Trane and American Standard Furnace 4 flashes: Open limit switch
Updated October 2026 · Sourced from Trane and American Standard documentation
🧭 Check a few things first
Trane and American Standard 4 flashes means different things on different series:
- Trane and American Standard single-stage furnaces serviced with the SiNi igniter control kit (*UH1, *DH1, *UD1, *DD1 families and M951): The control reports an open limit switch. The limit is a heat safety control.
- Trane and American Standard modulating communicating *UHM/*DHM furnaces (e.g. *UHMB060ACV3VB, *UHMC100ACV4VB, *UHMD120ACV5VB and matching DHM models): The alert group is an open high limit, reverse flow limit or rollout. The manual says the heat exchanger temperature is too high, which could be caused by low airflow or fan failure.
Is it safe to keep running? Check the steps below first. Call a licensed HVAC technician if it repeats after a new filter. Never bypass a limit switch.
4 flashes at a glance
| Code / how it shows | Meaning | Applies to |
|---|---|---|
| 4 flashes Red LED on the integrated furnace control blinks 4 times, then pauses | Open limit switch | Trane and American Standard single-stage furnaces serviced with the SiNi igniter control kit (*UH1, *DH1, *UD1, *DD1 families and M951) |
| 4 flashes Red fault LED on the integrated furnace control blinks 4 times | High temperature limit fault | Trane and American Standard modulating communicating *UHM/*DHM furnaces (e.g. *UHMB060ACV3VB, *UHMC100ACV4VB, *UHMD120ACV5VB and matching DHM models) |
Check this first
Only steps a homeowner can do safely. Do not open electrical panels or the burner area.
- Replace a dirty air filter, then restore power and see if the code returns.
- Make sure supply and return vents are open and not blocked by furniture or rugs.
- Turn the furnace power switch or breaker off for 30 seconds, then back on, and see if the code clears.
🛑 Stop and call a licensed pro if
- Call a licensed HVAC technician if it repeats after a new filter. Never bypass a limit switch.
- Call a licensed HVAC technician if it repeats. Never bypass limit or rollout switches.
- You smell gas. Leave the house, do not touch switches, and call your gas utility from outside.
- Flame rollout, limit or pressure switch codes that come back after one reset.
- Anything involving the control board, gas valve, flame sensor, igniter or venting.
- A carbon monoxide alarm sounds. Get everyone outside and call 911 or the fire department.
Safe first steps for any Trane and American Standard code
- Count the LED flashes through the blower door window before you switch anything off. Cutting power can clear the code.
- Replace or clean the furnace air filter.
- Check the thermostat: correct mode, set above room temperature, and fresh batteries.
- Make sure the furnace power switch and breaker are on. If the furnace has locked out, turning the power off and back on once restarts it.
- Make sure the gas valve is in the ON position, following the lighting instructions on the furnace label only.
- On high-efficiency furnaces, look at the outside vent and intake pipes for snow, ice, leaves or nests (look only, do not take pipes apart).
Sources: Trane Service Installer's Guide 18-CH93D1-1A-EN (Jul 2020), Integrated Furnace Control Diagnostic Codes; Trane Installer's Guide 18-CD30D1-20-EN (06/2020), Table 24 Furnace Alert Codes, p.55. Codes are series-specific, so your unit's own manual is the final word. Reviewed October 2026.
Shaun Cain · Certified HVAC & Refrigeration Technician · Ask an HVAC technician
Related Trane and American Standard codes
2 flashes : System lockout, retries or recycles exceeded3 flashes : Pressure switch fault5 flashes : Flame sensed when none should be present6 flashes : 115 volt power reversed or improper ground7 flashes : Gas valve circuit error8 flashes : Low flame sense signal9 flashes : Igniter relay fault10 flashes : Communication faultE2.1 : Retry exceeded (flame never sensed)E2.2 : Recycles exceeded (flame lost)E3.1 : Shorted pressure switch, 1st stageE3.2 : Open pressure switch, 1st stage