HVAC Code Lookup

Carrier Furnace: 2 Flashes

A Carrier furnace LED pattern starting with 2 flashes matches 5 documented codes: 21, 22, 23, 24, 25. Watch the full pattern (short vs. long flashes) to tell them apart — all 5 are explained below.

Code 21: Gas heating lockout

Hard lockoutShown as: 2 short flashes, 1 long flash

Gas heating lockout. The control detected 24 VAC on the gas valve when it should not be there. This lockout does NOT auto-reset.

Manual wording: “21 GAS HEATING LOCKOUT - Control will NOT auto reset. Check for: - Mis-wired gas valve. - Defective control (valve relay).”

Documented for: 58SC0A, 58MCB, 58MVP, 58UVB

Likely causes, most common first

  1. Stuck-closed gas valve relay on the control board
  2. Mis-wired or shorted gas valve wire

When to call a technician

Code 21 means the control can no longer guarantee it can shut the gas valve off. Shut the furnace off at the service switch and call a technician - this one does not clear on its own.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Full page for Carrier code 21

Code 22: Abnormal flame-proving signal

Safety faultShown as: 2 short flashes, 2 long flashes

Abnormal flame-proving signal (false flame). The control senses flame while the gas valve is de-energized. The inducer will keep running until the fault clears.

Manual wording: “22 ABNORMAL FLAME-PROVING SIGNAL - Flame is proved while gas valve is de-energized. Inducer will run until fault is cleared.”

Documented for: 58SC0A, 58MCB, 58MVP, 58UVB

Likely causes, most common first

  1. Leaky gas valve
  2. Stuck-open gas valve

When to call a technician

This code means gas may be burning when the control has commanded the valve closed. Turn the furnace off at the service switch and call a technician immediately. Do not touch or attempt to test the gas valve.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Full page for Carrier code 22

Code 23: Pressure switch did not open

WarningShown as: 2 short flashes, 3 long flashes

Pressure switch did not open. The control expected the vent pressure switch to be open at the start of the cycle and it was still closed.

Manual wording: “23 PRESSURE SWITCH DID NOT OPEN - Check for: - Obstructed pressure tubing. - Pressure switch stuck closed.”

Documented for: 58SC0A, 58MCB, 58MVP, 58UVB

Likely causes, most common first

  1. Obstructed pressure tubing
  2. Pressure switch stuck closed

When to call a technician

The pressure switch is a combustion safety device. A technician needs to inspect the tubing and switch - never bypass or jumper a pressure switch.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Full page for Carrier code 23

Code 24: The 3-amp secondary voltage fuse on the control board is blown, which means there is a short somewhere in the 24 V low-voltage wiring

Hard lockoutShown as: 2 short flashes, 4 long flashes

The 3-amp secondary voltage fuse on the control board is blown, which means there is a short somewhere in the 24 V low-voltage wiring.

Manual wording: “24 SECONDARY VOLTAGE FUSE IS OPEN - Check for: - Short circuit in secondary voltage (24V) wiring including thermostat leads.”

Documented for: 58SC0A, 58MCB, 58MVP, 58UVB

Likely causes, most common first

  1. Short circuit in the 24 VAC secondary wiring, including the thermostat leads

When to call a technician

Replacing the fuse without finding the short just blows the new fuse. A technician needs to isolate the shorted thermostat or accessory wire.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Full page for Carrier code 24

Code 25: Model selection or setup error

WarningShown as: 2 short flashes, 5 long flashes

Model selection or setup error. The model plug (PL4) is missing or wrong, or a setup switch is in the wrong position. If the code flashes only 4 times at power-up, the control is running on the model selection stored in memory.

Manual wording: “25 MODEL SELECTION OR SETUP ERROR - Either Indicates the model plug (PL4) is missing or incorrect or setup switch "SW1-1" or "SW1-6" is positioned improperly.”

Documented for: 58MVP, 58UVB and other variable-speed two-stage Carrier furnaces

Likely causes, most common first

  1. Model plug PL4 missing or incorrect (common after a replacement control board)
  2. Setup switch SW1-1 or SW1-6 left ON
  3. Two different furnace models twinned together

When to call a technician

This is a setup fault that appears after service work. The technician who last worked on the furnace needs to install the correct model plug and reset the setup switches.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Source

Full page for Carrier code 25

Other Carrier codes