If your gas furnace initiates the start cycle but doesn't fire up, it's likely because of a faulty, damaged, or dirty ignitor/sensor. This is a very common problem — you can try to clean the sensor and replace the ignitor.
It could be a few things. Check if the gas supply is on, the pilot light is lit (if applicable), and there are no blockages in the gas line. If everything seems okay, it might be a problem with the ignition system or the thermocouple.
Defective Igniter or Pilot Light
If it clicks but your furnace won't ignite after the combustion cycle, the electronic ignition systems probably need to be replaced. In cases where the furnace ignition starts normally but only stays on for a few minutes and then goes out, a new pilot light likely needs to be installed.
If your furnace won't kick on when you turn the thermostat up, it could be due to a variety of reasons such as a tripped circuit breaker, a faulty thermostat, or a problem with the furnace itself like a malfunctioning igniter or gas valve. Checking these components might help you figure out the issue.
Check the Ignition
Then, locate and remove the burner door. Switch the ignitor to the off position, and wait at least five minutes. If your ignitor has a reset button, you can use this option. Now, you can replace the burner door (ensuring that it's appropriately latched), and turn the circuit back on.
You can reset the ignition on your furnace simply. To do this you must cut the power to the furnace first by turning off the circuit that powers it. Once the power is off you can remove the burner door to reveal the ignitor. Turn this to “off” and leave it off for at least 5 minutes.
Your gas hob might not ignite properly if spilled food or dirt is clogging the ignition nozzles. Ignition may also fail if the nozzles, burner caps or ports are wet after cleaning. Dry the hob and the burner tops thoroughly after cleaning them.
If your heater doesn't start, chances are the problem stems from having a dirty furnace. Having dirt on your blower motor or flamer sensor could render your furnace unable to perform. When the blower motor is too dirty, it won't pass enough cool air to the heat exchanger, leading to equipment failure.
Checking your thermostat is the best place to start. Make sure it's set to “heat” and that you've set the temperature higher than the current room temperature. If that doesn't resolve the problem, try switching your fan to “on” to force the furnace to start working.
The Short Answer Is: Warning signs of a bad furnace ignitor include the furnace not starting or heating, a noticeable clicking sound without ignition, or frequent short cycling where the furnace turns on and off rapidly. Additionally, if the ignitor appears cracked or discolored, it may need replacement.
Furnace Not Igniting Because of Incorrect Thermostat Setting
Make sure the thermostat is set to heating and that the temperature is set high enough to actually turn the furnace on. It may be set at too low of a temperature to activate the furnace.
If your furnace won't come on, check the ignition sensor. A dirty sensor could be the reason why your furnace won't stay on or even why your furnace won't stop blowing cold air. Only if the furnace's heat exchanger — the part that actually heats the air — senses an ignition, will it power on the furnace.
Defective Igniter. The igniter is the part of the gas heater that starts the flame, and if it's defective, the heater won't work. Sometimes this can be fixed, but other times it might be best to replace the entire gas heater.
If your boiler isn't igniting, it's likely that there is no pilot light. A common cause of pilot light going out is a blocked jet. Pilot light's jet is small and can easily get blocked with even the slightest bit of debris. The good news is that the jet can be cleaned or easily replaced by a Gas Safe Engineer.
Most likely, your furnace is making a clicking noise and not providing heat because your furnace's spark igniter is trying to ignite your pilot light or burners but is failing to do so. In other words, there's an ignition system malfunction that needs to be fixed. Otherwise, your furnace won't start.
There are several reasons why your furnace may not turn on. The most common include a broken thermostat, gas line issues, clogged air filters, blocked ductwork, a tripped circuit breaker, and issues with the furnace itself.
Most modern gas furnaces can be reset by turning off the power, waiting around 20 seconds, then switching the power back on. Other furnace models may exit lockout after 1-2 hours and attempt to power on again.
Furnace is not producing heat.
Possible causes include a broken thermostat; the thermostat being set too low; an out pilot light; a blown fuse or circuit breaker; a closed gas valve; or electronic ignition problems.
At your electrical panel, make sure all switches related to the HVAC system are on. Reset any tripped breakers and replace all blown fuses, and your furnace should kick back on. Gas supply leaks: If you smell gas, leave the area immediately and contact your gas company for emergency services.
Here are some of the most common reasons why the ignitor fails. Using The Wrong Ignitor: Ignition failure will happen if your furnace uses an ignitor that has the incorrect voltage. The Limit Switch Becomes Too Hot: Furnaces have a built-in limit switch that detects high temperatures and prevents overheating.
If you don't hear anything at all, it could be a problem with the ignition switch and fuse, the starter solenoid, or bad electrical systems in your car, to name a few. If you can hear a clicking noise, you may have a partially discharged battery or a damaged starter motor.
Igniter. You'll pay $150 to $250 to repair the igniter. Igniters are crucial parts of gas ovens, as they are responsible for igniting the gas to give heat and cook your food.