What causes Trane Gas Furnace 4 flashes?

A dirty or clogged air filter, the most common cause of low airflow and overheating. Closed or blocked supply registers or return grilles, or a collapsed or restricted duct (Trane lists low airflow from excessive duct static). A blower motor that's weak, failed, or not being turned on, so heat builds up in the heat exchanger. A dirty blower wheel or a clogged evaporator (AC) coil sitting above the furnace. A tripped flame rollout switch (on models where rollout is in the limit circuit), which points to a venting, combustion air or heat exchanger problem. Corroded terminals, a broken wire, or a limit switch whose contacts have failed open; less commonly a faulty control board.

How do I fix Trane Gas Furnace 4 flashes?

Count the red LED through the sight glass on the lower door before you touch anything: 4 flashes and a pause is this code. Trane's chart lists slow flashing as normal with no call for heat and fast flashing as normal during a call for heat. Check the air filter and replace it if it's dirty or you can't remember when it was last changed. Use the nominal size printed on the old filter's frame. Walk the house and open every supply register, and make sure no return grille is blocked by furniture, rugs or curtains. Give the furnace time. A main limit that tripped from overheating resets on its own once it cools, so wait 15 to 30 minutes with the fan running or the thermostat off, then call for heat again. When it restarts, check that the blower comes on and moves a strong stream of air out of the registers. If the burners light but the blower never starts, shut it off and call a pro. Note your model number from the rating label inside the door before you call, since XR and XV names cover many models with different boards.

When should I call a pro for Trane Gas Furnace 4 flashes?

4 flashes comes back after a new filter and open vents, or the furnace keeps short-cycling on the limit. The blower doesn't run, runs weakly, or the board flashes 4 times with no heat at all even when the furnace is cold (that points to an open rollout switch, broken wire or failed limit). A rollout switch has tripped. Don't just keep pushing its reset button: it trips when flames or hot gases roll out of the burner area, which can mean a blocked vent, a bad inducer or a cracked heat exchanger. Testing the limit circuit with a meter, measuring temperature rise and static pressure, or replacing a limit, rollout switch, blower motor or control board. That's live-voltage work on a gas appliance. Never jumper or bypass a limit or rollout switch. They stop the furnace from overheating or burning outside the heat exchanger. If you smell gas or a CO alarm sounds, leave the house and call your gas utility from outside.

Code: 4 flashes🔥 Trane Gas Furnace

Trane Furnace Blinking 4 Times: Open Temperature Limit Circuit (Meaning, Causes, Fixes)

Q: What does Trane Gas Furnace 4 flashes mean?

A red LED blinking 4 times then pausing on a Trane (or American Standard) furnace control board means the temperature limit circuit is open. A heat safety switch has opened, usually because the furnace got too hot from poor airflow, so the board shuts off the gas. A dirty filter or closed vents is the most common reason. Trane's integrated furnace control (IFC) flash code chart lists 4 flashes as "Open Temperature Limit Circuit". The limit switch is a safety device that shuts off the gas valve if the furnace overheats, and Trane's installer's guide has the installer test it by blocking return air until the burners shut off. On many current Trane gas furnaces the same circuit also runs through the flame rollout switches and any reverse airflow switch: before each cycle the control sends 24 volts out its HLO terminal and checks that it comes back on HLI, which only happens if the main thermal limit, rollout switches and reverse airflow switches are all closed. If any one is open, you get this code and no heat. Furnaces with a two-digit display show the same fault as E04 (open limit: main thermal, rollout switch or reverse airflow switch). Trane's help center describes the high-limit fault on its ignition boards as excessive heat or low airflow, and says it resets automatically once the switch cools and closes. A rollout switch is different: Trane notes it needs a manual reset, and it trips for combustion problems, not just airflow.

