Lennox SLP98UHV Error Code Pressure Switch Calibration Failed: Pressure Switch Calibration Failed

What Does Code Pressure Switch Calibration Failed Mean?

Error code E228 on the Lennox IFC control board indicates that the combustion air inducer motor calibration routine has failed. On Lennox communicating furnaces, the IFC board performs a calibration check on the inducer motor to establish baseline operating parameters. During this calibration, the board measures the inducer motor's speed and the resulting pressure differential. If the measurements fall outside acceptable ranges, the calibration fails and E228 is generated.

This calibration typically occurs when power is first applied to the furnace or when the board initiates a diagnostic routine. If the exhaust vent or combustion air intake is partially blocked, the inducer cannot achieve the expected performance during calibration, causing a failure. Similarly, if the condensate drain is blocked on high-efficiency models, water in the inducer housing can throw off the calibration.

E228 can also be caused by an inducer motor that is starting to fail — running slower than expected or drawing abnormal current. However, vent blockages are the most accessible DIY check and should be investigated first. If the calibration failure persists, the board may escalate to E276 (calibration retries exceeded) after multiple failed attempts.

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

What You'll Need

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

Steps

1. 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.
2. Inspect the exhaust vent and intake pipe terminations Find where the exhaust vent and combustion air intake pipes exit your home. On high-efficiency Lennox furnaces (SLP98, EL296), these are PVC/CPVC pipes exiting through a side wall. Check both openings for blockages — nests, ice, snow, leaves, or debris. Remove any obstructions. Because calibration requires unobstructed airflow, even a partial blockage can cause E228.
3. Check the full vent pipe run for obstructions or disconnections Trace the vent pipes from the furnace to the exterior. Check that all joints are connected and sealed. Look for sagging sections, kinks, or areas where debris could have accumulated inside the pipe. Any disruption to the vent path can affect the inducer calibration.
4. Check the condensate drain and trap On high-efficiency Lennox furnaces, inspect the condensate drain and trap for blockages. Water backing up into the inducer housing affects the motor's ability to create proper vacuum and will cause calibration failure. Ensure the drain flows freely — pour warm water through the trap to verify.
5. Check the pressure switch hose Inside the furnace, inspect the rubber hose between the inducer and the pressure switch(es). A cracked, disconnected, or waterlogged hose will affect the calibration readings. Drain any water from the hose and ensure connections are tight.
6. 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. When power is restored, the IFC board will attempt the inducer calibration routine again. Set your thermostat to call for heat.

When to Call a Professional

Contact a licensed HVAC technician if:

• The exhaust vent and intake pipes are completely clear but E228 persists
• The inducer motor sounds weak, vibrates excessively, or makes grinding noises
• E228 escalates to E276 (calibration retries exceeded)
• There is visible damage or heavy corrosion on the inducer motor housing
• You notice a gas smell at any point during the process