Lennox SLP98UHV Error Code E 224: Low Pressure Switch Failed Closed
What Does Code E 224 Mean?
Error code E224 on the Lennox SLP98UHV means the low-fire pressure switch has failed in the closed position. Before a heating cycle begins, the SureLight Variable Capacity control checks that the pressure switch is open, confirming there is no draft yet. If the switch is already closed before the inducer motor starts, the board knows the switch cannot be trusted to prove draft and refuses to proceed with ignition.
This switch is a safety device that verifies the combustion air inducer is producing proper draft. On this modulating furnace there are two such switches — a low-fire and a high-fire switch — and E 224 is specific to the low-fire switch being stuck closed. When the contacts stick or weld shut, from age, moisture intrusion, or a defect, the board can no longer tell the difference between a genuinely safe draft condition and a failed switch, so it locks out ignition as a precaution.
This fault sits within the pressure-switch family on this board. It is the opposite failure mode of E 223, where the low-fire switch fails to close, and it is distinct from E 226, where the high-fire switch is stuck closed. Repeated pressure-switch trips during operation escalate the furnace to soft lockouts E 271 (pressure switch opening) and E 272 (pressure switch recycle).
What You'll Notice
- The furnace will not start and displays E 224 in the diagnostic history
- The burners never ignite, so there is no heat
- You may hear the inducer motor run but the furnace never proceeds to light
- The code can appear after the switch or tubing was exposed to condensate or moisture
- The furnace stays locked out until the switch is serviced
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Pressure switch contacts stuck or welded closed | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician tests the low-fire pressure switch for continuity with the inducer off. Finding the contacts already closed, when they should be open, confirms the switch has failed closed and needs replacement.
Before replacing the switch, the technician also inspects the pressure tubing for water or condensate, which can make a diaphragm stick, and checks that no moisture is being carried back from the vent or inducer housing. Continuity testing and switch replacement are pro-only tasks.
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:
- The furnace will not start and displays E 224
- The pressure switch or its tubing was recently exposed to condensate or moisture
- You hear the inducer motor running but the furnace never proceeds to ignition
- The code returns after power has been cycled
Frequently Asked Questions
What does it mean that the switch is stuck closed?
The pressure switch is supposed to be open at rest and close only when the inducer creates draft. Stuck closed means the contacts stay shut even with the inducer off, so the board can no longer use the switch to prove safe venting and refuses to ignite.
Can I reset the furnace to clear E 224?
Cycling power may temporarily clear the display, but if the switch has truly failed closed the code will return. A stuck switch needs to be tested and replaced by a technician, not reset away.
Could moisture have caused this?
Yes. Condensate or moisture in the pressure tubing or on the switch diaphragm can make the switch stick. A technician checks for and corrects any moisture source while replacing the switch.
✓ Verified against manufacturer service manual — March 2026