Carrier Furnace Code 12
StatusShown as: 1 short flash, 2 long flashes
Blower on after power up. Normal operation - if furnace power is interrupted and restored while the thermostat is calling for heat, the blower runs for 90 seconds.
Manual wording: “12 BLOWER ON AFTER POWER UP - (115 VAC or 24 VAC) - Blower runs for 90 seconds, if unit is powered up during a call for heat (R-W/W1 closed)”
Documented for: 58SC0A, 58MCB, 58MVP, 58UVB and most Carrier 58-series furnaces
Likely causes, most common first
- Power was interrupted and restored during a call for heat (R to W closed)
- The thermostat call ended during the blower on-delay period
What you can safely check yourself
- Nothing to fix - this is normal after a power blip or after you flip the furnace switch back on
When to call a technician
If code 12 keeps appearing without any power outages, have a technician check for a loose 115 V connection or a failing door switch.
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