Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Lennox Error Code 125 — What It Means
Lennox error code 125 indicates that the pressure switch opened or failed to close when the inducer transitioned to heat speed. On two-stage or variable-speed furnaces, the inducer runs at a lower speed during the pre-purge, then ramps up to full speed when heat is called. Code 125 specifically means the pressure switch did not confirm adequate pressure at the higher heat speed — suggesting an inducer performance issue, a switch fault, or a venting restriction that only manifests under load.
Common Causes
- Partially blocked vent or exhaust pipe — A partial blockage that allows the inducer to make sufficient pressure at low speed but not at heat speed will trigger a 125 fault while a low-speed pressure fault would not occur.
- Inducer motor losing performance — A motor whose bearings are wearing may run adequately at low speed but cannot sustain the higher RPM needed at heat speed.
- Wrong pressure switch for the installation — If the furnace was modified, the vent length extended, or a different switch installed, the switch setpoint may not match actual system requirements at heat speed.
- Condensate in the pressure switch or hose — Water accumulation that allows the switch to function marginally at low inducer speed but trips at the higher pressure differential of heat speed.
Step-by-Step Fix
- Inspect the vent pipe for restrictions — Check the entire flue and combustion air pipe run. Look for bird screens blocked with debris, ice in cold weather, or a collapsed or sagging vent section that traps condensate.
- Clear the pressure switch hose and condensate trap — Disconnect the pressure switch hose and blow it out. Drain and clean the condensate trap and collector box on high-efficiency models.
- Monitor inducer behavior at heat speed — After the furnace starts (low-speed pre-purge), listen and observe when the inducer ramps up to heat speed. A healthy inducer should be noticeably louder and produce clearly more airflow at heat speed. A worn inducer may not change significantly.
- Measure inducer suction at heat speed — Connect a magnehelic or digital manometer to the pressure switch port. Observe the reading when the inducer shifts to heat speed. It should meet or exceed the pressure switch setpoint labeled on the switch.
- Test the pressure switch — Use a multimeter in continuity mode across the switch terminals. The switch should close when inducer vacuum reaches its rated setpoint and open below it. A switch that stays open at correct pressure is faulty.
- Replace the failing component — Replace the pressure switch if it fails under correct pressure, or replace the inducer motor assembly if it cannot achieve proper pressure at heat speed.
- Reset the system — Restore power and verify the furnace completes a full heat cycle at both low and high stage without triggering code 125.
Parts Often Needed
| Part | Notes |
|---|---|
| Pressure switch (heat speed rated) | Verify correct setpoint for your Lennox model and vent configuration |
| Draft inducer motor | Replace if motor cannot maintain heat-speed pressure |
| Pressure switch hose | 5/16” rubber hose; replace if cracked or water-logged |
| Condensate trap assembly | Replace if cracked or blocked beyond cleaning |
Technician Tips
Lennox furnace error codes are displayed as a series of LED flashes on the SureLight or integrated control board. A slow flash usually indicates normal operation, while rapid flashing points to a specific fault code. Code 1 = no previous code (normal standby). Code 2 = pressure switch failed open — check the vent, condensate drain, and inducer motor. Code 3 = pressure switch failed closed — replace the switch. Code 4 = high temperature limit — check airflow. Lennox systems with the SureLight igniter are particularly sensitive to gas pressure — verify 3.5” WC on natural gas before troubleshooting further. A 5-minute gas pressure check can save an hour of unnecessary component swapping.
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
If inducer pressure tests and component replacements do not resolve code 125, a Lennox-authorized technician should perform a full system performance check. Vent sizing issues or heat exchanger restrictions may require redesign of the venting system.