Carrier Furnace: 4 Flashes
A Carrier furnace LED pattern starting with 4 flashes matches 8 documented codes: 41, 42, 43, 45, 45.1, 45.2, 45.3, 46.1. Watch the full pattern (short vs. long flashes) to tell them apart — all 8 are explained below.
Code 41: Blower motor fault
Blower motor fault. The blower failed to reach 250 RPM, or failed to report back to the control within the allowed time (30 seconds after start, or 10 seconds during steady operation).
Manual wording: “41 BLOWER MOTOR FAULT - Indicates the blower failed to reach 250 RPM or the blower failed to communicate within the prescribed time limits.”
Documented for: 58MVP, 58UVB and other variable-speed two-stage Carrier furnaces
Likely causes, most common first
- Failed blower motor or motor module
- Loose blower motor wiring or communication connection
- Blower wheel jammed or seized
What you can safely check yourself
- Make sure the blower access door is fully closed and latched
- Turn the furnace off at the service switch for a minute, then back on once, to force a fresh start
When to call a technician
Testing an ECM/variable-speed blower motor and its module requires a technician. Repeating this code usually means the motor or module is failing.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Troubleshooting Guide - Variable Speed, Two-Stage Gas Furnaces (Form 58-06T) (Carrier, 58MVP, 58UVB and other Carrier variable-speed two-stage gas furnaces (replaces 58MVP-10SM)) · checked 2026-08-25
Code 42: Inducer motor fault
Inducer motor fault. The inducer did not start within 20 seconds of a call for heat, its RPM is outside the valid range, or the RPM signal was lost for 5 seconds during operation.
Manual wording: “42 INDUCER MOTOR FAULT - Indicates the inducer has not started within 20 seconds after a call for heat, the inducer motor RPM is outside its valid range of operation, or the inducer RPM signal was lost for 5 seconds during operation.”
Documented for: 58MVP, 58UVB and other variable-speed two-stage Carrier furnaces
Likely causes, most common first
- Improper vent sizing
- Restricted combustion air supply
- Failed inducer motor
- Improper motor wiring
What you can safely check yourself
- Check the outdoor intake and exhaust pipes for snow, ice, or debris blocking them
When to call a technician
The inducer motor and its wiring have to be tested by a technician. A furnace that cannot run its inducer cannot vent combustion gases safely.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Troubleshooting Guide - Variable Speed, Two-Stage Gas Furnaces (Form 58-06T) (Carrier, 58MVP, 58UVB and other Carrier variable-speed two-stage gas furnaces (replaces 58MVP-10SM)) · checked 2026-08-25
Code 43: The low-heat pressure switch is open while the high-heat pressure switch is closed - a contradictory pressure switch state on a two-stage furnace
The low-heat pressure switch is open while the high-heat pressure switch is closed - a contradictory pressure switch state on a two-stage furnace.
Manual wording: “43 LOW-HEAT PRESSURE SWITCH OPEN WHILE HIGH-HEAT PRESSURE SWITCH IS CLOSED”
Documented for: 58MVP, 58UVB and other variable-speed two-stage Carrier furnaces
Likely causes, most common first
- Low inlet gas pressure (if a low gas pressure switch is used)
- Plugged condensate drain
- Improper pressure switch wiring
- Water in the vent piping from a sagging pipe
- Stuck-open low-heat pressure switch
- Disconnected or obstructed pressure tubing
What you can safely check yourself
- Check the condensate drain line and trap for a clog or standing water
- Check the outdoor vent terminations for blockage
When to call a technician
Pressure switch wiring and vent piping slope are technician diagnostics. Never jumper a pressure switch.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Troubleshooting Guide - Variable Speed, Two-Stage Gas Furnaces (Form 58-06T) (Carrier, 58MVP, 58UVB and other Carrier variable-speed two-stage gas furnaces (replaces 58MVP-10SM)) · checked 2026-08-25
Code 45: Control circuitry lockout
Control circuitry lockout. The board detected an internal failure and locks out, auto-resetting after 1 hour.
Manual wording: “45 CONTROL CIRCUITRY LOCKOUT - Auto-reset after 1 hour lockout due to: - Flame sense circuit failure. - Gas valve relay stuck open. - Software check error.”
