Carrier 58MVC Error Code 43: Low-Heat Pressure Switch Open While Medium-Heat Pressure Switch Is Closed
What Does Code 43 Mean?
Status code 43 on the Carrier 58MVC indicates that the low-heat pressure switch is open while the medium-heat pressure switch is closed. This is an abnormal condition because if the medium-heat switch can close (indicating sufficient draft at medium speed), the low-heat switch should also be able to close.
This inconsistency most commonly points to a plugged condensate drain. When condensate water cannot drain properly, it backs up and can interfere with the low-heat pressure switch operation while the higher suction from medium-heat mode is still sufficient to close its switch.
Other possible causes include a stuck open low-heat pressure switch, disconnected or obstructed pressure tubing, water trapped in the vent piping (possibly from a sagging pipe section), improper pressure switch wiring, or low inlet gas pressure if a low gas pressure switch (LGPS) is installed.
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 to Fix It: Clear the Condensate Drain
What You'll Need
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the gas shutoff valve to the OFF position (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
- Check the condensate drain Locate the condensate drain line — a small PVC pipe that carries water away from the furnace. Check if water is backing up or if the drain is clogged. If accessible, try clearing the drain by pouring warm water through it or using a wet/dry vacuum on the drain outlet. Also check the condensate trap — it may be full of sediment or debris.
- Check for water in visible vent piping Look along the vent pipes from the furnace. Check for any sagging sections where water could pool. Water trapped in the vent piping can affect pressure switch readings. Note any sagging for a technician to correct.
- Restore power and gas, then test Turn the gas supply valve to ON and flip the circuit breaker to ON. Set your thermostat to call for heat and monitor the furnace through a complete heating cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- The condensate drain is clear but code 43 persists
- You see water pooling around the furnace or in the vent piping
- You suspect a faulty low-heat pressure switch
- The pressure tubing appears disconnected or damaged
- The code returns after every heating cycle
✓ Verified against manufacturer service manual — March 2026