GAS FURNACE: CALL FOR HEAT, NO INDUCER, NO IGNITION, NO LED

#1 Failed control board (dead microprocessor, bad relay, or heat-cracked solder joints)

For: 24V arrives and nothing happens — no LED, no outputs, no response. Boards in hot blower compartments fail with age, and the week-old continuous-blower episode is a classic pre-failure symptom: a blower relay welding closed or a driver dying is often the first visible stage of the same board's decline, weeks before the processor quits entirely.

Against: The continuous blower cuts the other way too: a limit-tripped cool-down blower looks identical to a homeowner. And an open safety in the limit string (cause #2) can produce the same powered-but-silent behavior on some boards. Without the LED-by-design chart for this board model, 'no light' is pattern, not proof.

#2 Open safety in the limit string (high-limit or rollout switch tripped, or a spilled/failed rollout)

For: Safeties sit in series ahead of the gas valve and often ahead of the ignition sequence itself; an open limit on some designs means the board waits silently with no flash. The continuous blower a week ago fits a limit trip perfectly — boards run the blower to shed heat after a trip, and a dirty filter or a plugged coil can have been degrading ever since.

Against: The thermostat call is believed present but never proven at the board, and the critical measurement is missing: voltage across each safety. On most boards an open limit still lights the LED — often with a flash code — so a fully dark board argues against this being the whole story.

#3 No call is actually arriving (thermostat, thermostat wiring, or a blown fuse on the R-W path)

For: The homeowner 'hearing a click' is the homeowner's report, not a measurement. A broken thermostat wire, dead thermostat batteries, or a call that dies at a connector produces exactly this: 24V present at R-C, board idle, no LED, nothing runs.

Against: The R-to-W jumper test — one wire, ten seconds, done at the board — was not performed. But a blown low-voltage fuse would pull R dead, and R measures 24.2V live, which leaves only the W-path wiring or the thermostat itself, both cheap and both ruled in or out by the jumper.

#4 Door/interlock switch or its harness not making contact

For: The door switch was cycled to remove the panel — it is the one component the technician's own hands touched, and an old interlock that no longer fully closes de-powers the board while the transformer reads fine upstream.

Against: The door switch is in series ahead of the transformer's low-voltage output on most designs, so a dead interlock usually means no 24V at R-C — and 24V was measured. Not fully excluded, but it is a 30-second visual check, not a hypothesis worth ranking higher.

#5 Inducer motor or its harness failed (board fine, load dead)

For: A seized inducer or unplugged harness stops ignition dead, and the tech's attention is naturally on the board's dark LED.

Against: A dead inducer load does not explain a dark status LED, and it does nothing to explain the continuous-blower history. The board symptom and the load symptom would have to fail simultaneously by coincidence.

? What are the symptoms of a failing furnace control board?

The most common ones: the furnace never starts even though 24 volts arrive at the board from the thermostat; no status LED at all (a healthy board glows steady, or flashes a code); the inducer fan or igniter never fires on a call for heat; and the blower behaves on its own schedule — running constantly, or continuing long after the call ends — because the blower relay on the board has welded or its driver failed. Intermittent ignition followed by a total no-start is the classic arc: boards usually fail in stages, not all at once. The catch is that every one of these symptoms can also be caused by something cheaper in series with the board, which is why the board is condemned last, not first.

? Will a bad control board make the blower run constantly?

Yes — a blower relay welded closed or a failed tripper/driver keeps 120V on the blower motor regardless of what the thermostat asks, and the only way to stop it is cutting power at the disconnect or breaker. That symptom deserves a caveat, though: a control board can also run the blower continuously on purpose, as a cool-down after a limit switch trip, or when the thermostat is set to fan-on. Constant blower plus a no-heat complaint together is a different, stronger story than constant blower alone — one points at the board's relay, the other at a safety that has tripped.

? How do I tell if it's the control board or the thermostat?

Jumper R to W directly at the furnace board (with the door switch engaged and the low-voltage fuse in mind) — this creates the heat call at the board itself, bypassing the thermostat and its wiring entirely. If the furnace lights on the jumper, the board heard the call and the problem is upstream: thermostat, wiring, or the thermostat battery. If it stays dead on the jumper, the call is arriving and the problem is at or after the board: a safety in the limit string, the board itself, or its ground.

? How much does it cost to replace a furnace control board?

The board itself typically runs roughly $75–$300 depending on the brand and whether it is an OEM part or a universal retrofit; installed by a contractor, the total usually lands in the $300–$650 range with labor. That price is exactly why a careful tech proves the board dead before ordering it — a $5 door-switch or a $15 thermostat can produce the same no-response symptom, and a board bought on a guess is rarely returnable.