Documented for: 58SC0A, 58MCB, 58MVP, 58UVB
Likely causes, most common first
- Flame sense circuit failure
- Gas valve relay stuck open
- Software check error
What you can safely check yourself
- Turn the furnace off at the service switch for a minute and back on once - the manual says resetting power clears this lockout
When to call a technician
If code 45 comes back after a power reset, the manual's own instruction is to replace the control board - that is a technician job.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Troubleshooting Guide - Variable Speed, Two-Stage Gas Furnaces (Form 58-06T) (Carrier, 58MVP, 58UVB and other Carrier variable-speed two-stage gas furnaces (replaces 58MVP-10SM)) · checked 2026-08-25
- Installation, Start-Up, Operating and Service and Maintenance Instructions - 58SC0A/58SC1A (Carrier, 58SC0A/58SC1A single-stage 4-way multipoise condensing gas furnace) · checked 2026-08-25
- 58MCB 4-Way Multipoise Fixed-Capacity Condensing Gas Furnace - Service Manual (Carrier, 58MCB single-stage condensing gas furnace) · checked 2026-08-25
Code 45.1: Control failure - flame circuit fault, memory mismatch, or sequence error inside the control board
Control failure - flame circuit fault, memory mismatch, or sequence error inside the control board.
Manual wording: “45.1 CONTROL FAILURE - flame circuit fault, memory mismatch or sequence error.”
Documented for: 58SC0B/58SC1B
Likely causes, most common first
- Internal control board failure
What you can safely check yourself
- Turn the furnace off at the service switch for a minute and back on once - the manual says to cycle power
When to call a technician
The manual says that if code 45 repeats after a power cycle, replace the control. That is a technician job.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation, Start-Up, Operating and Service and Maintenance Instructions - 58SC0B/58SC1B (Carrier, 58SC0B/58SC1B single-stage condensing gas furnace with 3-digit status display) · checked 2026-08-25
Code 45.2: Control failure - the gas valve relay on the control board will not close
Control failure - the gas valve relay on the control board will not close.
Manual wording: “45.2 CONTROL FAILURE - gas valve relay will not close.”
Documented for: 58SC0B/58SC1B
Likely causes, most common first
- Failed gas valve relay on the control board
What you can safely check yourself
- Turn the furnace off at the service switch and leave it off until a technician arrives
When to call a technician
A gas valve relay that will not close means the board can no longer control gas flow reliably. Shut the furnace off and call a technician; the manual's fix is to replace the control.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation, Start-Up, Operating and Service and Maintenance Instructions - 58SC0B/58SC1B (Carrier, 58SC0B/58SC1B single-stage condensing gas furnace with 3-digit status display) · checked 2026-08-25
Code 45.3: Control failure - EEPROM memory issue on the control board
Control failure - EEPROM memory issue on the control board.
Manual wording: “45.3 CONTROL FAILURE - EEPROM memory issue.”
Documented for: 58SC0B/58SC1B
Likely causes, most common first
- Corrupted or failed memory on the control board
What you can safely check yourself
- Turn the furnace off at the service switch for a minute and back on once
When to call a technician
If the code repeats after cycling power, the manual says to replace the control board.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation, Start-Up, Operating and Service and Maintenance Instructions - 58SC0B/58SC1B (Carrier, 58SC0B/58SC1B single-stage condensing gas furnace with 3-digit status display) · checked 2026-08-25
Code 46.1: Momentary loss of power
Momentary loss of power. Heating stops and restarts on its own once the power fluctuation is resolved.
Manual wording: “46.1 MOMENTARY LOSS of power - Heating operation will stop and will restart once the power fluctuation is resolved.”
Documented for: 58SC0B/58SC1B
Likely causes, most common first
- Brownout or brief power interruption to the furnace
What you can safely check yourself
- Nothing to fix if you had a power blip
- If it happens often, check that the breaker is fully seated and the furnace service switch is not loose
When to call a technician
Frequent 46.1 codes with no utility outages point to a loose 115 V connection or failing door switch - have a technician check it.
A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation, Start-Up, Operating and Service and Maintenance Instructions - 58SC0B/58SC1B (Carrier, 58SC0B/58SC1B single-stage condensing gas furnace with 3-digit status display) · checked 2026-08-25