Furnace Troubleshooting

Use this guide to troubleshoot problems, look up error codes, and estimate furnace repair costs.

Start here: No heat diagnostic

Refer to the furnace glossary if you need help identifying furnace parts or terms.

If you have no heat and your furnace simply isn’t running, verify the following about the power supply, filter, and thermostat first:

Furnace filter is clean.

If it's dirty, replace it with a new filter or remove it (for now).

If all of the above check out, continue to find error codes for Mid- and high-efficiency furnaces or troubleshoot a Low-efficiency furnace.

Mid- and high-efficiency furnaces

Get help with furnace types.

Select your furnace brand to look up error codes and continue troubleshooting:

Low-efficiency furnaces

Get help with furnace types.

Most low-efficiency furnaces don’t display error codes. Do any of the following describe the issue with your furnace, either now or before it stopped working?:

Pilot light is off. It won't stay lit or I can't relight it.

Faulty thermocouple or gas valve.

Burners light, but only briefly. Low heat output. Burnt plastic smell. Grinding or buzzing noise. (Any combination)

Faulty blower motor or capacitor.

Pilot light is on and thermostat calls for heat, but burners do not light. Cold air comes through the vents.

Faulty thermostat or fan limit control.

None of these describe the issue.

Professional diagnosis required.

Secondary diagnostic

Only do this if directed, after finding the error code for your furnace make and model.

To further diagnose the issue with your furnace, follow these steps:

  1. Ensure thermostat is on and calling for heat.
  2. Ensure upper furnace cover is off.
  3. Turn power switch to the furnace off for 20 seconds, then turn it back on.
  4. Observe for 5 minutes and select from the descriptions below. 

Blower motor runs continuously. Inducer motor does not try to start. Error code may indicate limit/roll out switch is open.

Blocked furnace filter, faulty limit switch, or plugged heat exchanger. Book a professional repair or continue to the Limit switch diagnostic →

Inducer motor diagnostic

Only do this if directed from another troubleshooting step.

To further diagnose the inducer motor, follow these steps:

  1. Ensure thermostat is on and calling for heat.
  2. Ensure upper furnace cover is off.
  3. Turn power switch to the furnace off for 10 seconds, then turn it back on.
  4. Observe for 5 minutes and select from the descriptions below. (It’s normal for the blower motor to run for 1-2 minutes before the ignition sequence begins.)

Inducer motor runs for a few minutes, shuts off, and restarts. It sounds like there is water inside the motor.

Condensate trap blockage, intake or exhaust blockage, improper furnace venting, or back-sloped furnace level. Book a professional repair or continue to the Condensate trap diagnostic →

Inducer motor runs for a few minutes, shuts off, and restarts. Igniter does not glow orange. Error code might be "pressure switch failed to close".

Condensate trap blockage, intake or exhaust blockage, improper furnace venting, or back-sloped furnace level. Book a professional repair or continue to the Condensate trap diagnostic →

Inducer motor runs for a few minutes, shuts off, and restarts. Igniter does not glow orange. There is a slight natural gas smell.

Failed igniter or ignition control board. Book a professional repair or continue to the Igniter diagnostic →

Inducer motor makes a grinding, buzzing, or humming noise. It does not sound like normal operation.

Failed inducer motor. Book a professional repair.

Inducer motor runs normally. Igniter glows for 10-30 seconds, but burners do not light. After a few tries, the furnace locks out.

Failed gas valve. Book a professional repair or continue to the Gas valve diagnostic →

Thermostat diagnostic

If there is a problem with your thermostat or its settings, it may not be calling for heat from the furnace as expected.

To verify the thermostat is calling for heat, see these examples of common thermostats calling for heat, and follow these steps:

  1. Check or replace the thermostat batteries.
  2. Set thermostat mode to ‘heat’ or ‘auto’.
  3. Set fan mode to ‘auto’ (if available).
  4. Set the desired temperature to at least 2 degrees above the current indoor temperature, as shown on the thermostat.
  5. Wait up to 5 minutes for the thermostat to ‘call for heat’ and the furnace to turn on. Some thermostats have a built-in delay to prevent short cycling the furnace.

