Furnace Fault Codes

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.

Do the simple checks below, then stop. The simple checks listed above are genuinely homeowner work and worth doing first. Beyond them, this fault involves the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. If the simple checks do not resolve it, stop there and call a licensed technician rather than resetting the furnace repeatedly to keep the heat on. Make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on. The conditions that mean stop now.

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

How to diagnose it

  1. Note that the furnace may still be heating. Low flame current is a warning ahead of failure rather than a failure itself.
  2. Note whether the furnace has started short-cycling or occasionally losing flame, which is the signal falling further.
  3. 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.
  4. Note when the furnace was last serviced, since an uncleaned flame sensor is the usual explanation.
  5. Lennox references Service and Application Note H-14-06 on flame rectification — worth mentioning to a technician.

How to fix it

  1. Book a service call while the furnace is still working. The repair is usually inexpensive and this alert gives warning.
  2. Flame sensor cleaning, grounding measurement, and burner cleaning are technician work.
  3. If the vent termination is recirculating, that is an installation problem needing correction rather than a component replacement.
  4. 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.

Related

All codes for this manufacturer

Lennox furnace alert codes — every code, with what differs between models.

What the fix costs

This alert usually resolves at the cheapest line in the repair cost guide: flame sensor cleaning at $80–200, which the guide calls a ten-minute job and the classic easy fix. Lennox's longer checklist is what protects that $80 from becoming a subscription — the neutral-to-ground voltage check catches the poor ground that makes cleanings stop lasting (see alert 117), and the recirculation check catches a vent termination quietly degrading combustion. A fair quote for a first-time 240 is a cleaning plus those verifications inside one visit. A furnace on its third cleaning in two seasons has earned the fuller electrical diagnosis, not a fourth cleaning.

DIY or call a pro

This site draws its DIY line at the flame sensing circuit, and alert 240 lives on that circuit — the sensor sits at the burner assembly, and everything Lennox lists beyond wiping it (microamp readings, ground verification, burner face cleaning, gas valve wiring) is combustion-adjacent work. The homeowner checks that genuinely bear on a weak flame signal are outside the cabinet: a filter choked enough to disturb burner airflow, and the outside terminations positioned or blocked such that exhaust could recirculate into the intake — Lennox's own unusual check, and one you can eyeball in thirty seconds. The behavioral tell worth reporting: whether the furnace lights fine and drops out mid-run (this alert's signature) versus never lighting, because that distinction routes the visit before it starts.

The same fault on other brands

A flame that burns while its signal fades is coded across nearly every family the site tracks — the flame sensor hub collects them. Direct equivalents to this alert include Allied's 4 flashes (low flame signal — check the flame sensor), Trane's 8 flashes (flame current low but usable), and the Goodman/York boards' rapid amber flash. Lennox is also internally consistent here: the seven-segment boards report the identical condition as E240, one of the code numbers on which Lennox's two systems agree exactly.

What this looks like

From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — that walkthrough covers what to check in order, and when it stops being a homeowner job.

Sources

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.