You need a few sanity checks that tell you whether the engine is breathing, fueling and sensing reality properly.
Fuel trims (STFT/LTFT)
What they mean: how hard the ECU is correcting fueling. High positive trims = adding fuel (lean condition). High negative trims = removing fuel (rich condition).
- Lean at idle, improves with RPM: often an intake leak/unmetered air.
- Lean under load: suspect fuel delivery, MAF under‑reading, or restriction.
- Rich corrections: leaking injector, fuel pressure issues, MAF over‑reading, EVAP purge stuck open.
MAF sanity check
Different cars vary, but the question is simple: does airflow increase smoothly with RPM/load? Spikes, dropouts or implausible readings are a clue.
Related: P0101.
O2 / lambda behaviour
Upstream sensor should respond quickly to fueling changes. Downstream sensor is mainly for catalyst monitoring.
Don’t replace O2 sensors just because a catalyst code exists — check trims, misfires and exhaust leaks first. See P0420.
Boost target vs actual (turbo engines)
If actual boost can’t meet target, think leaks, stuck wastegate, vacuum supply, or charge pipe issues before “turbo failure”.
Related: P0299.
Coolant warm‑up (plausibility)
Slow warm‑up can point to thermostat issues. Unrealistic temp jumps can point to sensor/wiring faults. Always ask: does the value match reality?