Bryant 355MAV 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 Bryant 355MAV means the low-heat pressure switch is open while the high-heat pressure switch is closed at the same time. On a two-stage furnace these two switches should agree in a sensible way, so this simultaneous open/closed disagreement points to a partial draft or drainage problem that affects the more sensitive low-heat switch but not the high-heat switch.
Because the 355MAV is a two-stage condensing furnace, its low-heat pressure switch closes on a lighter draft and is therefore more easily thrown off by small restrictions or water backup. That sensitivity is exactly why a marginal condition shows up as this disagreement rather than as a clean high-heat failure.
The most common cause is a plugged condensate drain: backed-up water in the inducer housing lowers the draft just enough to keep the low-heat switch open. Checking and clearing the drain is a homeowner-safe first step. The manual also lists low inlet gas pressure (if an LGPS is used), improper pressure switch wiring, water trapped in a sagging vent pipe, a stuck-open low-heat switch, and disconnected or obstructed pressure tubing — those need a technician. This is distinct from code 32 (Low-Heat Pressure Switch Did Not Close or Reopened), which is the low-heat switch failing on its own rather than disagreeing with the high-heat switch.
What You'll Notice
- The furnace fails or shuts down when it should be running low-heat, while high-heat may still work
- Water may be visible backing up or pooling near the furnace base
- The status LED flashes code 43 (four short flashes, three long flashes)
- Heating may be inconsistent, with the furnace favoring high-heat
- Symptoms can worsen over time as a drain or sagging vent slowly collects more water
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Plugged condensate drain | Most common | ✓ DIY fix → |
| Stuck open low-heat pressure switch | Common | ✗ Call a pro → |
| Water in vent piping or sagging pipe | Common | ✗ Call a pro → |
How This Is Diagnosed
Checking and clearing the condensate drain and trap is the first step, since a clog is the most common cause on this condensing furnace, followed by scanning the vent piping for sagging sections holding water. If drainage is clear and the code persists, a technician examines the low-heat switch, its tubing, and wiring for a stuck-open switch, a disconnected hose, or a wiring error, measures low-heat draft to confirm the inducer develops enough suction, and checks inlet gas pressure if a low gas pressure switch is used.
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 including the quieter low-heat stage.
When to Call a Professional
Contact a licensed HVAC technician if:
- Code 43 persists after the condensate drain and trap have been cleared
- Water keeps pooling around the furnace base
- The low-heat pressure switch appears stuck or its tubing looks disconnected or cracked
- The furnace favors high-heat and low-heat operation is unreliable
- You are not equipped to test the pressure switches and their tubing
Frequently Asked Questions
How is code 43 different from code 32 on a Bryant 355MAV?
Code 32 is the low-heat pressure switch failing to close or reopening on its own. Code 43 is a disagreement — the low-heat switch is open at the same time the high-heat switch is closed. Both involve the low-heat switch but describe different conditions.
Why does a plugged drain cause a pressure switch code?
The 355MAV is a condensing furnace that makes water as it runs. If the condensate drain clogs, water backs into the inducer housing and drops the draft slightly — enough to keep the sensitive low-heat switch open and trigger code 43.
Sources
✓ Verified against manufacturer service manual — March 2026