An oil no-heat call gets a fixed sequence: tank gauge (run-outs are common and need delivery plus a bleed-and-reprime, not parts), reset history (one press is diagnostic; five presses is a flooded chamber), last service date (nozzle, electrodes, and filter are consumables — a burner two years past service is a lockout waiting for January), and filter behavior (filters clogging every few weeks point at the tank — rust, sludge, water — rather than the burner).
Gas calls get their own sequence — flame-sensing behavior, igniter history, condensate symptoms after cold snaps — but the principle is the same: the symptom plus the fuel plus the house style usually names the subsystem before anyone drives over. On site, the tech tests inside that subsystem with instruments: flame rectification current on gas, nozzle spray and cad-cell sighting on oil, expansion tank charge and feed valve behavior against the gauge on sealed systems. Findings come before authorization, always.