What it is
A gas furnace runs the same startup sequence every single cycle, in the same order, every time. That is the gift that makes a no-heat call diagnosable. You don't have to guess — you watch the furnace try to start, find the exact step where it stalls, and the part that owns that step is your suspect. The sequence is your diagnostic tree; the furnace walks it for you.
This applies to a standard hot-surface-ignition (HSI) furnace, and the same logic adapts to spark and intermittent-pilot units. Know the sequence cold and a no-heat call becomes "which step failed," not "which part do I throw at it."
How it works
When the thermostat calls for heat (W energized), a modern furnace runs an interlocked startup. Each step has to prove out before the next one is allowed. If any safety isn't satisfied, the board parks the sequence right there. That's deliberate — it keeps raw gas from ever entering a box that isn't proven safe to light.
So the diagnosis is: where in the chain did "prove out" fail? Pull the door (with the switch defeated only as needed and safely), pop a jumper on the thermostat call if needed, and watch.
In the field
Follow the sequence and note where it stops:
- Call for heat. Confirm 24V on W at the board. No call, no heat — check the stat, the W wire, batteries, setpoint. A flashing diagnostic LED on the board is free information; read the code chart on the door.
- Inducer (draft) motor starts. The combustion blower should spin up first. If it doesn't, check 24V to the inducer relay, the relay/board, and the motor itself. The inducer's job is to prove draft.
- Pressure switch proves. Once the inducer establishes airflow, the pressure switch closes and tells the board draft is good. This is the #1 no-heat stall point. A switch that won't close means: blocked flue or intake, cracked/disconnected hose, condensate-plugged trap or drain (very common on 90%+ condensing furnaces), weak inducer, or a bad switch. Check the actual pressure at the switch before condemning it — often the switch is fine and the draft is bad.
- Igniter heats (or spark/pilot lights). With draft proven, the HSI glows for its warm-up period. No glow? Check the igniter for an open element (it'll read a few dozen to ~150 ohms cold; an open one reads infinite) and check that the board is sending voltage to it.
- Gas valve opens / burners light. The valve energizes and gas hits the hot igniter. No light with everything before it good → check 24V at the valve, manifold pressure, gas supply, and that the gas is actually on.
- Flame sense proves the flame. The flame rod must detect the flame within a few seconds (typically ~4–7 sec trial) or the board closes the valve and may retry or lock out. This is the #2 stall point: it lights, runs a few seconds, then drops out. That pattern is almost always flame sensing — a dirty/oxidized flame rod, bad ground, cracked porcelain, or the rod not in the flame. Measure the flame-rectification microamps.
- Blower starts (heat blower delay). After the heat exchanger warms, the board energizes the indoor blower (G/heat). If burners run but no air moves, suspect the blower, its capacitor/ECM, or the board's blower output. If it short-cycles on the high limit, you likely have an airflow problem.
Normal values & targets
- Flame-sense current: typically ~2–6 microamps DC on a healthy rod; below ~1 µA and the board may drop the flame. Clean the rod with light abrasive (not heavy sandpaper) and recheck.
- HSI resistance (cold): roughly 40–150 ohms depending on the igniter; open = infinite = replace.
- Manifold gas pressure: about 3.5" W.C. for natural gas, ~10–11" W.C. for LP, on a typical single-stage valve — always set to the rating plate.
- High-limit: opens on overtemp from low airflow; if it cycles, fix airflow (filter, blower, ducting), don't just bypass it.
Always confirm against the unit's rating plate; these are typical, not universal.
Common faults & what they mean
- Pressure switch won't close: plugged condensate trap/drain, blocked flue/intake, weak inducer, bad hose, or bad switch. Single most common no-heat.
- Lights then drops out in seconds: flame-sensing failure — dirty rod, poor ground, cracked insulator.
- No igniter glow: open HSI element or no signal from board.
- Inducer never starts: relay, board output, or motor.
- Burners run, no warm air: blower or blower control; if it overheats and trips limit, it's an airflow problem.
- Locks out after several tries: count the retries and read the flash code — the board logged where it gave up.
Tech tips & gotchas
- Read the LED flash code first. The board already diagnosed itself; the code tells you which step failed before you touch anything.
- Don't jump straight to "bad board." Boards fail, but pressure switches, igniters, and dirty flame rods fail far more often. Clear the cheap, common stuff first.
- On condensing furnaces, a slow or plugged condensate drain causes intermittent pressure-switch trips that look random. Check the trap.
- A cracked heat exchanger is a safety condemnation, not a no-heat fix — but watch for flame distortion on blower start as a flag to investigate further. CO testing is part of a thorough no-heat call.
Safety / code notes
- This is fuel-gas and combustion work — verify there's no gas leak, and use a combustion analyzer to confirm safe operation, not just "it lit."
- Confirm venting and combustion-air provisions are intact and sized per the applicable mechanical/fuel-gas code sections (e.g., IMC §701 / IFGC) — never reproduce or improvise venting.
- Test for CO before you leave. A furnace that heats but spills combustion products is a callback and a hazard, not a completed repair.
- Never permanently defeat a safety (limit, pressure switch, rollout) to "make it run." If a safety is tripping, it's reporting a real condition — find it.