Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Carrier Furnace Error Codes — Quick Reference
Carrier gas furnaces use a blinking LED on the control board to communicate fault codes. Count the flashes in a repeating pattern — the number of blinks identifies the fault. Single-digit codes are common on older IFC boards; newer Carrier Infinity and Performance series use two-digit codes. Pull the furnace door and look through the sight glass or remove the lower access panel to see the LED.
| Code | Meaning | Quick Fix |
|---|---|---|
| 11 | No previous fault stored | Normal after power cycle |
| 12 | Blower on after power up / prior to heat | Check blower wheel; verify prior call |
| 13 | Limit device lockout (retries exceeded) | Check filter, airflow, heat exchanger |
| 14 | Ignition lockout (retries exceeded) | Check gas, igniter, flame sensor |
| 21 | Gas heating lockout | Manual reset; check gas pressure |
| 22 | Abnormal flame-proving signal | Clean flame sensor rod |
| 24 | Secondary voltage fuse open | Check for short in low-voltage wiring |
| 25 | Model plug missing or incorrect | Replace/reseat model plug |
| 31 | Pressure switch stuck open | Check inducer, hose, pressure switch |
| 32 | Pressure switch stuck closed | Check pressure switch hose routing |
| 33 | Limit or flame roll-out switch open | Check filter, ducts, roll-out switch |
| 34 | Ignition proving failure | Clean flame sensor; check gas valve |
| 40 | Inducer high-speed fault | Inducer motor issue |
| 41 | Inducer motor fault | Replace inducer motor |
| 42 | Inducer motor fault (secondary) | Verify inducer RPM feedback |
| 43 | Low-voltage control fault | Check transformer and 24V circuit |
| 44 | Blower motor fault | Check blower motor and capacitor |
| 45 | Control board fault | Replace IFC board |
| 46 | Check IFC | Board self-diagnosis fault |
| 52 | Duplicate serial number | IFC/comm setup issue |
| 54 | Soft lockout — low pressure switch open | Pressure switch or inducer fault |
| 58 | Indoor air quality sensor fault | Check IAQ sensor wiring |
Technician Tips
Carrier uses different diagnostic systems across furnaces, heat pumps, chillers, mini-splits, VRF equipment, rooftop units, and communicating controls. Confirm the full model number and whether the fault came from the indoor board, outdoor board, thermostat, or equipment controller before matching it to a repair path.
Start with power, wiring, airflow or water-flow, sensor, condensate, and safety-circuit checks before ordering control boards. If the same code returns after one reset and the basic cause is not obvious, test the named circuit instead of repeatedly cycling the equipment.
Common replacement parts for Carrier systems:
- Carrier flame sensor (part #HH18ZA641)
- Limit switch kit (part #HH18HA600)
- Pressure switch (part #HH18HA719)
Need Replacement Parts?
Before ordering, match the furnace part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Most Common Codes
Code 13: Limit Device Lockout
The furnace tried to heat multiple times but the high-limit switch tripped each time. Most often caused by a clogged air filter, closed supply/return registers, or a failing blower motor. Replace the filter first. If that doesn’t solve it, check for blocked ducts and verify the blower wheel spins freely.
Code 14: Ignition Lockout
The furnace failed to establish a flame after the maximum number of ignition attempts. Check that the gas supply valve is open, verify gas pressure (3.5” WC minimum for natural gas), inspect the hot-surface igniter for cracks, and clean the flame sensor rod with fine steel wool or 400-grit sandpaper.
Code 31: Pressure Switch Stuck Open
The inducer motor started but the pressure switch never closed. Most common causes: cracked pressure switch hose, a clogged condensate drain trap (on 90%+ furnaces), weak inducer motor, or a failed pressure switch itself. Start by inspecting the rubber hose between the inducer and the pressure switch.
Code 33: Limit or Roll-Out Switch Open
A safety limit opened during operation. On 80% furnaces this usually means airflow is restricted — check the filter and blower. On 90%+ furnaces, also check the secondary heat exchanger. If the roll-out switch tripped, do not reset it until you find the cause — that switch protects against flue gas spillage.
Code 34: Ignition Proving Failure
The igniter lit the burner but the flame sensor didn’t confirm a flame reliably. Clean the flame sensor rod (the single metal rod that sits in the burner flame). If cleaning doesn’t fix it, check the microamp signal; healthy flame signal is typically 2–8 µA.
Code 45: Control Board Fault
The IFC board has diagnosed itself as faulty. Before replacing, verify all wiring connections are tight, there are no shorted low-voltage wires, and the 3-amp fuse on the board is intact.
When to Call a Pro
If you’ve reset the furnace and the same code returns immediately, or if codes 21 or 33 involve a tripped roll-out switch, contact a licensed HVAC technician — these situations can indicate cracked heat exchangers or flue gas spillage, which are safety hazards.