Furnace
Furnace Not Igniting: Flame Sensor and Igniter
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Prefer to read it? Quick checklist
- What safe pattern do you observe?
- Other fuel appliances are also affected: A pilot or fuel issue needs professional attention: A furnace with a pilot has no flame or other fuel appliances are also affected. (possible)
- Furnace starts then stops: A flame-sensor fault is possible: The burner lights briefly and shuts down or the furnace enters a lockout. (possible)
- No sustained ignition: Is there a visible code or repeated safety shutdown?
- Yes, a code or shutdown returns: What does the model label indicate?
- Limit or pressure switch: A limit or pressure switch has stopped the sequence: A model-specific fault code returns after the normal checks. (possible)
- Control-board fault: A control-board fault needs diagnosis: Ignition stops and a board-related code remains. (possible)
- No visible code: The igniter may have failed: The unit calls for heat but the normal ignition sequence does not produce sustained heat. (possible)
- Yes, a code or shutdown returns: What does the model label indicate?
Last updated
A failed ignition is not a safe reason to open a furnace. The sequence may involve a pilot or igniter, followed by a flame-sensor check and safety control. Use this guide with the furnace troubleshooting hub to describe the pattern, then let a qualified technician diagnose the internal parts.
Describe what happened
Carrier describes a sequence in which the draft motor runs, the igniter glows, a burner lights and the flame sensor confirms heat. If the furnace starts and then shuts down, a flame-sensor fault is one possibility. PickHVAC says a dirty sensor can fail to register heat, after which the control stops the gas flow. That secondary-source description does not prove the sensor is the cause in a particular furnace.
Bryant says igniters and flame sensors are common early failures, but their diagnosis needs specialized tools. The useful clue is the order you can hear from outside the cabinet: whether the furnace starts, whether it makes a short attempt, whether the blower runs and whether a code appears. Do not remove a panel to watch the burner or look for soot.
Checks you can make without opening anything
Trane’s furnace-not-igniting page starts with four checks that happen outside the cabinet, and each one takes only a few minutes.
- Thermostat. Confirm it is set to Heat and that the target is above the current room temperature. Trane notes that someone else in the household may have changed the setting.
- Power. Check the switch near the furnace, which can look like an ordinary light switch, and look for a tripped breaker in the panel. Trane says not to keep resetting a breaker that has tripped, because it probably tripped because of a furnace problem. Call a professional instead.
- Filter. Trane tells homeowners to replace a dirty filter and says filters need changing every one to three months depending on quality. A badly clogged filter can overheat the furnace and stop the ignition sequence through a safety switch.
- Gas shutoff. Trane describes a shutoff valve within a few feet of the furnace, on a black or copper pipe with a knob or lever, and says to confirm it is in the ON position. If you are unsure which valve it is or which position is open, stop and call a professional rather than guessing.
Trane also separates furnaces by age. Models made since about 2010 usually light with an electronic ignitor, and Trane treats ignitor and flame-sensor problems on those as work for a technician. Older furnaces with a standing pilot may have a pilot that has gone out. Trane links a separate relighting guide for that case; this page does not repeat those steps, because the correct procedure depends on the exact model and your owner’s manual controls it.
1. A pilot or fuel issue needs professional attention
A pilot or fuel issue needs professional attention
A furnace with a pilot has no flame or other fuel appliances are also affected.
- Difficulty
- Pro
- Time
- 60 minutes
- Tools
- None listed
Carrier says some furnaces have a pilot light and that a pilot which will not stay lit may point to an orifice or thermocouple problem. It also lists an interrupted gas, oil or propane supply. These are not owner repair steps.
If other gas appliances also do not work, Carrier says to contact the gas company. If you smell gas, leave and call from outside. Do not open a valve, light a pilot or purge a line based on a general guide.
2. A flame-sensor fault is possible
A flame-sensor fault is possible
The burner lights briefly and shuts down or the furnace enters a lockout.
- Difficulty
- Pro
- Time
- 60 minutes
- Tools
- None listed
- Part cost
- $68–$127Source: Furnace Repair Cost (Fixr) · Checked October 12, 2026
Carrier says soot on a sensor or cracked porcelain can be clues, but it also says cleaning or replacement needs a professional. Bryant says diagnosis requires specialized tools. Keep the panel closed and tell the technician whether the burner appears to start and then stop.
PickHVAC describes a soft lockout pattern: the draft motor starts, the igniter glows, the burner lights briefly and the gas valve closes. Treat that as a secondary explanation, not a diagnosis. Some furnaces may require an hour or more before another reset after a hard lockout; use the exact manual and do not keep resetting the system.
$68–$127Source: Furnace Repair Cost (Fixr) · Checked October 12, 20263. The igniter may have failed
The igniter may have failed
The unit calls for heat but the normal ignition sequence does not produce sustained heat.
- Difficulty
- Pro
- Time
- 60 minutes
- Tools
- None listed
- Part cost
- $96–$178Source: Furnace Repair Cost (Fixr) · Checked October 12, 2026
Bryant lists the igniter among common early failures. Carrier says the igniter is part of the startup sequence. If the furnace calls for heat but does not reach sustained heat, record that pattern. Do not inspect the igniter through an open cabinet.
