Lennox SLP98UHV Error Code E 225: High Pressure Switch Failed Open
What Does Code E 225 Mean?
Error code E225 on the Lennox SLP98UHV means the high-fire pressure switch failed to close during a call for heat. Because this is a modulating, variable-capacity furnace, it proves safe draft at more than one firing rate and uses two separate switches: a low-fire pressure switch and a high-fire pressure switch. The high-fire switch requires greater draft to close because it must confirm safe venting at the higher combustion rate.
When the inducer runs at high speed for high-fire operation, it should create a stronger vacuum that closes the high-fire switch. If that switch does not close, the SureLight Variable Capacity control will not allow high-fire ignition. Depending on the situation the furnace may still be able to run on low fire if the low-fire switch closes normally, but it cannot safely step up to full output.
The most common cause is a partial restriction in the exhaust vent or combustion air intake that is enough to keep the higher draft threshold from being reached — a partly blocked screen, light ice, or debris. A failing inducer motor that can no longer reach full speed is another common cause. This code is distinct from E 223, where the low-fire switch fails to close, and from E 226, where the high-fire switch is stuck closed. Repeated pressure-switch failures escalate to soft lockouts E 271 (pressure switch opening) and E 272 (pressure switch recycle).
What You'll Notice
- The furnace runs on low fire but never steps up to full heat on cold days
- The home heats slowly or the furnace seems to lack its usual output
- The 7-segment display shows E 225 in the diagnostic error history
- Symptoms may worsen when ice or debris partly restricts the vent
- In some cases the furnace fails to light at all if draft is too weak
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Blocked or restricted exhaust vent preventing proper draft at high fire | Most common | ✗ Call a pro → |
How This Is Diagnosed
A homeowner-safe first step is to check the exhaust and intake vent terminations outside for partial blockages such as frost, light ice, leaves, or nests, since even a small restriction can keep the high-fire switch from closing. If the terminations are clear and the code returns, diagnosis moves to the technician.
A technician measures the draft pressure in inches of water column with the inducer at high speed and compares it to the high-fire switch's specification, which distinguishes a venting restriction from an inducer that can no longer reach full speed or a faulty switch. They also inspect the pressure switch hoses for cracks, kinks, or water. Pressure measurement, inducer 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 vent and intake terminations outside are clear but E 225 persists
- The inducer motor sounds different at high speed than at low speed
- The furnace runs on low fire but cannot switch to high fire
- The pressure switch hoses are intact but the high-fire switch will not close
- You also see E 271 or E 272 (soft lockouts related to the pressure switches)
Frequently Asked Questions
Why does the furnace still make some heat with E 225?
This modulating furnace has separate low-fire and high-fire pressure switches. If only the high-fire switch fails to close, the furnace may still prove draft and run on low fire, but it cannot safely step up to full output until the problem is fixed.
Can I check anything myself before calling a technician?
Yes. You can safely inspect the exhaust and intake vent terminations outside and clear frost, ice, leaves, or nests. Even a partial blockage can trigger E 225. Draft measurement and any work inside the furnace should be left to a technician.
How is E 225 different from E 223?
E 225 is the high-fire pressure switch failing to close at full firing rate, while E 223 is the low-fire pressure switch failing to close. They are two different switches proving draft at two different combustion rates on this variable-capacity furnace.
✓ Verified against manufacturer service manual — March 2026