Most furnaces built in the last 25 years keep a diary, and they'll read it to you for nothing: the small LED visible through the panel window blinks in patterns — three flashes, pause, repeat — and the legend translating those patterns is printed right on the inside of the front door. Before you call anyone (including us), thirty seconds of counting turns "it's broken" into "it's flashing 3-then-2," which meaningfully sharpens the visit.
How to read it
Watch the LED through the little window (or with the upper door off — note the blower door has a safety switch and the furnace won't run with it removed on most models). Count: steady flashes in groups, sometimes two-digit codes as X fast + Y slow. Match against the door legend. Codes differ by manufacturer, but the vocabulary is shared across nearly all of them.
The shared vocabulary, translated
Pressure switch open/stuck (very common): the furnace can't prove its venting is drafting — blocked intake/exhaust, failing inducer motor, or on high-efficiency units, a clogged condensate trap backing water into the switch tubing. Ignition failure / lockout: three failed light attempts — usually the igniter or flame sensor from our other guides; a power cycle grants one retry, and if it locks out again, stop retrying. Flame lost / weak flame signal: the sensor-cleaning story. Limit switch open: overheating — filter and airflow first. Polarity/grounding fault: an electrical supply issue that truly calls for a professional. Rapid or steady-on with no heat: often a control board fault code of its own — the door legend rules.
What's DIY and what isn't
Fair game: reading the code, changing the filter, checking that intake/exhaust pipes outside aren't blocked by leaves, snow or an ambitious bird, and one power-cycle retry. Not fair game: bypassing any switch the code names — every one of those switches is a safety testifying about combustion, draft or heat, and jumpering them converts a no-heat evening into an actual hazard.
Why the code helps us help you
Tell us the flash pattern and the furnace brand on the phone and we arrive already narrowed to a subsystem, often with the likely part in hand — which is how a no-heat call turns into a one-visit repair. A written quote follows once we verify the code's story with a meter; boards do occasionally lie, and we bill for the truth, not the rumor.