York furnace 4 red flashes: Open high limit switch

Four red flashes means the high limit switch opened because the furnace overheated. The cause is almost always restricted airflow, and the most common single reason is a dirty filter.

Applies to: York, Luxaire and Coleman gas furnaces using the UTEC or W/R integrated furnace control boards documented in technical service letter ST-045-09. Stored in the error code array.

What it means

The high limit opens when air passing over the heat exchanger is not carrying heat away fast enough. The control shuts off the gas and runs the blower to cool things down. This is protective rather than incidental: a heat exchanger that repeatedly overheats will eventually crack, and a cracked heat exchanger can leak combustion products, including carbon monoxide, into the air circulating through the house. The switch is usually working correctly and reporting something true. York distinguishes this from the eleven-flash code, which is a limit open beyond five minutes attributed specifically to blower failure.

Most common causes

A clogged air filter — by a wide margin the most common cause.
Closed, blocked, or covered supply registers.
A blocked return air grille.
Undersized or restrictive ductwork, or a duct that has come apart.
A failing blower motor, or a blower wheel packed with dust.
Blower speed set too low for the furnace's output.
Overfiring, with gas pressure set too high for the airflow available.

How to diagnose it

Check the air filter first. Grey, matted, or opaque to light means you have very likely found it.
Walk the house and count closed or blocked supply registers, and check the return grille.
Note whether the blower runs at all. If the furnace fires and the blower never starts, overheating is certain and the eleven-flash code may be more apt.
Note whether it only happens on long runs, which is the marginal-airflow signature.
Have the stored fault history replayed to see whether this has been recurring.

How to fix it

Replace the air filter with the correct size and type and check it monthly during the heating season.
Open all supply registers and clear the return grille.
Blower motor work, duct repair, speed changes, and gas pressure adjustment are technician tasks.
If the code returns after a filter change, stop resetting it and have the furnace inspected. Repeated overheating is what damages heat exchangers.

Notes

Worth taking seriously despite its usually trivial cause. The switch exists because sustained overheating cracks heat exchangers, which is the most dangerous fault a gas furnace can develop.

All codes for this manufacturer

York, Luxaire and Coleman furnace flash codes — every code, with what differs between models.

This code is not a homeowner repair. Faults in this group involve 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. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

Johnson Controls Unitary Products technical service letter ST-045-09 — York UTEC furnace boards, fault code tables and retrieval procedure

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.