Usually on the smaller side
A dirty flame sensor. The furnace lights, can’t sense the flame, shuts the gas off and eventually locks out. Cleaning or replacing the sensor is usually a small repair and one of our most common heating calls. A thermostat problem. Batteries, settings or a failed thermostat; see thermostat repair and installation. A clogged condensate drain. On a high-efficiency furnace, a blocked drain can keep it from firing.
In the middle
A hot surface igniter. It glows to light the burners and wears out with use; a cracked one means clicking and no flame. A pressure switch. It confirms the vent is drawing before the burners light. Sometimes the switch has failed, sometimes its tubing or the vent is blocked, and the price follows the cause. A control board. It costs more than a sensor, and a board that failed because of another fault will fail again, so we find that fault first.
Usually the larger repairs
An inducer motor. The small fan that pushes exhaust out the vent. A blower motor. It moves air for heat and, when the furnace is part of the AC system, for cooling too. It takes more labor, especially in a tight attic. A gas valve. Replacing it is gas-safety work, priced with the testing that goes with it.
A cracked heat exchanger
The heat exchanger keeps the burner’s exhaust, including carbon monoxide, out of your house air. A crack is a safety problem, not something to run and watch, and on most furnaces it means replacement. Check whether the manufacturer’s warranty still covers it. The U.S. Consumer Product Safety Commission recommends a yearly professional inspection of gas heating equipment and a CO alarm on every level of the home. If one sounds, get everyone outside and call 911.
Heat pumps and AUX heat
Backup heat strips that have burned out, or the relay that switches them on, leave the house cold on a freezing night. Defrost problems let the outdoor coil ice over. A sticking reversing valve can keep the system from switching into heating. A compressor is the costliest heat pump part, and on an older system it’s often where repair stops making sense. If AUX is on all the time, see heat pumps.
What else moves the price
Access. Many homes near our shop have the furnace in the attic beside the AC coil, and a cramped attic takes longer than a closet. Warranty. A manufacturer’s warranty may cover the part but usually not the labor. Timing. The National Weather Service puts Houston’s average first freeze around December 9. Heaters that sat all year tend to fail in the first cold week, so a fall check finds worn parts before the rush.
Sources: U.S. CPSC, home heating equipment and carbon monoxide · NWS Houston/Galveston, freeze statistics. Updated November 2026.
