Furnace lights then shuts off after seconds

The burners light, run for four to ten seconds, and go out. The furnace tries again, does the same thing two or three more times, and then stops trying. This is one specific failure, and it is not the burner.

What is actually happening

What you are watching is the control shutting the gas off on purpose. After the burners light, the control has a few seconds to confirm flame is actually present. It does that by flame rectification: it puts an AC voltage on a metal rod sitting in the flame, and the ionised gas conducts current in one direction only, returning a few microamps of DC. If that current does not arrive, or arrives too small, the control assumes there is unburnt gas in the heat exchanger and closes the valve immediately.

What causes it

The single most common cause is a flame sensor coated in a thin oxide film, which builds up normally over a heating season and is not visible as dirt. The rod still sits in the flame; the current can no longer get through it. Other causes are a cracked or misaligned sensor, a poor ground or reversed line voltage polarity — flame rectification is referenced to neutral and chassis ground and cannot work without both — a burner flame not actually touching the rod, or a partially blocked burner. The timing is the tell: a furnace that never lights at all is a different fault, and a furnace that runs for minutes and then stops is usually a limit trip.

The codes that report this

9 codes across 7 control families. Each links to what that manufacturer says about it.

Brand	Code	What it reports
Armstrong	3 flashes	Burner failed to light, or lost flame during a heat demand
Armstrong	4 flashes	Low flame signal — check the flame sensor
Goodman	7 flashes	Flame signal is too weak
Goodman	E6	Flame signal is too weak
Goodman	rapid amber flashing	Low flame sense current
Heil	6 + 2 flashes	Soft lockout — flame lost during run, cycling pressure switch, or blocked condensate
Trane	8 flashes	Low flame sense signal
York	8 red flashes	Lockout on recycles, flame dropouts
York	Rapid amber flashing	Low flame sense current

Worth knowing

Count the flashes or read the code, then find it in the table below. Several manufacturers publish a numeric threshold that turns this from a judgement into a measurement — Carrier gives a flame sense minimum of 0.5 microamps DC against 4.0–6.0 nominal, and Trane specifies under two volts between line neutral and 24 volt common.

Other symptoms

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.