Furnace short cycling then locking out

The furnace starts, runs briefly, stops, tries again, and after a few rounds gives up completely for about an hour. That hour is not the furnace recovering. It is a timer.

What is actually happening

Every modern furnace control counts its own failures. When a heat call ends without a proven flame, the control records a retry; when a flame is proven and then lost, it records a recycle. Both counters have a fixed limit, usually three or four. Once the limit is reached the control stops trying and enters a lockout — the manufacturers call it a soft lockout, a watchguard, or a one-hour lockout depending on the brand. It clears itself after roughly an hour and starts the whole sequence over.

What causes it

This is why a short-cycling furnace so often looks like it is working intermittently, and why it seems to fix itself overnight. It is not intermittent. It is failing the same way every time and retrying on a schedule. The lockout code tells you the counter that ran out, not the original fault, so the useful information is in which counter it was: retries exhausted points at ignition, gas supply or flame sensing; recycles exhausted points at a flame that lights and then goes out; a pressure switch cycle lockout points at draft that comes and goes; a limit-related soft lockout points at airflow.

The codes that report this

12 codes across 6 control families. Each links to what that manufacturer says about it.

Brand	Code	What it reports
Armstrong	5 flashes	Watchguard — burner failed to light, retries or recycles exceeded
Goodman	6 red flashes	Pressure switch cycle lockout
Goodman	8 red flashes	External lockout — recycles exhausted
Heil	6 + 1 flashes	Soft lockout — failed to light within four trials for ignition
Heil	6 + 2 flashes	Soft lockout — flame lost during run, cycling pressure switch, or blocked condensate
Heil	6 + 3 flashes	Soft lockout — airflow proving circuit opened during the run
Heil	6 + 4 flashes	Soft lockout — limit circuit opened during the run
Heil	7 flashes	Soft lockout — limit trips taking longer than two minutes to reset
Rheem	1 blink	Soft lockout, one hour
Trane	2 flashes	System lockout — retries or recycles exceeded
York	6 red flashes	Pressure switch cycle lockout
York	8 red flashes	Lockout on recycles, flame dropouts

Worth knowing

Cutting power to clear the lockout and get one more cycle of heat is the most common thing people do here, and it hides the pattern that identifies the fault. If the furnace has locked out more than once, note which code it showed before you reset it — that code is the diagnosis.

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.