Carrier 58MVP Furnace 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.
Clearing a plugged condensate drain is a task a homeowner can safely handle and often resolves code 43 on its own. Diagnosing pressure-switch wiring, tubing, and any water or sag in the vent piping remains combustion-safety work for a technician.
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 | ✓ DIY fix → |
| Water in vent piping or sagging pipe | Common | ✗ Call a pro → |
How This Is Diagnosed
The homeowner-checkable cause comes first: inspect the condensate drain line, trap, and pump for a clog or standing water, and confirm the condensate is draining freely.
If the drain is clear and code 43 persists, a technician checks the vent piping for water, sagging, or improper slope, and verifies the pressure-switch wiring and tubing, confirming each switch opens and closes correctly and replacing a stuck-open low-heat switch if needed. That remaining work is professional combustion-safety work.
How to Fix It: Clear the Condensate Drain
What You'll Need
Steps
- Turn off power at the furnace switch or breaker and shut off the gas supply Set the furnace disconnect to OFF (or trip its breaker) and close the manual gas shutoff valve before inspecting the furnace. If you smell gas, leave the house immediately and call your gas company from outside.
- Find the condensate drain path Follow the small drain hose from the furnace to its trap and then to a floor drain or condensate pump. Look for standing water, sludge, or a slimy buildup that signals a clog.
- Clear the drain line and trap Gently flush or clear the line. A wet/dry vacuum placed on the outlet end can pull a clog free. Catch any water in a bucket, and if there is a condensate pump, confirm it is running and not full.
- Check for a sagging or water-filled vent pipe Scan the visible vent piping for a low spot where water could pool. You cannot re-pitch the pipe yourself, but noting a sag helps a technician if the code returns.
- Restore power and test Turn the gas back on, restore power, and call for heat. Cycle the furnace switch if needed to clear the fault, then watch a full cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- The condensate drain is clear but code 43 keeps returning
- You see water inside the burner or control compartment
- The pressure tubing looks disconnected, cracked, or full of moisture
- The furnace continues to short-cycle after the drain is cleared
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, which a homeowner can check and clear, or water in the vent piping, which needs a technician.
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.
Can I fix code 43 without a technician?
If it stems from a clogged condensate drain, clearing the drain often resolves it. If the code returns after that, a stuck switch, wiring issue, or obstructed tubing needs professional service.
Sources
✓ Verified against manufacturer service manual — March 2026