Furnace clicks but will not ignite

Critical gas safety steps

Before troubleshooting a furnace that clicks without lighting, you must put gas safety first. Fuel gas can leak if components fail or if fittings loosen.

If you smell gas at any time, do not flip light switches, use phones inside, or attempt repairs. Leave the house right away and contact your local gas utility from outside. Always turn off both the electric power at the breaker and the gas supply valve before inspecting internal parts.

The furnace start-up order

A modern gas furnace follows an exact sequence of events every time the thermostat calls for heat. Knowing this order helps you pinpoint where the sequence stops.

First, the thermostat sends a 24 V control signal to the furnace control board. The draft inducer motor turns on to clear residual exhaust gases from the heat exchanger and venting pipes. Next, the pressure switch verifies proper draft airflow. Once airflow is proven, the board powers the igniter. Finally, the gas valve opens, releasing fuel across the hot igniter to create a steady flame.

1. The thermostat calls for heat using a 24 V signal.
2. The draft inducer motor starts and purges the heat exchanger.
3. The pressure switch closes to confirm adequate draft pressure.
4. The igniter receives power and begins to glow hot.
5. The gas valve opens to send fuel to the burners.
6. The flame sensor confirms the presence of flame to keep the gas valve open.

Pressure switch and inducer problems

The pressure switch is an essential safety device. It lets the furnace light only when the vent is clear and the inducer is running.

If the inducer motor fails to run, or if the vent pipe or drain line is blocked, the pressure switch will stay open. A cracked or clogged pressure switch hose can also make a good switch look bad. When the switch does not close, the sequence stops completely. The furnace may click as relays engage, but it will never advance to the ignition stage.

Never bypass a pressure switch to force the furnace to ignite. Jumping this switch can allow dangerous carbon monoxide into your home.

Igniter problems

The igniter heats up to light the fuel. If it is cracked, broken, or worn out, the furnace tries to start but never lights.

Most furnace hot surface igniters run on 120 V, though some systems use 80 V models. These two types are not interchangeable. Many older systems use silicon carbide igniters, while newer designs use durable silicon nitride. Silicon nitride igniters last longer than silicon carbide igniters.

Igniters are very brittle. Never touch the glowing element with your bare fingers, as natural skin oils can shorten its working life. If the igniter does not glow during the start-up sequence, it may have burned out and need replacement.

Gas valve and flame sensor issues

If the igniter glows brightly but no fire appears, the gas valve might not be opening. The click you hear could be the control board sending 24 V to the valve coils, or the valve solenoid attempting to actuate.

A gas valve must match the specific fuel supply, whether natural gas or propane, as well as the pipe size and BTU rating of your equipment. For instance, some valves handle natural gas with intermittent pilot systems, while others fit propane applications.

If the burners do light briefly but shut off after 3 to 10 seconds, the flame sensor is often dirty or worn. The flame sensor proves there is a flame. When it fails to detect the fire, the control board quickly shuts the gas valve off to prevent unburned gas from accumulating.

Furnace lockouts and control boards

When a furnace tries to ignite several times and fails, the control board enters a safety lockout mode. This prevents the system from endlessly opening the gas valve or cycling the igniter.

During a lockout, the furnace will not attempt to light again until the control board resets or its built-in lockout period ends. You can inspect the diagnostic LED light on the control board through the blower sight glass. The flashing light pattern signals which safety component stopped the sequence.