How to Troubleshoot a Furnace

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Project Overview
  • Working Time: 15 mins
  • Total Time: 15 mins

Modern high-efficiency furnaces are complex pieces of machinery, and most major repairs are best left to a professional furnace technician. But before calling the furnace company and incurring a substantial service call charge, it makes sense to check out three very common problems that are often easy to remedy yourself: power supply interruptions, thermostat malfunctions, and problems with the condensate drain system.

What You'll Need

Equipment / Tools

  • Flashlight


  • Batteries (optional)
  • Condensate pump (optional)


Check for Power Interruption

Just like any other appliance in your home, a furnace that suddenly stops running may be experiencing an electrical power interruption. Begin by making sure the toggle switch is in the "ON" position. If it is, follow these steps.

  1. Check the Circuit Breaker

    Locate your main electric service panel and open the door. Find the circuit breaker that controls the furnace, and check to see if it has tripped to the OFF position.

  2. Reset the Circuit Breaker

    To reset this circuit breaker, first turn the thermostat to the lowest setting to prevent the furnace from starting up at the instant you reset the breaker. Then flip the furnace's circuit breaker back to the ON position. Set the thermostat to a high-temperature setting, and observe how the furnace functions. In many cases, simply resetting the breaker will fix the problem.


    Occasionally a circuit breaker will trip for no apparent reason. But if it continues to trip, consider why. Consistent tripping can indicate a breaker on the verge of failure, a laboring furnace motor (such as you might find with a badly clogged furnace filter), a bad capacitator, or an electrical short in the furnace wiring. Replacing the filter is an easy solution you can try, but if the circuit breaker trips repeatedly, it is time to call a professional to examine the furnace or circuit breaker for more serious problems.

Check the Thermostat Function

When a furnace stops working, often the problem is not the furnace, but the thermostat. Follow these steps to troubleshoot and fix your thermostat.

  1. Inspect the Clock Time

    If you have a programmable thermostat, start by simply checking the automatic clock. If the clock is off or set incorrectly, then the schedule will not operate as it should. Resetting the clock may return the furnace to normal operation.

  2. Replace the Batteries

    Newer programmable thermostats usually have onboard batteries designed to keep the inner clock and program intact in the event of a power outage. If the batteries go dead, the thermostat may stop working. In many cases, simply replacing the batteries will return the thermostat—and your furnace—to perfect operating condition.


    Many older thermostats operate with low-power current from the wire that attaches to the thermostat, so they will have no batteries.

  3. Examine the Wire Connections

    Your thermostat may malfunction if any of the low-voltage wire connections have come loose. Carefully detach the thermostat body from the wall and check each connection. Depending on the complexity of your system, there can be as many as six low-voltage wire connections linking your thermostat to the furnace, or as few as two wires. Make sure each low-voltage wire coming out of the wall is securely attached to its corresponding terminal on the thermostat.

  4. Examine the Wire Connections at the Furnace

    It's also possible that the thermostat wire connections might have loosened at the other end—at the furnace itself. Locate the control panel on the furnace and check the low voltage thermostat wire connections to make sure they are all secure.


    The thermostat might malfunction because of a problem with the transformer that steps down 120-volt power to the 24-volt current required for the thermostat. Replacing the transformer is usually a matter for a furnace technician, as it involves handling 120-volt line voltage wires, testing the transformer with a multimeter, and disconnecting electrical connections. Don't attempt this unless you are very experienced with electrical wiring.

Check the Condensate Pump Function

Modern high-efficiency furnaces have a two-stage heat exchange design that creates condensed water. This water is removed from the furnace by a drain line. This drain line may include a small pump if the furnace installer ran a tube for the condensate water to a utility sink or other plumbing fitting. (If there is a handy floor drain near the furnace, the installer may have installed a simple gravity drain tube from the furnace to the floor drain.)

But if the condensate pump becomes clogged or malfunctions, the condensate water may simply spill onto the floor around the base of the furnace. This can be an alarming symptom, but it's actually not all that serious. It's usually easy enough to get the pump working again—or even replace it entirely.

Fun Fact

If your furnace is equipped with an electric condensate pump, you may have heard a faint buzzing—a sound similar to that produced by the pump for an aquarium or decorative water fountain. This sound is usually heard after a long heating cycle—the sound runs for a few minutes, then stops.

  1. Make Sure the GFCI Outlet is Working

    The condensate pump and pan are about the size of a shoebox, usually located on the side of the furnace. Sometimes, it will be powered by an auxiliary outlet next to the furnace. If this is a GFCI outlet (it should be), check to make sure the GFCI hasn't tripped. If it has, merely resetting the outlet may return the pump to operation.

  2. Check the Condensate Tube

    Sometimes the condensate pump is hard-wired and powered directly by the furnace. If so, turn off the circuit breaker (or breakers) controlling the furnace before examining the pump.

    Check the tube that runs from the condensate pump to the outside and make sure that it is not pinched or blocked.

  3. Fix the Automatic Switch

    Open the lid to the condensate pan (usually by removing the condensate pump in the process). Check to see if the draw tube is still at the bottom. The condensate pump pan might be overly full and leaking on the floor. If so, it means the automatic switch on the pump isn't working correctly. Manually flip the switch back and forth a few times to loosen it.

  4. Replace the Condensate Pump

    If the condensate pump still does not work, it will need to be replaced. Condensate pumps are relatively inexpensive and easy to replace. If the pump is hard-wired into the furnace, you may need to hire an electrician.