Reviewed 2026-10-22 · Based on 5 manufacturer & service sources

What the code means

Trane's integrated furnace control (IFC) flash code chart lists 4 flashes as "Open Temperature Limit Circuit". The limit switch is a safety device that shuts off the gas valve if the furnace overheats, and Trane's installer's guide has the installer test it by blocking return air until the burners shut off. On many current Trane gas furnaces the same circuit also runs through the flame rollout switches and any reverse airflow switch: before each cycle the control sends 24 volts out its HLO terminal and checks that it comes back on HLI, which only happens if the main thermal limit, rollout switches and reverse airflow switches are all closed. If any one is open, you get this code and no heat. Furnaces with a two-digit display show the same fault as E04 (open limit: main thermal, rollout switch or reverse airflow switch). Trane's help center describes the high-limit fault on its ignition boards as excessive heat or low airflow, and says it resets automatically once the switch cools and closes. A rollout switch is different: Trane notes it needs a manual reset, and it trips for combustion problems, not just airflow.

Likely causes

A dirty or clogged air filter, the most common cause of low airflow and overheating
Closed or blocked supply registers or return grilles, or a collapsed or restricted duct (Trane lists low airflow from excessive duct static)
A blower motor that's weak, failed, or not being turned on, so heat builds up in the heat exchanger
A dirty blower wheel or a clogged evaporator (AC) coil sitting above the furnace
A tripped flame rollout switch (on models where rollout is in the limit circuit), which points to a venting, combustion air or heat exchanger problem
Corroded terminals, a broken wire, or a limit switch whose contacts have failed open; less commonly a faulty control board

Safe DIY checks

⚠ Before you start

Turn off power at the furnace/HVAC service switch and the breaker before opening panels. Shut the gas valve before any work near the burners, and never disconnect or alter the gas line. If you smell gas, leave the house and call your gas utility.

Count the red LED through the sight glass on the lower door before you touch anything: 4 flashes and a pause is this code. Trane's chart lists slow flashing as normal with no call for heat and fast flashing as normal during a call for heat.
Check the air filter and replace it if it's dirty or you can't remember when it was last changed. Use the nominal size printed on the old filter's frame.
Walk the house and open every supply register, and make sure no return grille is blocked by furniture, rugs or curtains.
Give the furnace time. A main limit that tripped from overheating resets on its own once it cools, so wait 15 to 30 minutes with the fan running or the thermostat off, then call for heat again.
When it restarts, check that the blower comes on and moves a strong stream of air out of the registers. If the burners light but the blower never starts, shut it off and call a pro.
Note your model number from the rating label inside the door before you call, since XR and XV names cover many models with different boards.

🛠 When to call a pro

4 flashes comes back after a new filter and open vents, or the furnace keeps short-cycling on the limit.
The blower doesn't run, runs weakly, or the board flashes 4 times with no heat at all even when the furnace is cold (that points to an open rollout switch, broken wire or failed limit).
A rollout switch has tripped. Don't just keep pushing its reset button: it trips when flames or hot gases roll out of the burner area, which can mean a blocked vent, a bad inducer or a cracked heat exchanger.
Testing the limit circuit with a meter, measuring temperature rise and static pressure, or replacing a limit, rollout switch, blower motor or control board. That's live-voltage work on a gas appliance.
Never jumper or bypass a limit or rollout switch. They stop the furnace from overheating or burning outside the heat exchanger. If you smell gas or a CO alarm sounds, leave the house and call your gas utility from outside.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Furnace air filter (your exact size)

The cheapest and most common fix. Use the nominal size printed on your current filter's frame.

[affiliate link slot: AFFILIATE_SLOT_AIR_FILTER]

⚙

Limit switch (exact OEM part for your model)

Only after airflow is ruled out. Match the part number and temperature rating on your existing limit, never a higher-temperature one.

[affiliate link slot: AFFILIATE_SLOT_LIMIT_SWITCH]

⚙

Flame rollout switch (exact OEM part for your model)

Only after a pro finds why it tripped. Match the part number and rating on the existing switch.

[affiliate link slot: AFFILIATE_SLOT_ROLLOUT_SWITCH]

⚙

Circulator blower motor

Model-specific; usually a pro install.

[affiliate link slot: AFFILIATE_SLOT_BLOWER_MOTOR]

Sources

Last reviewed 2026-10-22. Always confirm against the manual for your exact model number.