Bryant Furnace 24 Error Code: Meaning, Causes & Fix
Secondary voltage fuse is open — the fuse protecting the control's 24-volt (secondary) circuit has opened because there is a short circuit in the secondary-voltage wiring. On the Evolution 987M the fuse is an automotive-type 3-amp fuse located on the control, and Bryant states the control LED will display status code 24 when the fuse needs to be replaced.
Code 24 is the Bryant control protecting itself, and it starts with what 'secondary voltage' means: the transformer steps line voltage down to the 24 VAC that runs everything the furnace thinks with — thermostat circuit, limit circuit, pressure switches, gas valve, and accessories such as the humidifier terminal. When a wire in that 24-volt web shorts to ground or common, the fuse on the control opens before the short can take the transformer or board with it. The furnace then presents confusingly: the thermostat may be blank (its power came through the same circuit), yet the control still flashes 24 at the blower-door indicator — alive on its line-voltage side, reporting its low-voltage side down. Bryant's 355MAV troubleshooting for 24 is built on one lesson: do not hunt the short — isolate it. The technician disconnects the User Interface (ABCD) connector or all thermostat leads (including anything on the HUM terminal), fits a new fuse, restores power, then re-energises the circuits one call at a time — heat, high-heat, cooling — watching which stage makes 24 return. Each branch has a named suspect: the limit circuit's red wires to chassis ground convict the limit circuit; the pressure-switch wires convict their circuits in turn; the gas valve's blue wire to ground convicts its wiring or the valve; a 24 arriving only in cooling convicts the outdoor-unit wiring or contactor; a 24 arriving at blower start sends the flow to the humidifier's draw — and finally to Bryant's most experience-soaked note in the guide: loose or torn insulation shorting intermittently against the limit switches. The fuse is the messenger and the sacrifice; the short is the fault.
What causes the 24 code on a Bryant furnace?
Short in the thermostat wiring
The cause Bryant's labels name outright: a short in the secondary-voltage wiring 'including thermostat leads' (the 393AAV label adds 'shorting to ductwork or furnace cabinet'). Thermostat cable is thin, stapled, and run through walls and duct chases — chafed insulation, a screw or staple through the bundle, or a bare conductor on sheet metal is the classic 24.
Short in the limit switch circuit
Named in Bryant's isolation flow: continuity between either red wire of the limit circuit and chassis ground puts the short in the limit circuit — limit and rollout switch wiring. The flow's closing note adds the intermittent variant: loose insulation shifted against the limit switch(es), shorting only when blower vibration moves it.
Short in a pressure-switch circuit
The same flow tests both pressure switch circuits wire by wire (the low-heat switch's yellow and orange wires; the high-heat switch's gray wire, and the brown wire shared toward the gas valve). Any of them grounded produces a 24 when that stage is energised — which is why a furnace can heat on low fire and blow the fuse only on high-heat.
Gas valve circuit short
If 24 appears when the gas valve is energised, Bryant's flow disconnects the valve's blue wire: if the pattern changes (a 34 appears instead), the valve is condemned; if the blue wire shows continuity to chassis ground, the wire is shorting and is repaired or replaced. A coil shorted internally opens fuses exactly like a chafed wire.
Outdoor unit wiring or contactor (cooling-mode 24s)
A 24 during the cooling cycle sends Bryant's flow to the thermostat-to-outdoor-unit wiring, the outdoor contactor, or a heat pump's reversing valve. The flow adds the experienced caveat: a contactor failing to pull in cleanly can draw excessive current on the low-voltage circuit — intermittently, which is why some 24s happen only on the hottest afternoons.
Humidifier circuit excessive draw
The HUM terminal is energised when the blower turns on, so Bryant's flow checks it near the end: if 24 follows blower start, disconnect the humidifier and check the solenoid wiring, the diode bridge (if used), and the solenoid for excessive draw. An accessory added years after the furnace is a prime suspect in a new 24.
Checks to run before you replace anything
- SAFETY: low voltage is not no voltage — the same cabinet holds 115V (and the gas train). Power OFF at the furnace switch before opening panels or touching the fuse, every time. Gas and board work are licensed-technician territory; the checks below are observation and isolation logic, not an invitation to rewire circuits.
- Prove the fuse before believing the code — Bryant's flow starts there: power off, blower access panel removed, check the fuse's continuity. A genuinely open fuse confirms the report. If the fuse is NOT blown while 24 flashes, Bryant's next step is not another fuse: it is replacing the control.
