Carrier 58MVC Error Code 43: Low-Heat Pressure Switch Open While Medium-Heat Pressure Switch Is Closed
What Does Code 43 Mean?
The 58MVC is a step-modulating furnace with separate pressure switches that prove proper draft at its low- and medium-heat stages. In normal operation those switches follow a predictable pattern as the furnace ramps up. Code 43 is set when the pattern is contradictory: the low-heat pressure switch is open while the medium-heat pressure switch is closed, a combination that should not occur if both switches and their tubing are healthy.
Because this is a condensing furnace, it produces condensate whenever it runs. The most common homeowner-checkable cause of code 43 is a plugged condensate drain — backed-up water can disturb the low-heat switch or its tubing while the medium-heat side still reads closed. Clearing the drain often resolves it.
Other causes the manual lists are a stuck-open low-heat pressure switch, disconnected or obstructed pressure tubing, water sitting in a sagging vent pipe, improper pressure-switch wiring, and low inlet gas pressure where a low-gas-pressure switch is used. Those are internal or gas-side issues that require a technician.
Code 43 sits alongside the other pressure-switch codes on this board: code 32 (low-heat switch did not close or reopened) and code 31 (medium-heat switch or HPSR relay). Where 32 and 31 report a single switch failing, code 43 specifically flags the two switches disagreeing with each other.
What You'll Notice
- The furnace starts but shuts down instead of running a normal cycle
- Short-cycling or repeated failed starts during a call for heat
- Water pooling near the furnace base, suggesting a backed-up condensate drain
- The inducer runs but the burners do not stay lit
- Little or no heat despite the thermostat calling
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Plugged condensate drain | Most common | ✓ DIY fix → |
| Stuck open low-heat pressure switch | Common | ✗ Call a pro → |
| Disconnected or obstructed pressure tubing | 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 inspects the low- and medium-heat pressure-switch tubing for obstruction or moisture, checks for water in a sagging vent run, tests the low-heat pressure switch for a stuck-open condition, and verifies the switch wiring. Opening the cabinet to work on switches, tubing, or wiring is not a homeowner task.
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 58MVC?
It means the low-heat pressure switch is open while the medium-heat pressure switch is closed — the two switches are giving contradictory readings. On this condensing furnace, a plugged condensate drain is the most common cause you can check yourself.
How is code 43 different from code 32?
Code 32 is a single low-heat pressure switch that failed to close or reopened. Code 43 specifically flags a disagreement between the low-heat and medium-heat switches, which points more toward a drain, tubing, or stuck-switch problem.
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 or obstructed tubing needs professional service.
Why does my condensing furnace make water?
High-efficiency condensing furnaces like the 58MVC extract so much heat that flue gas cools and condenses into water, which must drain away. If that drain clogs, it can back up and trip pressure-switch codes like 43.
✓ Verified against manufacturer service manual — March 2026