Bryant 355MAV 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. 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. 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. These checks are technician work on this model.
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 | ✗ Call a pro → |
| 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
A technician checks the condensate drain and trap first, since a clog is the most common cause on this condensing furnace, then inspects the vent piping for sagging sections holding water. Because the code is a disagreement between two switches, the low-heat switch, its tubing, and its wiring are examined specifically for a stuck-open switch, a disconnected hose, or a wiring error.
Draft at low-heat is measured to confirm whether the inducer develops enough suction for the low-heat switch to close, and inlet gas pressure is checked if a low gas pressure switch is used. Accessing the drain, switches, and tubing and interpreting these readings require tools and training, so this is handled by a qualified technician.
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:
- Code 43 persists and the condensate drain, trap, or vent piping needs service
- 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