- Note exactly WHEN the fuse opened — the most diagnostic fact on this code. At heat-call start? When high-heat engaged? Only in cooling? Only when the blower (and HUM terminal) came on? Bryant's whole isolation flow is organised by that timing, because each stage energises a different branch of the 24-volt web.
- Do the disconnect test the labels themselves direct: 'Disconnect thermostat leads to isolate short circuit.' With the leads off the control and a fresh fuse fitted (technician procedure), a 24 that no longer occurs puts the short in the thermostat runs or thermostat; one that still occurs puts it inside the furnace's own circuits.
- Never fit a larger fuse, and never keep feeding fuses into an unrepaired short. Bryant (987M instructions) specifies the replacement exactly: use ONLY a 3-amp fuse. A fuse that opens again instantly has given you information — the short is present and localised to whatever was energised — not a cue to try a third.
How to fix bryant furnace error code 24?
- Repair or replace the shorted wire run — thermostat cable, limit-circuit wiring, pressure-switch wiring, gas valve lead or accessory (humidifier) circuit, as the isolation proves; a humidifier solenoid or diode bridge drawing excessively is replaced. A wire that has chafed through once is replaced or properly re-insulated and re-routed, not taped against the same edge.
- Limit-circuit branch: repair the short, and deal with Bryant's named intermittent cause — loose or torn insulation shifted against the limit switch(es) is secured or replaced so blower vibration cannot walk it back into contact.
- Gas valve branch: if Bryant's blue-wire test condemns the valve (the pattern flips to a 34 with the wire off), the gas valve is replaced — gas-train work for a licensed technician. If the wire itself is grounded, the wire is repaired or replaced.
- Cooling branch: repair the thermostat-to-outdoor-unit wiring, or replace the outdoor contactor (including the failing-to-pull-in, excessive-draw case Bryant flags), or the reversing valve circuit on heat pumps.
- Only after the short is repaired: fit the new fuse — automotive-type, 3-amp, exactly as Bryant specifies — restore power, and verify by operating the furnace in heating AND cooling from the thermostat, through the stage that originally opened the fuse. Bryant's flow does not call the job done until both modes run clean.
The part that usually fails: The 3-amp fuse — but only as the last step, after the short that opened it is repaired
The fuse is the cheapest part in the story and the only one Bryant lets you name a size for: automotive-type, 3-amp, located on the control (987M instructions: 'If fuse replacement is required, use ONLY a 3-amp fuse'). The part that actually failed is whichever wire, switch circuit, gas valve, contactor or solenoid the isolation convicts — buying fuses in bulk is a subscription to the same fault.
How to reset bryant furnace error code 24?
Code 24 has no timer and no auto-reset: it clears when — and only when — the short is repaired and the fuse replaced. Bryant's order is strict: isolate and repair first, fuse last, because a fuse fitted into a live short simply opens again (and that re-opening, staged circuit by circuit, is how the isolation flow localises the fault). The one non-fuse outcome Bryant documents: 24 flashing with the fuse proven NOT blown means the control itself has failed and is replaced. There is nothing to wait out on a 24.
Bryant's lockouts run on documented timers, and they are not interchangeable. Limit lockout (13) and ignition lockout (14) auto-reset after 3 hours. Blower motor lockout (15, variable-speed boards) also auto-resets after 3 hours. Control circuitry lockout (45) auto-resets after 1 hour, and Bryant adds: reset power to clear the lockout — if 45 repeats, replace the control. Gas heating lockout (21) is the exception with NO auto-reset: Bryant's instruction is to turn off power and wait 5 minutes to retry. The pressure-switch behaviour under 31/32 (inducer shuts off for 15 minutes after the switch has been open 5 minutes, then retries) is a retry pause, not a lockout. Everything else on the Bryant chart is a condition code that clears when its condition clears: 33 ends when the limit or rollout switch remakes as the furnace cools (the rollout switch itself is manual-reset hardware — no timer resets it, and Bryant requires its cause to be corrected before it is reset), and 24 ends only when the 24-volt short is repaired and the fuse replaced. Interrupting power restarts a Bryant control, but on the older boards it also erases the stored fault history — read the history first, and never treat a cleared timer as a repaired furnace: an unfixed cause simply walks the control back into the same lockout.
Why does the 24 code keep coming back?
