What glow plugs are, how OBD-II sees their state via P0670, P0671-P0678, why one dead plug ruins the whole cold start, and how much replacement costs on a 4-cyl diesel.
What IMA/injector correction code is, how EDC17 ECU learns each injector, codes P0263-P0272, what OBD-II reads, and why "just replace the injector" on diesel doesn't work.
What a DPF is, how it regenerates, which DTCs signal a problem (P2002, P244A, P244B), how to read soot levels via OBD-II, and why short trips kill diesel engines.
When passive/active DPF regen fails: how to run forced/stationary regen via VCDS, ISTA, Xentry or universal Autel. Safety, time, warnings.
What DTC severity level means, how to tell "light from dust on the MAF" from "stop the engine now", which codes can wait, which are critical right now.
What OBD-II shows when oil pressure drops: P0520-P0524, difference between "high" and "low", when you can't drive another meter, worn pump, clogged filter or worn bearings.
What OBD-II can and can't do on electric vehicles: hybrid diagnostics for Toyota Prius, reading battery health on Nissan Leaf, Tesla's limitations, and the role of manufacturer CAN.
What Short-Term (STFT) and Long-Term (LTFT) Fuel Trim mean, which values are normal, and how to diagnose a vacuum leak, dirty MAF or clogged injector from Fuel Trim numbers alone.
What OBD-II shows when the odometer says 50 000 mi but the real number is 150 000: ECU log audit, mismatch between ABS, ECM and cluster, motohours vs mileage, DTC history analysis.
Step-by-step guide to reset the "Service in 500 mi" reminder on VW, BMW, Mercedes, Toyota. Button method vs diagnostic method, when online activation is needed, and why the dealer charges $80 for a 30-second job.
Cheap "ELM327 v2.1" for $5 from AliExpress is a fake. How to spot the clone: "ATZ" test, protocol support, firmware version, real Elm Electronics chip vs Chinese clone.
Dual-zone climate control via OBD: codes B12xx, blend-door actuator state, mode-door, vent output temperature. How to diagnose "warm on the right — cold on the left".