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Error code reference

York Furnace Error Code 4 Flash

Open limit circuit (includes Primary Limit, Auxiliary Limit, and Rollout Switches).

York codesYork furnace codes

Open limit circuit (includes Primary Limit, Auxiliary Limit, and Rollout Switches).

First step from the service manual

Check for dirty filter, improperly sized duct system, incorrect blower speed selection, incorrect firing rate, faulty blower motor, or faulty control.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
  • Verified part numbers when replacement is required
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What 4 Flash means on a York furnace

A 4 Flash error on a York furnace indicates an open limit circuit. This circuit includes three protective devices: the primary limit switch, the auxiliary limit switch, and the rollout switches. Each of these components is a normally closed safety switch that opens when it detects an unsafe temperature condition. When any one of them opens, the control board reads the interruption in the circuit and enters a lockout state, halting burner operation to prevent damage or fire hazard.

The primary limit switch monitors heat exchanger temperature and trips when it senses overheating, which is almost always caused by restricted airflow. Common airflow culprits include a clogged air filter, an undersized or improperly configured duct system, or an incorrect blower speed setting on the control board. A faulty blower motor that runs slowly or fails intermittently can also cause heat to build up beyond the limit switch threshold. If the firing rate is set too high for the equipment, the heat exchanger will similarly overheat and trip the primary limit.

Rollout switches are located near the burner assembly and trip when flames roll out beyond the combustion chamber, which can indicate a cracked heat exchanger, blocked flue, or a combustion problem. The auxiliary limit switch provides additional protection in specific zones of the air handling section. After correcting the root cause, most limit switches automatically reset once they cool down, but repeated tripping will cause them to fail permanently and require replacement. A faulty control board that misreads the circuit can also generate a false 4 Flash condition.

Source: manufacturer service documentation.

Common causes of 4 Flash

  1. 01

    Dirty or clogged air filter

    Part replacement

    A restricted filter is the most frequent cause of limit switch trips. It starves the heat exchanger of airflow, causing temperature to rise until the primary limit opens. Inspect and replace the filter if it appears clogged or has not been changed recently.

  2. 02

    Faulty or incorrectly configured blower motor

    Wiring / connection

    If the blower motor is failing, running at reduced speed, or set to the wrong speed tap on the control board, it cannot move enough air across the heat exchanger. Check that the blower starts promptly after ignition and that the speed setting matches the system requirements.

  3. 03

    Improperly sized or restricted duct system

    Wiring / connection

    A duct system with too little return air capacity, closed registers, or collapsed flex duct will restrict airflow even when the filter and blower are functioning correctly. Inspect all supply and return registers and check accessible ductwork for obstructions or damage.

  4. 04

    Incorrect firing rate

    Wiring / connection

    If the gas valve or orifices are configured for a higher firing rate than the system is rated for, the heat exchanger will overheat and trip the primary limit. Verify the input BTU rate matches the rating plate specifications.

  5. 05

    Faulty limit or rollout switch

    Part replacement

    A limit or rollout switch that has tripped repeatedly may fail to close even after temperatures return to normal, or may have an internal fault causing it to read open at all times. Use a multimeter to check for continuity across each switch in the circuit and replace any that do not close when cool.

See the test procedure for each cause

Frequently asked questions about 4 Flash

What does 4 Flash mean on a York furnace?
A 4 Flash error means the control board has detected an open limit circuit. This circuit includes the primary limit switch, auxiliary limit switch, and rollout switches, all of which are safety devices that interrupt furnace operation when unsafe temperatures are detected. The furnace will not fire until the underlying cause is resolved and the circuit is restored to a closed state. Common causes include restricted airflow, a faulty blower, or a tripped rollout switch.
Can I fix a York furnace 4 Flash error myself?
Some causes of the 4 Flash error are straightforward to address without professional help, such as replacing a dirty air filter or opening closed supply and return registers. However, diagnosing a faulty blower motor, verifying the firing rate, or testing individual limit and rollout switches with a multimeter requires a basic level of technical skill and comfort working near gas and electrical components. If flames are rolling out of the combustion chamber or you suspect a cracked heat exchanger, contact a licensed HVAC technician immediately.
Is a York furnace 4 Flash error dangerous?
The 4 Flash condition itself means the furnace has already shut down to protect itself and your home, so the immediate danger is mitigated. However, the root cause can be serious, especially if rollout switches have tripped, which can indicate flames escaping the combustion chamber due to a blocked flue or cracked heat exchanger. Do not repeatedly reset and restart the furnace without identifying and correcting the cause, as this can damage components or create a fire or carbon monoxide hazard.
Will resetting the furnace clear the 4 Flash error?
Cycling power to the furnace may clear the error temporarily if the limit switches have cooled and reset themselves. However, if the underlying cause of the overheating or rollout condition has not been corrected, the furnace will trip again, often within the same heating cycle. Repeated trips can permanently damage limit switches, requiring replacement. Address the root cause before resetting.
How much does it cost to repair a York furnace with a 4 Flash error?
Repair costs vary widely depending on the cause. A new air filter is typically a low-cost fix in the range of $5 to $30. Replacing a faulty limit or rollout switch generally runs between $100 and $250 including parts and labor. Blower motor replacement can range from $300 to $600 or more depending on the motor type and local labor rates. A thorough diagnostic visit from an HVAC technician typically costs between $75 and $150 before any parts are factored in.

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