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 means the low-heat pressure switch is open at the same time the high-heat pressure switch is closed — a contradictory state the control does not expect. Carrier's guide lists several checks: low inlet gas pressure (if a low-gas-pressure switch is used), a plugged condensate drain, improper pressure-switch wiring, water in the vent piping or a sagging pipe, a stuck-open low-heat pressure switch, and disconnected or obstructed pressure tubing.
Because the 58MVP uses separate low- and high-heat pressure switches to prove venting at each stage, this code specifically catches the case where those two switches disagree. A plugged condensate drain or water pooling in a sagging vent pipe can hold the low-heat switch open while the high-heat switch stays made, which is why drainage and vent slope are prime suspects.
Diagnosing pressure-switch wiring, condensate drainage, and vent piping is combustion-safety work for a technician, not a homeowner repair.
What You'll Notice
- The furnace heats erratically or fails to run properly
- The board LED flashes code 43
- Signs of a plugged condensate drain or water pooling near the furnace
- A sagging section of vent pipe that could trap condensate
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Plugged condensate drain | Most common | ✗ Call a pro → |
| Water in vent piping or sagging pipe | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician clears and tests the condensate drain, checks the vent piping for water, sagging, or improper slope, and verifies the pressure-switch wiring and tubing. They confirm each switch opens and closes correctly, replacing a stuck-open low-heat switch if needed. This is professional combustion-safety work.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The board repeatedly flashes code 43
- There are signs of a plugged condensate drain or water in the vent piping
- A vent pipe section sags or lacks proper slope
- The problem followed recent service to the pressure switches or venting
Frequently Asked Questions
What does code 43 mean on a Carrier 58MVP?
The low-heat pressure switch is open while the high-heat switch is closed — a disagreement between the two switches. It commonly points to a plugged condensate drain or water in the vent piping.
Why do drainage and vent slope matter for code 43?
Water backing up from a plugged drain, or pooling in a sagging vent pipe, can hold the low-heat pressure switch open, creating the mismatch this code detects.
Sources
✓ Verified against manufacturer service manual — March 2026