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Error code reference

Coleman Furnace Error Code 4 FLASH

Limit switch open; a primary or auxiliary limit switch has opened its normally closed contacts, possibly…

Coleman codesColeman furnace codes

Limit switch open; a primary or auxiliary limit switch has opened its normally closed contacts, possibly caused by dirty filter, improperly sized duct system, incorrect blower speed setting, incorrect firing rate or faulty blower motor.

First step from the service manual

Check filter, duct system sizing, blower speed setting, firing rate, and blower motor.

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 Coleman furnace

The 4 FLASH error code on a Coleman furnace indicates that a limit switch has opened its normally closed contacts. Limit switches are safety devices mounted on or near the heat exchanger that monitor temperature. When the air surrounding the heat exchanger rises above a safe threshold, the limit switch opens the circuit, shutting down the burners to prevent overheating. The furnace control board detects this open circuit and signals the fault with four consecutive LED flashes. Both primary and auxiliary limit switches can trigger this code, and the control board does not distinguish between them in this diagnostic signal.

The root cause is almost always restricted airflow or an imbalance between heat production and heat distribution. A clogged air filter is the single most common culprit because it reduces the volume of return air flowing across the heat exchanger, causing heat to build up rapidly. An improperly sized duct system, whether undersized supply runs or blocked registers, creates the same effect. If the blower motor speed is set too low for the installed duct configuration, the system cannot move enough conditioned air to keep temperatures within limit. Overfiring caused by an incorrect gas pressure or orifice sizing can generate more heat than the air system can absorb.

A faulty blower motor is another significant cause. If the motor is failing, running at reduced RPM due to a failing capacitor, or cutting out intermittently, airflow will be insufficient even if the filter and ductwork are in good condition. Diagnosing this code requires a systematic check of the filter condition, register positions, blower motor operation and speed tap wiring, static pressure in the duct system, and the firing rate against rated input specifications.

Source: manufacturer service documentation.

Common causes of 4 FLASH

  1. 01

    Dirty or clogged air filter

    Part replacement

    A heavily restricted filter reduces return airflow across the heat exchanger, causing temperatures to spike and trip the limit switch. Inspect the filter and replace it if it is visibly loaded with dust and debris.

  2. 02

    Blower motor failure or weak performance

    Part replacement

    A failing blower motor or a defective run capacitor can cause the motor to spin at reduced speed or not at all, starving the heat exchanger of airflow. Check that the blower wheel is spinning freely and that the motor reaches full speed during operation.

  3. 03

    Incorrect blower speed setting

    Wiring / connection

    If the blower motor speed tap is wired to a lower speed than required by the duct system, airflow will be insufficient to match the firing rate. Verify that the speed tap wiring matches the specification for the installed duct configuration.

  4. 04

    Improperly sized or blocked duct system

    Wiring / connection

    Undersized supply ducts, closed registers, or obstructed return air grilles restrict airflow system-wide and cause heat to accumulate around the heat exchanger. Check that all registers are open and that static pressure in the duct system falls within the furnace design range.

  5. 05

    Incorrect firing rate

    Wiring / connection

    If the gas pressure or orifice sizing causes the furnace to fire above its rated input, it will produce more heat than the air system can absorb, repeatedly tripping the limit. Verify manifold gas pressure against the rating plate specifications.

See the test procedure for each cause

Frequently asked questions about 4 FLASH

What does 4 FLASH mean on a Coleman furnace?
A 4 FLASH code on a Coleman furnace means a limit switch has opened its normally closed contacts, which is a safety response to excessive heat around the heat exchanger. The furnace shuts the burners down to prevent damage. Common causes include a dirty air filter, a failing blower motor, incorrect blower speed, restricted ductwork, or an incorrect firing rate. The furnace will continue to attempt restarts and flash this code until the underlying cause is corrected.
Can I fix the 4 FLASH error myself, or do I need a technician?
Some causes are straightforward DIY fixes. Start by replacing the air filter and verifying that all supply and return registers are fully open. If those steps do not resolve the fault, diagnosing blower motor performance, blower speed tap wiring, duct system static pressure, and gas firing rate typically requires test instruments and technical knowledge. Those steps are best handled by a qualified HVAC technician.
Is it safe to keep running my furnace when it shows a 4 FLASH code?
The limit switch is a built-in safety device designed to prevent heat exchanger overheating and potential damage. When it opens, the furnace shuts the burners down automatically, so the immediate risk is controlled. However, repeated limit trips over time can stress and crack the heat exchanger, which is a serious safety concern. You should not ignore this code or attempt to bypass the limit switch.
Will resetting the furnace clear the 4 FLASH error?
Cycling power to the furnace may temporarily clear the fault and allow the furnace to restart, but if the root cause has not been addressed, the limit switch will trip again and the 4 FLASH code will return. A reset is useful for confirming whether the fault is intermittent, but it is not a repair. Correct the underlying airflow or mechanical issue to permanently resolve the code.
How much does it cost to repair a Coleman furnace showing a 4 FLASH error?
The repair cost depends entirely on the cause. Replacing an air filter is inexpensive, typically in the range of $10 to $30 for the part alone. A blower motor capacitor replacement generally runs $50 to $150 in parts plus labor. A full blower motor replacement can cost $200 to $600 or more depending on the motor and local labor rates. Duct system modifications or gas pressure corrections may add further costs. A diagnostic service call is usually required to pinpoint the exact cause before pricing the repair.

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