Carrier 58MVP Error Code 43: Low-Heat Pressure Switch Open While High-Heat Pressure Switch Is Closed
What Does Code 43 Mean?
Status code 43 on the Carrier 58MVP indicates that the low-heat pressure switch is open while the high-heat pressure switch is closed. This inconsistency suggests a partial blockage or drainage issue that affects the lower draft levels used in low-heat operation but not the higher draft levels used in high-heat.
The Carrier 58MVP is a two-stage condensing furnace with separate pressure switches for low-heat and high-heat operation. The low-heat pressure switch requires less draft to close, making it more sensitive to conditions that restrict airflow or drainage.
The most common cause is a plugged condensate drain. When the drain is clogged, water backs up into the inducer housing and reduces the draft enough to prevent the low-heat switch from closing. Other possible causes include low inlet gas pressure (if an LGPS is used), improper pressure switch wiring, water trapped in the vent piping (possibly from a sagging pipe), a stuck open low-heat pressure switch, or disconnected/obstructed pressure tubing.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Plugged condensate drain | Most common | ✗ Call a pro → |
| Water in vent piping or sagging pipe | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- Code 43 returns after clearing the condensate drain
- You see water trapped in the vent piping that you cannot drain
- The low-heat pressure switch appears stuck or physically damaged
- Pressure switch tubing appears disconnected or cracked
- Water keeps pooling around the furnace base despite clearing the drain
✓ Verified against manufacturer service manual — March 2026