Lennox furnace Alert code 240: Low flame current in run mode
This alert covers either low flame current or a loss of flame during a run. Lennox's checklist is the most detailed of any manufacturer's for this fault and includes checking neutral to ground voltage.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
Flame detection works by rectification — a small current passes through the flame to ground, measured in microamps. This alert means that current is low, or the flame was lost mid-run. Lennox's guidance is notably thorough: check the microamps using the thermostat or control board, clean or replace the flame sensor, measure voltage of neutral to ground to confirm a good unit ground, clean the face of the burner assembly, confirm the vent termination is not recirculating, and check for loose wiring at the gas valve. That recirculation check is unusual and worth knowing — a vent terminating where exhaust can be drawn back into the intake degrades combustion and the flame signal with it.
Most common causes
- A flame sensor coated with oxide or carbon, which Lennox addresses first.
- Poor unit grounding, which Lennox has you verify by measuring neutral to ground voltage.
- A dirty burner assembly face.
- A vent termination that recirculates exhaust back into the intake.
- Loose wiring at the gas valve.
- A flame sensor out of position or damaged.
- Low gas pressure producing a weak flame.
How to diagnose it
- Note that the furnace may still be heating. Low flame current is a warning ahead of failure rather than a failure itself.
- Note whether the furnace has started short-cycling or occasionally losing flame, which is the signal falling further.
- Look at the outside vent termination and consider whether exhaust could be drawn back into the intake — poor placement, a sheltered corner, or a later extension or enclosure.
- Note when the furnace was last serviced, since an uncleaned flame sensor is the usual explanation.
- Lennox references Service and Application Note H-14-06 on flame rectification — worth mentioning to a technician.
How to fix it
- Book a service call while the furnace is still working. The repair is usually inexpensive and this alert gives warning.
- Flame sensor cleaning, grounding measurement, and burner cleaning are technician work.
- If the vent termination is recirculating, that is an installation problem needing correction rather than a component replacement.
- Ask for the neutral to ground measurement Lennox specifies if a cleaned sensor has not resolved it — grounding is a recurring cause across manufacturers.
Notes
The recirculation check is the standout item here. A furnace can develop this fault years after installation because a fence, an extension, or a shrub changed the airflow around a vent termination that used to be fine.