Fixr’s secondary estimate appears below. It is a planning range and cannot show whether the failed component is the igniter, a sensor, a limit or another control.
$96–$178Source: Furnace Repair Cost (Fixr) · Checked October 12, 20264. A limit or pressure switch has stopped the sequence
A limit or pressure switch has stopped the sequence
A model-specific fault code returns after the normal checks.
- Difficulty
- Pro
- Time
- 60 minutes
- Tools
- None listed
- Part cost
- $66–$122Source: Furnace Repair Cost (Fixr) · Checked October 12, 2026
A furnace safety switch can interrupt ignition. Carrier lists safety controls among reasons to call, and Bryant warns against testing internal components. A recurring code or shutdown should be read from the exact model label and checked by a professional.
Do not bypass a switch or open the panel to find it. If the filter is dirty, replace it at its normal access point, then call if the furnace still stops. A filter change does not confirm the condition of a safety switch.
$66–$122Source: Furnace Repair Cost (Fixr) · Checked October 12, 20265. A control-board fault needs diagnosis
A control-board fault needs diagnosis
Ignition stops and a board-related code remains.
- Difficulty
- Pro
- Time
- 90 minutes
- Tools
- None listed
- Part cost
- $390–$725Source: Furnace Repair Cost (Fixr) · Checked October 12, 2026
Carrier explicitly says not to repair or replace the control board yourself. A code can point toward a control problem, but a code is not permission to handle wiring or board components. Record the visible message or flashes through the normal sight glass.
Ask the technician to identify the part and explain what test supports the diagnosis. Fixr’s range is shown as a secondary estimate, not a quote for your model.
$390–$725Source: Furnace Repair Cost (Fixr) · Checked October 12, 2026If the burner lights and then goes out
Trane’s page on a main burner that shuts off after ignition lists four causes: a dirty or clogged filter that overheats the heat exchanger and trips the limit switch, burners that are cracked or rusted on older furnaces or clogged with dirt on newer ones, a faulty or dirty flame sensor that shuts the furnace off early, and a control board problem. The board manages the gas valve, igniter, flame sensor and, on newer units, blower control.
That list explains why the same symptom can lead to different repair bills. The only homeowner step Trane gives is checking and changing the filter. Everything else it lists, including burner cleaning or replacement and flame-sensor service, is a repair-visit item. Trane describes the flame sensor as the part that prevents gas from building up when there is no flame to burn it, which is the reason a technician should handle it with the cabinet closed during your own checks.
Who handles each cause
| Cause listed by Trane | Homeowner | Technician |
|---|---|---|
| Thermostat setting | Set to Heat and raise the target | Replace a faulty thermostat |
| Tripped breaker or no power | Check the switch and panel once | Find why it tripped |
| Dirty filter | Replace it | None, unless a limit switch tripped |
| Gas shutoff closed | Check the valve position | Confirm if unsure |
| Electronic ignitor | None | Test and replace |
| Flame sensor | None | Clean or replace |
| Standing pilot out | Follow the owner’s manual for your model | Repeated outages |
| Dirty burners | None | Clean; DIY work can affect a warranty |
Trane notes that opening the furnace to clean burners yourself could void a warranty, so even a handy owner should compare that risk with the cost of a service call.
What to record for the service call
Write down the thermostat setting, the time of the failed heat call, whether the blower moved air, whether the furnace tried to ignite and any visible code. If the furnace has a lockout, do not keep cycling the thermostat. PickHVAC describes some hard lockouts as requiring a wait; the exact timing depends on the model, so check the manual or contact service.
The furnace no-start hub covers thermostat, switch and filter checks. The blinking-light guide explains why the same code number can mean something different across brands. For repair-versus-replacement context, see the furnace cost guide.
Leave the ignition system closed
Do not sand a sensor, remove a burner, replace an igniter or adjust a gas valve. Carrier recommends professional work on the flame sensor; Bryant says internal diagnosis needs specialized tools. The sources differ from PickHVAC, which describes a cleaning procedure, but that secondary procedure does not override the manufacturer warnings.
Call promptly if the furnace shuts down repeatedly, a code returns, there is soot or the heat exchanger is suspected. If you smell gas, leave the building and contact the gas utility or emergency service from outside. Do not attempt to make the furnace run again before it has been checked.
If the furnace also makes a bang, rumble or click while trying to start, the furnace noise guide explains which sounds Trane treats as normal and which need attention.
Frequently asked questions
Why does my furnace light and then shut off?
Carrier and PickHVAC describe a possible flame-sensor problem and lockout. That pattern can have other causes and needs professional diagnosis.
Is a furnace igniter safe to replace myself?
Bryant says igniter and flame-sensor diagnosis requires specialized tools. Do not remove panels or replace ignition parts yourself.
How much does a furnace igniter cost to repair?
Fixr's listed estimate appears below as a secondary planning range. It does not identify the cause or include every local service charge.
Sources
- Gas Furnace TroubleshootingBryant
- Furnace Blowing Cold AirCarrier
- How to Clean a Furnace Flame SensorPickHVAC (secondary)
- Furnace Repair CostFixr Editorial Team (secondary)
- Furnace Not IgnitingTrane
- Main Furnace Burner Shuts Off After IgnitionTrane