Lennox Furnace Keeps Shutting Off — What’s Happening
A Lennox furnace that keeps shutting off is entering a safety shutdown or lockout because a fault was detected. The exact problem depends on the LED flash pattern or display code on the control board. Common scenarios include the furnace lighting then shutting off after a few seconds, repeating ignition attempts and locking out, or not starting at all.
The control board monitors flame sensing, pressure switches, rollout switches, polarity, and voltage to protect against unsafe operation. When one of these safeties trips or a circuit fails to prove, the furnace cuts power to the gas valve and stops the heating cycle. Identifying the LED pattern before any repair is the key to pinpointing the root cause.
Most Likely Causes
- Dirty or misaligned flame sensor The most common cause of repeated shutdowns is a flame sensor rod coated with combustion residue or positioned too far from the burner, resulting in weak flame rectification and low flame signal that drops the gas valve after ignition.
- Open rollout switch A rollout switch trips when flame escapes the heat exchanger due to blocked venting, improper combustion air, a cracked exchanger, or dirty burners, and the furnace will not restart until the switch cools and the underlying hazard is corrected.
- Pressure switch or inducer problems Blocked intake or exhaust venting, clogged condensate drain, failed inducer motor, or disconnected pressure-switch tubing prevent the pressure switch from closing, stopping ignition or causing mid-cycle shutdowns.
- Reversed line polarity or wiring fault Incorrect hot and neutral wiring at the furnace connection or a short circuit in low-voltage wiring can trigger polarity or short-circuit fault codes and prevent safe operation.
- No power or blown fuse If the LEDs are completely off, the furnace is not receiving power due to a tripped breaker, open service disconnect, or blown control-board fuse.
- Gas supply or ignition failure Closed gas valve, low gas pressure, fouled ignitor, or a stuck gas valve can prevent flame establishment or cause unstable flame that leads to repeated lockouts.
- Control board failure After external causes are ruled out, a failed control board with damaged relays or corrupted firmware may misread flame signal or fail to energize components correctly, though this is a last-step suspect.
How to Diagnose and Fix
- Observe the exact LED flash pattern on the Lennox control board before touching any components and write it down.
- If the LEDs are completely off, check the furnace circuit breaker, service disconnect switch, and any control-board fuse for continuity.
- If one light is solid and the other slow-flashing, inspect the rollout switch for an open condition and do not bypass it. Check venting, burners, heat exchanger integrity, and combustion air path for blockages or flame rollout before replacing the switch.
- If the furnace lights then shuts off after a few seconds, remove the flame sensor rod, clean it with fine emery cloth or steel wool until shiny, verify it sits in the flame zone about 1/2 inch from the burner, and reinstall securely.
- Verify the inducer motor starts and runs smoothly, then check that the pressure switch closes by listening for a click or testing continuity. Inspect vent pipes, condensate trap, and pressure-switch tubing for blockages or disconnections.
- Confirm correct line polarity at the furnace using a multimeter or outlet tester, and inspect all low-voltage wiring for shorts, loose connections, or damaged insulation.
- Check gas supply by confirming the manual valve is open and gas pressure at the valve inlet meets manufacturer specification. For ignitor and burner inspection or gas-valve replacement, call a qualified HVAC technician.
- Clear any lockout by cycling furnace power off for 30 seconds, then restore power and observe whether the same fault recurs. If the fault repeats with the same LED pattern, the original cause was not fully corrected.
Parts You Might Need
| Part | Notes |
|---|---|
| Flame sensor rod | Amazon | Verify exact Lennox model compatibility before ordering. |
| Rollout switch | Amazon | Match temperature and mounting style to original. Only replace after correcting the root cause of rollout. |
| Pressure switch | Amazon | Confirm pressure setpoint matches your furnace model and venting configuration. |
Related Error Codes
If your appliance also shows a code on the display, these match this problem:
- Lennox Furnace E117 error code
- Lennox Furnace E125 error code
- Lennox Furnace E204 error code
- Lennox Furnace E205 error code
- Lennox Furnace E206 error code
- Lennox Furnace E227 error code
- Lennox Furnace E241 error code
- Lennox Furnace E250 error code
- Lennox Furnace E270 error code
- Lennox Furnace E275 error code
- Lennox Furnace E290 error code
When to Call a Pro
Call a licensed HVAC technician if the rollout switch has tripped (rollout events indicate unsafe combustion or venting), if you smell gas or suspect a gas-valve or ignition problem, if the inducer motor does not start, or if the fault code persists after cleaning the flame sensor and clearing blockages. Technicians have the tools to measure flame microamps, static pressure, gas pressure, and heat-exchanger integrity. Any work involving gas piping, burner adjustment, or heat-exchanger replacement requires professional certification and proper combustion analysis. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.