Thermostat calls for heat and furnace runs normally.

Nice work!

Thermostat does not call for heat or is malfunctioning.

Faulty thermostat. Book a professional repair.

Power diagnostic

Power to the furnace can be interrupted or accidentally switched off in multiple ways. Check the following to make sure:

Electrical panel — Find the circuit breaker for the furnace and ensure it’s switched on.

Power switch — Most furnaces also have a power switch on them or nearby. If you have one, ensure it’s switched on.

Lower furnace cover — For safety reasons, some furnaces automatically disable the power when the lower cover is open. Ensure it is closed and secure.

Some low-efficiency furnaces have a manual switch to run the blower motor (fan). If this doesn’t work, the furnace probably isn’t getting power.

Once you’ve verified that power to the furnace is on, don’t turn it off again while troubleshooting — unless instructed otherwise. Turning off the power clears the error codes used to diagnose furnace problems.

Furnace powers up and is working normally.

Nice work!

Furnace still does not power up.

Book a professional repair or continue to the Power transformer and control board fuse diagnostic →

Filter blockage diagnostic

Only do this if directed from another troubleshooting step.

A dirty furnace filter could be the problem. To diagnose further, follow these steps:

  1. Ensure thermostat is on and calling for heat.
  2. Remove the furnace filter.
  3. With filter out, wait 5 minutes, then reset the furnace by turning the power switch to the furnace off for 20 seconds, then turn it back on.
  4. Observe for 5 minutes and select from the descriptions below. (It’s normal for the blower motor to run for 1-2 minutes before the ignition sequence begins.)

Furnace powers up and is working normally.

Nice work! Don't forget to replace your furnace filter with a new one.

Inducer motor turns on, igniter glows orange, and the burners light for 1-5 seconds — then abruptly cut out.

Faulty flame sensor/rod, dirty burner assembly, or failed control board. Book a professional repair or continue to the Flame sensor diagnostic →

Furnace runs for 1-20 minutes but shuts off before house reaches set temperature. Error code may indicate an open limit switch.

Blocked furnace filter, heat runs or returns, plugged heat exchanger, or faulty blower motor. Book a professional repair or continue to the Limit switch diagnostic →

Burners light, but only briefly. Low heat output. Burnt plastic smell. Grinding or buzzing noise. (Any combination)

Failed blower motor. Book a professional repair or continue to the Blower motor diagnostic →

None of these describe the issue.

Professional diagnosis required.

On this page:

Furnace repairs should only be performed by a certified technician.

This website is for informational purposes only. Advanpro Ltd. is not liable for property damage, personal injury, or death caused directly or indirectly by actions taken as a result of the information provided here.

Repair Flame Sensor/Rod, Burner Assembly, or Control Board

$239 to $999 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Blower Motor

$999 to $1499 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Thermostat

$239 to $550 + GST

Pricing depends on thermostat supplied. Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Inducer Motor

$999 to $1499 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Igniter or Ignition Control Board

$419 to $1499 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Blockage, Back Sloping, or Venting

$239 to $1799 + GST

Back sloping and venting repair pricing depends on difficulty and complexity. Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Filter, Limit Switch, or Heat Exchanger

$109, $429, or $1799 + GST

Pricing assumes heat exchanger is still in warranty. Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Thermostat or Fan Limit Control

$239 to $649 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Blower Motor or Capacitor

$999 or $239 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Gas Valve

$999 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Professional Diagnosis Required

$109.95 regular hours call-out & diagnosis fee

Our technician will identify the problem and tell you how much it will cost to repair. Once you know the problem and the cost, you can decide if you want to proceed.

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Thermocouple or Gas Valve

$239 or $999 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Repair Pressure Switch

$419 + GST

Mention this diagnosis from the troubleshooting guide when you book:

(403) 873-7779

Call us 24 hours, 7 days a week

[email protected]

Email us, get a reply in 1 business day

Furnace types

High-efficiency furnaces have white or black plastic vent pipes, which are 2″ – 3″ in diameter.

Mid-efficiency furnaces are usually less than 25 years old and have no pilot light.

Low-efficiency or “Standard” furnaces are more than 25 years old and usually have a pilot light.