Carrier furnace code 43: Low-Heat Pressure Switch Open in High Heat
Try DIY first, then call a pro
What does code 43 mean on a Carrier furnace? On the two-stage variable-speed control, a call for high heat closed the high-heat pressure switch but the low-heat switch stayed open. The control shuts down, stops the inducer, keeps the gas off, and retries after 2 minutes. Official checklist: low inlet gas pressure (if LGPS used), plugged condensate drain, pressure switch wiring, water or sag in the vent piping, a stuck-open low-heat switch, disconnected or obstructed pressure tubing.
Before you touch anything: the six signs that mean stop troubleshooting and call a pro →
GOOD NEWS: THE FIRST CHECKS ARE FREE AND TAKE MINUTES.
Try these first — in this order
Drain trap, vent pipe slope, and the pressure tubing to both switches.
Switch testing and a wiring check, since the two switches are easy to cross. DIY part cost: $30–60 — a pro visit for this class of fault typically runs $150–350.
What causes it (in order of likelihood)
- Plugged condensate drain, or water pooling in a sagging vent pipe DIY-CHECKABLE
- Disconnected or obstructed pressure tubing DIY-CHECKABLE
- Low-heat pressure switch stuck open, or the two switches miswired TECH
- Low inlet gas pressure (if LGPS used) TECH
Parts referenced by this code
Always match parts against your model's parts list — part numbers vary within a series.
Questions about this code
What does code 43 mean on a Carrier furnace?
On the two-stage variable-speed control, a call for high heat closed the high-heat pressure switch but the low-heat switch stayed open. The control shuts down, stops the inducer, keeps the gas off, and retries after 2 minutes. Official checklist: low inlet gas pressure (if LGPS used), plugged condensate drain, pressure switch wiring, water or sag in the vent piping, a stuck-open low-heat switch, disconnected or obstructed pressure tubing.
How do I fix Carrier furnace code 43?
Drain, vent piping, tubing, and switch wiring checks are inside the furnace, so this is technician work.
How much does it cost to fix Carrier furnace code 43?
Cost depends on whether a plugged drain, sagging vent pipe, tubing, gas pressure, or switch wiring is responsible. A technician's inspection determines which.
Applies to
58MVP (Series 170/180) · 58MVB · 58UVB
Different Carrier generations use different code maps. If your series isn't listed, don't assume this meaning — check your blower-door label.
Verified against Carrier 58MVP/58MVB/58UVB Troubleshooting Guide (58-06T), p.8/30 — “LOW-HEAT PRESSURE SWITCH OPEN WHILE HIGH-HEAT PRESSURE SWITCH IS CLOSED – Check for: Low inlet gas pressure (if LGPS used). Plugged condensate drain. Improper pressure switch wiring. Water in vent piping, possible sagging pipe. Stuck open low-heat pressure switch. Disconnected or obstructed pressure tubing.”
Verified against Carrier 58MVP/58MVB/58UVB Troubleshooting Guide (58-06T), p.30 — “If the high heat pressure switch HPS makes and the low heat pressure switch LPS is still open the variable speed furnace control starts flashing status code 43, shuts unit down, and waits 2 minutes before restarting the heating cycle.”
Verified against Carrier 58MVB Service and Maintenance Instructions (58MVB-2SM), p.17 — “Check for: - Low-heat pressure switch stuck open. - Disconnected or obstructed pressure tubing. - Improper pressure switch wiring. - Low inlet gas pressure (if LGPS used). - Plugged condensate drain. - Water in vent piping, possible sagging pipe.”
Last verified 2026-09-05 · page updated 2026-10-03 · how we verify
Other Carrier furnace codes
These links include other Carrier model families. Code meanings can change by model and control board, so confirm your model and service manual before using a definition.