Error Code High Pressure Switch Failed Open

Lennox SLP98UHV Error Code High Pressure Switch Failed Open: High Pressure Switch Failed Open

TL;DR

Your Lennox furnace's high-stage pressure switch failed to close, meaning the inducer motor isn't creating enough draft at high fire. Check the exhaust vent and combustion air intake pipes for blockages — this is the high-fire counterpart to E223.

Disclaimer

This article is for informational purposes only and does not guarantee the accuracy or completeness of the information provided. Always turn off power and gas supply before attempting any repairs. If you smell gas, leave immediately and call your gas company. Consult a licensed HVAC technician for diagnosis and repair. Any actions taken based on this information are at your own risk.

What Does Code High Pressure Switch Failed Open Mean?

Error code E225 on the Lennox IFC control board means the high-stage pressure switch failed to close during the furnace startup or operation. On two-stage Lennox furnaces, there are typically two pressure switches — one for low fire and one for high fire. The high-fire pressure switch requires a stronger vacuum to close because more combustion air and exhaust flow is needed at the higher firing rate. E225 indicates this higher threshold wasn't met.

This code can appear in two scenarios: either the furnace was trying to start in high-fire mode and the pressure switch never closed, or the furnace was running in low fire and attempted to ramp up to high fire but the high-stage switch wouldn't close. In either case, the inducer motor is not generating enough airflow through the venting system to safely support high-fire operation.

The causes mirror those of E223 (low-stage pressure switch failure) — blocked exhaust vents, obstructed combustion air intake, condensate drain blockages, or a cracked pressure switch hose. However, because the high-fire switch has a tighter threshold, partial blockages that don't affect low-fire operation can trigger E225. A vent pipe that is partially obstructed may allow enough airflow for low fire but not enough for high fire.

How to Fix It: Check Exhaust Vent and Combustion Air Intake

⚠ Safety First

Always turn off the furnace at the power switch or breaker and shut off the gas supply before beginning. Do not proceed if you smell gas — leave the area and call your gas company immediately.

What You'll Need

Flashlight
Step ladder (if vent terminations are above ground level)
Mirror or phone camera (to look inside vent pipes)

Steps

Turn off power AND shut off gas supply Switch the furnace power off at the disconnect switch near the unit AND at the circuit breaker. Locate the gas shutoff valve on the gas line leading to the furnace and turn it to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave the area immediately and call your gas company from outside.
Locate the exhaust vent and combustion air intake pipes Find where the exhaust vent and combustion air intake pipes exit your home. On high-efficiency Lennox furnaces (SLP98, EL296, etc.), these are PVC or CPVC pipes that typically exit through a side wall. Look for two pipes — one for exhaust and one for fresh air intake. On standard-efficiency models, the exhaust is a metal flue going up through the roof.
Inspect and clear the vent terminations Check both pipe openings for any obstructions — bird nests, wasp or mud dauber nests, ice/snow buildup, leaves, or debris. Because E225 is the high-fire switch, even a partial blockage that allows low-fire operation can trigger this code. Make sure both pipes are completely clear. Use a flashlight to look inside the pipes as far as possible. Remove any obstructions.
Check the vent pipe runs for damage or disconnection Trace the vent pipes from the furnace to the exterior. Look for any disconnected joints, cracks, or sagging sections. On PVC vent systems, check that all joints are properly cemented. A leak in the exhaust pipe can prevent the inducer from building the higher vacuum needed for the high-fire pressure switch.
Check the condensate drain (high-efficiency models only) On high-efficiency Lennox furnaces (90%+ AFUE), check the condensate drain line and trap for blockages. A clogged drain can cause water to back up into the inducer housing, reducing its ability to create vacuum. Pour a small amount of warm water through the trap to verify it drains freely.
Inspect the pressure switch hose Inside the furnace, locate the rubber hose connected from the inducer housing to the high-fire pressure switch. On two-stage models, there may be two hoses going to two separate switches. Check that the hoses are not cracked, kinked, disconnected, or holding water. Drain any water from the hose if present.
Restore power and test Replace the access panel. Turn the gas supply back on (valve handle parallel to the pipe). Restore power at the disconnect switch and breaker. Set your thermostat to call for heat. If possible, set the temperature several degrees above room temperature to ensure the furnace attempts high-fire operation.

How to Verify

After restoring power, the furnace should complete its startup sequence and run at high fire without E225 returning. On Lennox communicating systems with iComfort, you can verify the current stage on the thermostat. The furnace should be able to reach and maintain high-fire operation. If E225 only appears when the furnace tries to ramp to high fire, a partial vent blockage or weak inducer motor may be the cause.

When to Call a Professional

Contact a licensed HVAC technician if:

The exhaust vent and intake are completely clear but E225 continues to appear
The furnace runs fine at low fire but always fails when attempting high fire
The inducer motor sounds weak or labored
There is condensate leaking from the inducer or vent connections
You notice a gas smell at any point during the process