Intermittent 24s are the ones that come back: Bryant's flow ends by warning that loose or torn insulation can short intermittently against the limit switches when vibration or temperature moves it — and that a failing outdoor contactor can draw excessively only sometimes. A 24 returning weeks later, in a different mode, usually means the short neighboured the repaired one (same cable run, same chafe point). Describe the mode and stage pattern precisely when you call; on this code, the pattern is the diagnosis.
How to stop 24 coming back
- Short in the thermostat wiring: The cause Bryant's labels name outright: a short in the secondary-voltage wiring 'including thermostat leads' (the 393AAV label adds 'shorting to ductwork or furnace cabinet'). Thermostat cable is thin, stapled, and run through walls and duct chases — chafed insulation, a screw or staple through the bundle, or a bare conductor on sheet metal is the classic 24.
- Short in the limit switch circuit: Named in Bryant's isolation flow: continuity between either red wire of the limit circuit and chassis ground puts the short in the limit circuit — limit and rollout switch wiring. The flow's closing note adds the intermittent variant: loose insulation shifted against the limit switch(es), shorting only when blower vibration moves it.
- Short in a pressure-switch circuit: The same flow tests both pressure switch circuits wire by wire (the low-heat switch's yellow and orange wires; the high-heat switch's gray wire, and the brown wire shared toward the gas valve). Any of them grounded produces a 24 when that stage is energised — which is why a furnace can heat on low fire and blow the fuse only on high-heat.
Retrieving stored fault codes on Bryant furnaces
Every Bryant status code in these manuals is a two-digit number read off the status LED at the indicator in the blower door: the first digit is the number of SHORT flashes, the second digit the number of LONG flashes (33 = three short, three long). Three steady states frame the codes: LED continuously ON means the control has 24 VAC power; continuously OFF means check for 115 VAC at L1 and L2 and 24 VAC at SEC-1 and SEC-2; RAPID flashing means line-voltage polarity is reversed. Stored history is played back, not guessed — and the method is generation-specific. On the oldest labels (330JAV/393AAV), briefly remove and reconnect one main limit wire without removing power or the blower door; those boards erase stored codes when power is interrupted or 48 hours after each fault is cleared. The 340AAV red-LED boards and the variable-speed 355MAV erase stored codes automatically after 72 hours; the 355MAV recalls history by disconnecting the R lead (or the ABCD User Interface connector), resetting power and putting setup switch SW1-1 ON, and clears it by jumpering R, W/W1 and Y/Y2 simultaneously until code 11 flashes. The Evolution 987M changed the rule: its stored codes are NOT erased when 115- or 24-volt power is interrupted — the control stores up to the last 7 status codes in order of occurrence; with no thermostat signal present and all blower-off delays complete, set SW1-1 ON and the control flashes up to 7 codes, the 8th code always being 11, and a continuously lit amber LED afterwards confirms proper operation. On Evolution systems the Connex wall control adds its own reporting layer on top of the board: a Last 10 Events record and service screens — including Furnace Status, and, on compatible models, an Advanced Diagnostics view naming the most likely root causes of the most recent fault. The component test (setup switch SW1-6) runs the inducer, igniter and blower in sequence and reports back in codes (11, 25, 41 and 42 on the 355MAV; 11, 25, 35, 41 and 42 on the 987M). Whatever the generation, the service label printed inside your own Bryant's blower door is the chart of record for that furnace.
