Furnace Clicks But Will Not Light: The Ignition Sequence Explained
A modern furnace runs a specific startup sequence. Knowing the steps tells you roughly where it is failing before anyone opens a panel.
A modern gas furnace starts in a fixed sequence: thermostat call, inducer motor, pressure switch closure, igniter warm-up, gas valve opening, flame proving by the flame sensor, and blower start. A furnace that clicks and tries but never lights is failing at one of those steps, and a dirty flame sensor is the single most common cause.
The sequence, step by step
Watching and listening during a start attempt tells you a great deal, because each step makes a distinct sound at a predictable time.
- Thermostat calls for heat. Low-voltage signal to the control board.
- Inducer motor starts. A small fan that clears the heat exchanger and establishes draft. You hear a whirring sound for 15 to 30 seconds.
- Pressure switch closes. A safety device confirming the inducer is actually moving air. You may hear a faint click.
- Igniter energizes. A hot surface igniter glows orange for a few seconds, or a spark igniter clicks rapidly.
- Gas valve opens. A solid click, then a soft whoosh as the burners light.
- Flame sensor proves the flame. Within a few seconds the control must see the flame or it shuts the gas off.
- Blower starts. After a short delay so the heat exchanger warms first.
It lights and then shuts off after a few seconds
This is the most common no-heat pattern we see, and it almost always means the flame sensor is dirty.
The flame sensor is a thin metal rod sitting in the flame path. It works by flame rectification — the flame itself conducts a tiny current, on the order of microamps, from the rod to ground. The control board watches that current to confirm a flame exists.
Over time the rod develops a coating of oxidation and combustion residue. That coating insulates it, the measured current falls below threshold, and the board concludes there is no flame — so it closes the gas valve as a safety response. The furnace lights, runs three to five seconds, and shuts down. Often it will retry two or three times before locking out.
The fix is cleaning the rod and verifying the microamp signal. It is a small job, and it is the reason we clean flame sensors at every fall maintenance visit rather than waiting for the failure.
Where else it fails, by symptom
| What you observe | Likely cause |
|---|---|
| Nothing happens at all | Thermostat, control board, transformer, blown low-voltage fuse, or power |
| Inducer runs, nothing further | Pressure switch not closing — blocked flue, cracked hose, failing inducer |
| Igniter glows, no gas | Failed gas valve, gas supply issue, or the board not sending the signal |
| Lights then shuts off in seconds | Dirty or failed flame sensor (most common) |
| Runs then shuts off after minutes | High limit tripping — usually restricted airflow |
| Igniter never glows | Cracked hot surface igniter — brittle and a common failure |
What you can safely check
- Filter. A restricted filter causes the furnace to overheat and trip the high limit, which reads exactly like a failing furnace.
- Thermostat. Set to HEAT, setpoint above room temperature, fresh batteries.
- Power. Furnaces have a service switch that looks like a light switch near the unit. People turn them off and forget.
- Gas supply. Is the gas on elsewhere in the house — water heater, range?
- Condensate. High-efficiency condensing furnaces produce condensate. A blocked drain trips a safety switch and prevents operation.
What you should not do is bypass a safety switch, jumper a pressure switch, or repeatedly reset a system that keeps locking out. Those switches exist because the failure modes they prevent are dangerous.
Why Arizona furnaces fail on the first cold night
Because they sat idle from March to October. Dust settles on the flame sensor and burner assembly, spiders build in the inducer housing and pressure switch tubing, and igniters that were already brittle finally crack on the first thermal cycle.
Every one of those is found and fixed in a twenty-minute fall maintenance check. That is the entire argument for October service in a climate where nobody thinks about heat until they suddenly need it.
Frequently asked
Why does my furnace light then shut off after a few seconds?
Almost always a dirty flame sensor. The sensor confirms a flame exists by measuring a tiny current the flame conducts. Oxidation on the rod insulates it, the current falls below threshold, and the control board shuts the gas off as a safety response.
What does it mean when a furnace clicks but does not start?
Clicking without ignition usually means the control board is attempting the sequence and a step is failing. Common causes are a failed igniter, a pressure switch not closing, a gas valve not opening, or a dirty flame sensor causing repeated lockouts.
Can I clean my furnace flame sensor myself?
Technically it is a simple part to access on many furnaces, but it involves working around a gas valve and electrical connections, and the job is not finished without verifying the microamp signal afterward. We would rather do it correctly as part of a fall tune-up than have someone reassemble a gas appliance uncertainly.