Furnaces

3 components2 common failuresCarrier, Bryant, Goodman, Amana, Daikin, Trane, American Standard, Lennox, Rheem, Ruud, York, Heil, Tempstar

90%+ condensing gas furnace: field service manual

High-efficiency condensing furnaces (about 90-98% AFUE) with a secondary heat exchanger, PVC venting and a condensate trap; single-stage, two-stage and modulating

3 components3 common failuresGoodman, Carrier, Trane, Rheem, Lennox, York, Nordyne/Intertherm

Electric furnace / air handler with heat strips: field service manual

Electric furnaces and air handlers with resistance heat strips, sequencers or relays, and limits/thermal links

7 components3 common failuresCarrier, Bryant

Carrier Performance 96 / Comfort 96 gas furnaces (59TP6, 59SP6, 59SC6): quick sheet

Carrier condensing gas furnaces: Performance 96 two-stage 59TP6 (variable 25-speed blower), Performance 96 single-stage 59SP6, and Comfort 96 single-stage 59SC6 (multi-speed blower); earlier 59SC5 / 59TP5 generation is similar. Bryant sells equivalent platforms.

7 components4 common failuresGoodman, Amana, Daikin

Goodman GM9S / GMVC96 condensing gas furnaces: quick sheet

Goodman 96% condensing furnaces: GM9S96 / GM9S80 (single-stage, LED flash codes) and GMVC96 (two-stage, variable-speed). Amana and Daikin badge similar platforms. Some newer boards use a 7-segment display instead of a single LED.

6 components2 common failuresTrane, American Standard

Trane S9V2 (96 Two-Stage Variable Speed) gas furnace: quick sheet

Trane S9V2 96% two-stage variable-speed condensing furnace (now marketed as the Trane '96 Two-Stage Variable Speed'); the S9V2-VS (97%, variable-speed inducer) is a separate model now sold as the '97'. American Standard sells equivalents.

Diagnostic guides · 14

Blower runs constantly

Check the thermostat fan setting first, then whether the board is holding the blower on for a safety (open limit) or a stuck relay. On electric furnaces a stuck sequencer can do the same.

4 steps ~30 mineasy

Burners light, then shut off after a few seconds (flame sensor)

The board must prove flame by flame rectification through the flame sensor. A dirty or oxidized sensor, poor ground or reversed polarity makes the board shut the gas off right after ignition.

5 steps ~30 mineasy

Burning smell when the heat first comes on

Dust settled on the heat exchanger or electric heat strips burns off at the first firing and fades within a few hours. A smell that lingers, or smells like plastic, rubber, hot wiring or gas, is not normal and needs diagnosis.

4 steps ~30 mineasy

Condensate trap and drain problems (90%+ furnaces)

Condensing furnaces produce acidic condensate that drains through a trap. A clogged or unprimed trap, sagging drain line, frozen line or failed condensate pump causes water backup and pressure switch trips.

5 steps ~30 mineasy

Furnace blowing cold air on a heat call

The common causes are a fan set to ON, a dirty filter tripping the high limit so the burners stop while the blower runs, a full condensate drain on a high-efficiency furnace, or an ignition lockout. Gas valve and burner work is blocked; stop at diagnosis and refer.

Furnace runs no heat (blower only or nothing)

Start with the board LED code, then work the call path: thermostat, 24 V, safeties, inducer, ignition, flame proving. Most no-heat calls end at a filter, a flame sensor, a pressure switch issue or gas being off.

5 steps ~45 minmoderate

Furnace short cycling (starts and stops quickly)

Short cycling is usually a safety opening mid-cycle (limit from low airflow, flame sensor dropping out, pressure switch opening) or a thermostat issue. Read the code and time the cycle.

Furnace won't turn on (no LED, nothing runs)

A dark board means no 120 V or no 24 V. Check the service switch, breaker, door switch, board fuse and transformer in that order.

4 steps ~30 mineasy

Igniter glows but burners never light

If the igniter glows and no flame appears, the problem is on the gas side (supply off, valve not opening, valve not receiving 24 V). You can confirm the gas shutoff is open and whether the board sends the valve signal; the valve and gas pressure are licensed-technician work.

3 steps ~30 minadvanced

Limit trips / overheating (airflow)

The limit protects the heat exchanger from overheating. The fix is almost always airflow: filter, blower, ducts. Rollout switch trips are different and must be referred.

Noisy furnace (squeal, rattle, roar, boom)

Mechanical noises (squeal, grind, rattle) usually come from the blower or inducer and can be checked with power off. A loud boom at ignition or roaring flame is a gas/combustion problem that must be referred.

Pressure switch open / inducer problems

The pressure switch proves the inducer is moving combustion gases. It stays open when the vent/intake is blocked, the hose or port is plugged, the condensate trap backs up, or the inducer is weak.

5 steps ~45 minmoderate

Whole-house humidifier leaking or running nonstop

Leaks come from a blocked drain line, loose tubing, or a scaled distribution tray. Constant water flow or too much humidity points to a setpoint that is too high, a humidistat or sensing problem, or a stuck-open valve.

Whole-house humidifier not adding moisture

Bypass and fan-powered humidifiers need water, airflow and a call from the humidistat. Check the season damper, water valve, scaled water panel and humidistat before testing the 24 V solenoid valve.