Lennox SLP98UHV Error Code E 224: Low Pressure Switch Failed Closed
What Does Code E 224 Mean?
Error code E224 on the Lennox SLP98UHV indicates that the low-fire pressure switch has failed in the closed position. Before the furnace starts a heating cycle, the control board checks that the pressure switch is open (no draft present). If the switch is already closed before the inducer motor starts, the board knows something is wrong and will not proceed with ignition.
The pressure switch is a safety device that verifies the combustion air inducer is creating proper draft to vent exhaust gases. When the switch contacts become stuck or welded closed — due to age, moisture intrusion, or a manufacturing defect — the control board cannot distinguish between a safe operating condition and a fault condition.
A technician will need to test the pressure switch contacts and replace the switch if it is stuck closed. They should also check the pressure tubing for water or moisture, which can sometimes cause the switch diaphragm to stick.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Pressure switch contacts stuck or welded closed | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The furnace will not start and displays E224
- The pressure switch was recently exposed to condensate or moisture
- You hear the inducer motor running but the furnace does not proceed to ignition
✓ Verified against manufacturer service manual — March 2026