Other Bryant furnace error codes
| Code | Meaning | First fix |
|---|---|---|
| 11 | No previous code — not a fault: 11 is the end marker of the stored-history playback, showing no fault is stored (or none remains) | No repair needed. Bryant's label directs: run the system through a heating or cooling cycle to check it |
| 12 | Blower on after power up — normal operation, not a fault: the blower runs 90 seconds when the furnace is powered up during a call for heat (R–W closed), or the call ends during the blower on-delay | No repair needed. If 12 shows with no power event and no heat call, have the thermostat call and control checked |
| 13 | Limit circuit lockout — lockout occurs if the limit or flame rollout switch is open longer than 3 minutes, or (variable-speed Bryant wording) 10 successive limit trips occurred during high-heat. The control will auto-reset after 3 hours. Bryant's label cross-refers every 13 to code 33 for the diagnosis. | Fix code 33's cause — that is the whole of 13's repair, by Bryant's own cross-reference. Airflow first: filter, registers, returns, duct restriction corrected at the source. |
| 14 | Ignition lockout — the system failed to ignite gas and prove flame in 4 attempts (the trial plus code 34's three retries). The control auto-resets after 3 hours | Do not keep resetting it. Diagnose as code 34 (see the ignition codes guide); gas and flame-sense work is licensed-technician work |
| 15 | Blower motor lockout (variable-speed boards) — the blower failed to reach 250 RPM, or to communicate within 30 seconds of switch-on, in two successive heating cycles. Auto-resets after 3 hours | Technician work — code 41 twice in a row. The repair is the proven blower, wiring or control, not another reset |
| 21 | Gas heating lockout — the control has locked out of gas heating and will NOT auto-reset. Bryant: turn off power and wait 5 minutes to retry | Power off, wait 5 minutes, retry once only. A returning 21 is a control or gas-valve wiring fault — licensed-technician work |
| 22 | Abnormal flame-proving signal — flame is proved while the gas valve is de-energised. The inducer runs until the fault is cleared | Do not run the furnace. Technician test: shut the external manual gas valve — if 22 stops, the gas valve is replaced; if not, sensor or control |
| 23 | Pressure switch did not open — a switch contact is still made (closed) when a call for heat begins, before the inducer runs; the control flashes 23 until the switch opens, then the cycle can begin | Never bypass the switch. A technician clears or replaces the tubing and replaces a switch that stays made |
| 25 | Model selection or setup error (variable-speed/Evolution boards) — the model plug (PL4) is missing or incorrect, or a setup switch is misplaced. If 25 flashes only 4 times on power-up, the control is defaulting to the model stored in memory and runs as if the plug were fitted | Technician work: fit the correct plug for the exact model (per the wiring diagram; read only on power-up) and return setup switches to OFF |
| 31 | Pressure switch did not close or reopened — on two-stage/variable-speed boards, the HIGH-heat switch (or its relay); single-stage boards print the same 31 for their one switch. Open over 5 minutes: the inducer shuts off 15 minutes before retry | Clear the outdoor vent/intake terminations first. Draft, switch and inducer diagnosis is technician work — never jumper the switch |
| 32 | Low-heat pressure switch did not close or reopened (two-stage/variable-speed boards; the label also covers the draft safeguard or aux-limit switch where used). Opens within 5 minutes after ignition: the next cycle is restricted to high-heat | Clear terminations and check the condensate trap/drain on condensing models; the rest is measured draft and switch diagnosis by a technician — never jumper it |
| 33 | Limit circuit fault — a limit switch or flame rollout switch in the limit circuit is open. The blower runs for 4 minutes or until the open switch remakes, whichever is longer; open longer than 3 minutes, the code changes to lockout 13; remade sooner, 33 keeps flashing until the blower shuts off. The rollout switch requires manual reset. | Restore airflow: new filter, all registers and returns open, and the duct restriction corrected at its source. |
| 34 | Ignition proving failure — flame was not sensed during the trial-for-ignition period. The control repeats the sequence 3 more times before lockout 14; a flame signal lost in the blower on-delay brings the blower on for the off-delay | Confirm the gas valve and shut-off are on; sensor (cleaned with fine steel wool), igniter, pressure and ground checks are technician work — see the ignition guide |
| 41 | Blower motor fault (variable-speed boards) — the blower failed to reach 250 RPM, or to communicate within the time limits: 30 seconds after switch-on, 10 seconds in steady-state operation. In the component test, 41 means the blower failed the test | Technician work: prove wheel freedom, wiring and control before replacing motor or module. Two successive cycles of 41 escalate to lockout 15 |
See the full list on the Bryant Furnace error codes hub.
Codes 24 gets confused with
13 — Limit circuit lockout — lockout occurs if the limit or flame rollout switch is open longer than 3 minutes, or (variable-speed Bryant wording) 10 successive limit trips occurred during high-heat. The control will auto-reset after 3 hours. Bryant's label cross-refers every 13 to code 33 for the diagnosis. Code 13 is code 33 that did not clear in time — Bryant prints the relationship on every service label ('Refer to status code #33') because 13 has no separate cause list: it is the lockout the limit circuit escalates to.
