Trane Furnace: 4 Flashes
Code 4 Flashes: Open high limit device
Soft lockoutShown as: 4 flashes, pause, repeat
Open high limit device. A temperature limit switch has opened because the furnace got too hot, which closes the gas valve and stops the burners.
Manual wording: “4 Flashes --- Open High Limit Device”
Documented for: *UH1 / *DH1 condensing furnaces (XR 95 family) and other Trane furnaces using the LED-only integrated furnace control
Likely causes, most common first
- Restricted airflow across the heat exchanger - the manual's own limit checkout deliberately trips the limit by 'blocking the return air', which is what a dirty filter or blocked returns do in service
- Blocked vent or combustion air inlet terminations (the manual's burner-box limit reset step says 'The vent and combustion air inlet terminations should be checked for blockage')
- Faulty limit switch
What you can safely check yourself
- Check the air filter and clean or replace it if it is dirty
- Check that supply registers and return grilles are open and not blocked by furniture or rugs
- Check the outdoor vent and combustion air intake terminations for blockage
When to call a technician
If the code comes back after a clean filter and open vents, the furnace is overheating for a reason a technician must diagnose. The burner-box manual-reset limit 'must be reset by a qualified servicer' per the manual - do not press or bypass safety limits yourself.
A Trane furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Installer's Guide - Upflow/Horizontal, Downflow/Horizontal, Gas-Fired, Direct Vent Condensing Furnaces (18-CD29D1-14-EN) (Trane, *UH1B/*UH1C/*UH1D upflow and *DH1B/*DH1C/*DH1D downflow condensing gas furnaces (XR 95 family; first letter A or T)) · checked 2026-08-25
- Owner's Guide - Indoor Units: Air Handlers, Gas Furnaces (32-5064-WEB01) (Trane, Trane residential gas furnaces and air handlers) · checked 2026-08-25