Live data: the eight readings that solve most engine codes

Codes name symptoms; live data names causes. These eight PIDs, available on nearly any scanner or dongle app, are the working set this site's write-ups reach for again and again.

1. Short + long term fuel trim (per bank) — the ECU's running confession of how wrong its fuel model is. Combined within ±8% is healthy. High positive: unmetered air or starved fuel. The idle-versus-2,500 rpm comparison splits those two — see the P0171 guide.
2. Coolant temperature — should climb steadily from cold and hold near the thermostat's rating (~90 °C). Plateauing low is a stuck-open thermostat wasting fuel; implausible-at-cold readings are sensor lies.
3. Intake air temperature — cross-check against coolant and ambient after an overnight soak; all three within a couple of degrees or someone is lying.
4. MAF grams/second — rough health check: about 1 g/s per litre of displacement at hot idle, rising sharply and smoothly with revs. Low everywhere with an oiled filter history: film on the element.
5. Front O2 sensor voltage — should switch busily between roughly 0.1 and 0.9 V at cruise. Stuck lean, stuck rich, or lazy switching each tell their own story, and trims give the context.
6. Rear O2 sensor voltage — flat and boring near 0.6–0.7 V is a healthy catalyst; mirroring the front one is P0420 forming in real time.
7. Engine load + rpm together — the context stamp for everything else, and the pair to match against a freeze frame when reproducing a fault on a road test.
8. Fuel system status — open loop when warm at cruise means the ECU has stopped trusting a sensor; find out which before chasing anything downstream of it.

The habit that makes these useful: record a short log of your car healthy. The day something breaks, "compared to last month" beats every generic specification on the internet.