HeatingCodesYork TM9V furnace 4 red flashes: main limit switch open (and 5 and 11 red)
What 4 red flashes means
York's manual says: "4 red flashes: this indicates that the main limit switch has opened its normally closed contacts. The control turns on the supply air blower and inducer. Check for a dirty filter, incorrectly sized duct system, incorrect blower speed setting, incorrect firing rate, loose limit switch wiring, or faulty blower motor."
Because the blower runs, you may hear and feel air with no heat. That is the furnace trying to cool itself.
How 4 red escalates
| Situation (per York) | What the LED does |
|---|---|
| Main limit open, fewer than 5 minutes | 4 red flashes while the limit is open. |
| Limit has not closed within 5 minutes | The control assumes the blower is not working, starts a hard lockout, and flashes 11 red flashes. 11 red usually means a failed blower motor or blower wheel. Power must be cycled to reset. |
| Limit still open after 15 minutes | The control assumes a manual-reset rollout switch has opened and flashes 5 red flashes. This is a hard lockout; power must be cycled after the problem is corrected. |
| Main limit opens five times in a single call for heat | 4 red flashes and a 1-hour soft lockout. |
For 5 red flashes York says to check for correct combustion air, correct inducer operation, a primary heat exchanger failure or a burner problem. That is a more serious list than a dirty filter.
What a homeowner can do
The first item in York's list is a dirty filter, and it is the one a homeowner can fix in a minute. Open registers and returns also matter. If a new filter and open registers do not stop the code, the remaining causes (duct sizing, blower speed setting, firing rate, wiring and the blower motor) are for a technician.
Safe homeowner checks for York furnace 4 red flashes
- Turn the thermostat offLet the furnace cool.
- Replace a dirty filterInstall a clean filter of the correct size.
- Open registers and grillesMake sure supply and return vents are not blocked.
- Restart and watchIf the LED shows 4, 5 or 11 red flashes again, turn the furnace off and call a technician.
Frequently asked questions
Is 4 red flashes serious?
The furnace is protecting itself. If it repeats, or if it escalates to 5 or 11 red flashes, it needs a technician. Never bypass the limit.
What does 11 red flashes mean?
York says it means the limit circuit has stayed open more than 5 and less than 15 minutes, usually from a failed blower motor or blower wheel. It is a hard lockout that needs power cycled after the repair.
What does a steady LED mean?
York says a steady LED of any color is a control failure. Turn power off and on, and if the code returns, the board needs replacement.
Sources
- Installation Manual 5750149-UIM-I-0423: Two-Stage Variable Speed ECM Residential Gas Furnaces, TM9V*C series (96% AFUE), Furnace control diagnostics (Johnson Controls (York)) Covers: York TM9V*C two-stage variable-speed 96% furnaces. Other York and Coleman furnace series use different boards and flash codes; check the label on your furnace.
Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.