Carrier Furnace Code 43
WarningShown as: 4 short flashes, 3 long flashes
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