A furnace start is a chain of proofs. The thermostat calls. The inducer motor runs and a pressure switch proves the vent is clear. The igniter heats or sparks. The gas valve opens. The flame sensor proves flame within seconds or the valve closes. The blower waits for warm air, then delivers it. Clicking without heat is one link failed, and the technician's job is finding which link, in order, with a meter.
The Englewood no-heat walkthrough runs two sequences at once: the furnace's start sequence, and the house's freeze sequence. While the igniter and sensor questions proceed, so do the cabinets under sinks on exterior walls, opened, and the faucets set to drip, because the local emergency doctrine treats the pipes as the second patient. A guide written for this market that omits the plumbing half has forgotten what sub-zero actually threatens.
The common failed links, roughly by frequency: a flame sensor filmed over with oxide (cleaning or a modest part), a cracked hot surface igniter, a failing inducer, a clogged condensate trap on high efficiency units that mimics bigger faults, and control boards at the expensive end. Filters starve airflow and cause the overheating shutdowns that look like random failures.
The safety lines outrank the sequence. Gas or rotten egg smell: leave, then the utility or 911. A carbon monoxide alarm sounding: the same. A cracked heat exchanger is the finding that ends a furnace's life for safety reasons, and any technician who finds one will shut the system down and show you the evidence. That is not a sales tactic; it is the rule the trade works under.
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