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Error code reference

York Furnace Error Code 6 Flash

1LP has opened five times during one call for heat.

York codesYork furnace codes

1LP has opened five times during one call for heat.

First step from the service manual

Check for restricted vent system, vent system exceeding specifications, or high wind conditions.

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

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What 6 Flash means on a York furnace

The 6 Flash error code on a York furnace indicates that the limit protection device designated 1LP has opened five times during a single call for heat. The 1LP is a high-limit switch, a thermal safety device that monitors heat exchanger or plenum temperatures and opens its circuit when temperatures exceed a safe threshold. When this switch trips repeatedly within the same heating cycle, the furnace control board locks out and displays the 6 Flash code to signal a persistent overheating condition rather than an isolated event.

The primary function of the high-limit switch is to protect the heat exchanger from thermal stress and prevent dangerously high temperatures from reaching the duct system or surrounding structure. When the 1LP opens five consecutive times in one call for heat, the control board interprets this as a systemic problem requiring diagnosis rather than a random nuisance trip. This repeated cycling causes the furnace to shut down the burners each time, attempt to restart, and trip again until the lockout threshold is reached.

The most common root causes involve restrictions to combustion air supply or flue gas exhaust. A blocked or undersized vent system forces exhaust gases to linger longer in the heat exchanger, driving temperatures up. High wind conditions can create backpressure in the vent that disrupts proper draft. Internal airflow issues such as a dirty air filter, blocked return air path, or a failing inducer motor can also cause heat to build up rapidly. Any condition that reduces airflow across the heat exchanger or impairs venting will cause the 1LP to trip repeatedly.

Source: manufacturer service documentation.

Common causes of 6 Flash

  1. 01

    Restricted or blocked vent system

    Wiring / connection

    Debris, bird nests, ice, or crushed vent pipe can obstruct flue gas exhaust, causing heat to build up rapidly in the heat exchanger and repeatedly trip the 1LP limit switch. Inspect the entire vent run from the furnace to the termination point for blockages or damage.

  2. 02

    Vent system exceeding design specifications

    Part replacement

    A vent pipe run that is too long, has too many elbows, or uses incorrect diameter pipe creates excessive static pressure that impairs draft. This causes exhaust gases to back up and overheat the heat exchanger. Compare the installed vent configuration against the specifications for the unit.

  3. 03

    High wind conditions causing backpressure

    Part replacement

    Strong or gusty winds blowing directly into the vent termination can create backpressure that disrupts combustion and traps heat in the system. If the error code appears only during windy weather, the vent termination location or style may need to be addressed.

  4. 04

    Restricted airflow across the heat exchanger

    Wiring / connection

    A clogged air filter, blocked return air grilles, or closed supply registers reduce the airflow needed to carry heat away from the heat exchanger, causing limit trips. Check and replace the air filter and confirm that all return and supply vents are open and unobstructed.

  5. 05

    Failing or weak inducer motor

    Part replacement

    The inducer motor is responsible for pulling combustion gases through the heat exchanger and out the flue. If the motor is running slowly or intermittently due to worn bearings or a failing capacitor, draft is insufficient and temperatures climb until the 1LP opens. Check inducer motor speed and current draw.

See the test procedure for each cause

Frequently asked questions about 6 Flash

What does 6 Flash mean on a York furnace?
The 6 Flash code on a York furnace means the high-limit switch designated 1LP has opened five times during a single call for heat. This is a lockout condition triggered by repeated overheating events within one heating cycle. The furnace shuts down to prevent damage to the heat exchanger and to ensure safe operation. A diagnostic inspection of the vent system and airflow path is required before resuming normal operation.
How do I fix a 6 Flash error on a York furnace?
Start by inspecting the vent system for blockages, ice, debris, or improper pipe configuration. Check and replace the air filter if it is dirty, and confirm all return and supply vents are open. If the vent system appears clear and airflow is adequate, check the inducer motor for proper operation and inspect the 1LP high-limit switch for continuity or signs of failure. If the vent run does not meet specifications for the unit, the piping may need to be modified.
Is a York furnace 6 Flash code a safety concern?
Yes, this code indicates a real safety condition involving repeated overheating of internal components. The high-limit switch is tripping because temperatures inside the furnace are exceeding safe levels. Operating the furnace without resolving the root cause can stress or crack the heat exchanger over time, which is a serious concern. Do not bypass or disable the limit switch to restore operation.
Will resetting the furnace clear the 6 Flash error code?
Resetting the furnace may temporarily clear the lockout and allow it to attempt another heating cycle, but the 6 Flash code will return if the underlying cause has not been corrected. Because the code requires five limit trips to trigger, simply resetting the unit without addressing airflow or venting issues will result in the same lockout. A full diagnostic inspection is needed to resolve the fault permanently.
How much does it cost to repair a York furnace showing a 6 Flash code?
Repair costs depend entirely on the root cause. If the fix involves clearing a blocked vent or replacing an air filter, costs are minimal, typically in the range of $20 to $100 including a service call. If the inducer motor needs replacement or the vent system requires reconfiguration, costs can range from $200 to $700 or more depending on parts and labor rates in your area. A technician will need to diagnose the specific cause before an accurate estimate can be given.

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