33 — Limit circuit fault — a limit switch or flame rollout switch in the limit circuit is open. The blower runs for 4 minutes or until the open switch remakes, whichever is longer; open longer than 3 minutes, the code changes to lockout 13; remade sooner, 33 keeps flashing until the blower shuts off. The rollout switch requires manual reset. Code 33 is Bryant's overheating-and-rollout report, with identical wording across the whole Bryant line — limit or flame rollout switch open, blower runs 4 minutes or until the switch remakes, 3 minutes to lockout 13 — printed on the single-stage 340AAV, 330JAV and 393AAV service labels, in the 577D control section, and in the variable-speed 355MAV guide.
Frequently asked questions
What does code 24 mean on a Bryant furnace?
Secondary voltage fuse is open: the fuse protecting the control's 24-volt circuit has opened because of a short circuit in the secondary-voltage (24VAC) wiring — Bryant's labels add 'including thermostat leads'. On the Evolution 987M it is an automotive-type 3-amp fuse on the control, and the LED displays 24 when it needs replacement. The short is the fault; the fuse is the evidence.
Can I just replace the fuse on my Bryant furnace?
You can replace the part — power off, ONLY a 3-amp automotive-type fuse, per Bryant — but understand what you are testing: if the short is still present, the new fuse opens too, sometimes instantly. Bryant's procedure isolates the short first (starting with disconnecting the thermostat leads) and fits the fuse last. One replacement as a test is reasonable; a second blown fuse is a technician call, not a shopping trip.
Why is my thermostat blank when the furnace shows code 24?
Because the thermostat's power comes through the same 24-volt circuit the fuse protects. With the fuse open, the thermostat goes dark, calls never reach the furnace, and safeties and gas valve lose control power — while the control board, fed on its line-voltage side, still flashes 24 at the blower-door indicator. Blank thermostat plus flashing 24 is a low-voltage short story, not a dead thermostat.
Why does my Bryant blow its fuse only in cooling mode?
Bryant's own flow anticipates exactly this: a 24 during the cooling cycle points to the thermostat-to-outdoor-unit wiring, a short in the outdoor contactor or (on heat pumps) reversing valve — or a contactor failing to pull in and drawing excessive current on the low-voltage circuit, which Bryant notes can be intermittent. The circuit energised only in cooling is where the short lives.
What if code 24 flashes but the fuse isn't blown?
Bryant's 355MAV flow covers it in two steps: check the fuse's continuity to be sure; if it is genuinely not open and 24 still flashes, the instruction is to replace the furnace control. With the sacrifice intact, the reporter itself has failed — a board diagnosis for a technician; do not keep swapping good fuses into it.
What does bryant furnace error code 24 mean?
Secondary voltage fuse is open — the fuse protecting the control's 24-volt (secondary) circuit has opened because there is a short circuit in the secondary-voltage wiring. On the Evolution 987M the fuse is an automotive-type 3-amp fuse located on the control, and Bryant states the control LED will display status code 24 when the fuse needs to be replaced. Code 24 is the Bryant control protecting itself, and it starts with what 'secondary voltage' means: the transformer steps line voltage down to the 24 VAC that runs everything the furnace thinks with — thermostat circuit, limit circuit, pressure switches, gas valve, and accessories such as the humidifier terminal.
How to fix bryant furnace error code 24?
Start here: Repair or replace the shorted wire run — thermostat cable, limit-circuit wiring, pressure-switch wiring, gas valve lead or accessory (humidifier) circuit, as the isolation proves; a humidifier solenoid or diode bridge drawing excessively is replaced. A wire that has chafed through once is replaced or properly re-insulated and re-routed, not taped against the same edge. Limit-circuit branch: repair the short, and deal with Bryant's named intermittent cause — loose or torn insulation shifted against the limit switch(es) is secured or replaced so blower vibration cannot walk it back into contact.
How to reset bryant furnace error code 24?
Code 24 has no timer and no auto-reset: it clears when — and only when — the short is repaired and the fuse replaced. Bryant's order is strict: isolate and repair first, fuse last, because a fuse fitted into a live short simply opens again (and that re-opening, staged circuit by circuit, is how the isolation flow localises the fault). The one non-fuse outcome Bryant documents: 24 flashing with the fuse proven NOT blown means the control itself has failed and is replaced. There is nothing to wait out on a 24.
Related guides
Facts on this page were checked against manufacturer documentation: shoredist.com · manuzoid.com · www.manualshelf.com · www.manualsdir.com · www.manualshelf.com · manualsdump.com · manualsdump.com · www.manualsdir.com · www.manualslib.com · manuals.plus