Trane Furnace Error Code 4 Flash

A 4 Flash code on a Trane furnace means a high temperature limit fault has occurred. This fault is triggered when the high limit switch, auxiliary limit switch, or rollout switch opens due to excessive heat or a combustion anomaly. The furnace shuts down as a protective measure to prevent equipment damage or a fire hazard. You can use the Scroll User Interface to check the Last 4 Faults log and identify exactly which switch tripped.

In most cases, a high limit fault caused by a dirty filter or restricted airflow is not immediately dangerous, as the safety switch is working as intended. However, a rollout switch fault is a more serious condition that can indicate combustion gases escaping the heat exchanger into living spaces. If the Last 4 Faults log shows ROLLOUT OPEN, do not restart the furnace until a qualified technician has inspected the heat exchanger and flue system.

Repair costs vary widely depending on the cause. Replacing an air filter costs only a few dollars and is a straightforward DIY task. A blower motor or capacitor replacement typically ranges from $150 to $600 including labor. If the fault involves a cracked heat exchanger or failed inducer motor, repair costs can range from $500 to over $1,500, and in some cases heat exchanger replacement may make a full furnace replacement more cost-effective.

Start by checking and replacing the air filter, then inspect the blower wheel for dirt buildup and confirm the blower motor operates correctly. Use the furnace control board's Last 4 Faults log to determine whether the fault is a HIGH LIMIT, AUX LIMIT, or ROLLOUT OPEN, since each requires a different repair approach. A rollout switch fault should be evaluated by a technician before the furnace is restarted, as it can indicate a cracked heat exchanger or flue blockage.

Resetting the furnace may temporarily clear the fault and allow the unit to restart, but the error will return if the underlying cause is not corrected. Some limit switches reset automatically once the component cools, while rollout switches are typically manual-reset devices that require physical intervention. Clearing the fault without fixing the root cause risks repeated shutdowns and potential damage to the heat exchanger.