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Kevin Thigpen

Technical Services Manager North Alabama District

Why am I getting a 25 Fault Code?

When a Bryant furnace displays Error Code 25, it indicates that the control board has detected a configuration mismatch between the furnace model and the settings or components used to identify it. This fault commonly arises from issues with the model plug (PL4), incorrect DIP-switch configurations, or incompatibility in replacement control boards or firmware versions. Although the alert can appear serious, it is typically related to identification or setup errors rather than a mechanical failure. The following article provides a detailed explanation of the possible causes of Code 25 and outlines systematic troubleshooting steps to accurately diagnose and resolve the issue.  

What Does Code 25 Mean?

  1. Model Plug (PL4) Missing or Incorrect
    • Bryant furnace control boards use a model plug (sometimes called PL4) — it’s a small jumper/resistor configuration that tells the control board what model of furnace it’s connected to.
    • If the model plug is missing, or if what’s installed doesn’t match the furnace model, you’ll get this code.
    • In some cases, if the furnace was repaired and the control board replaced, the new board may not have the correct plug installed.
  2. DIP Switch Settings Incorrect
    • There are dip switches on the control board (sometimes labeled SW1). Certain switches (notably SW1-1 and SW1-6) are used for service/testing, and if they’re left “ON” in normal operation, it can trigger this error.
    • Sometimes both switches are ON when they shouldn’t be, which causes a configuration error.
  3. Wrong Control Board or Firmware Version
    • If a replacement control board was installed, but it’s not the correct type (or doesn’t have the right software/firmware), it can “think” it’s controlling a different model than what’s actually installed.
    • For example, manuals mention needing a modulating furnace control with a certain software version (e.g., V17 or later) or you’ll get this fault.
  4. Model Plug Changed While Powered
    • In some Bryant/VFD systems, if the model plug value changes while the system is powered, the board flags a “model plug changed” error.
    • The system may reject the change, or go into a lockout if the plug doesn’t match expected values.

What to Do (Troubleshooting Steps)

Here are some recommended steps to try and resolve Code 25:

  1. Power Down Safely
    • Turn off power to the furnace before touching the control board — safety first.
  2. Check the Model Plug (PL4)
    • Locate the PL4 “personality” plug on the control board.
    • Make sure it’s physically present and seated correctly. Some technicians say to remove it and re-insert it to ensure a good connection.
    • Confirm the plug matches your exact furnace model. If you have documentation/manual for your furnace, you can cross-reference the correct model plug.
  3. Inspect / Reset DIP Switches
    • Look at the SW1 bank of dip switches. Turn SW1-1 and SW1-6 OFF (if they’re on), unless they’re specifically needed for a test mode.
    • After adjusting, power the furnace back on and see if the error clears.
  4. Reset the Control Board
    • After making corrections, remove power again, wait a moment, then restore power. This can force the board to re-read the model plug and the switch settings.
    • If the code “25-24” or similar (model changed) was triggered, sometimes removing and reinstalling the plug + power cycling clears the code.
  5. Replace the Control Board (if needed)
    • If everything looks correct (plug in place, switches correct) but the error persists, it may be a faulty control board: either a bad connection/solder, or the board isn’t recognizing the plug.
    • Make sure any replacement board is correct for your furnace model and has the right firmware/software.

As you can see, there are only a limited number of issues that will trigger a Code 25 fault, and most of them relate directly to how the control board identifies the furnace model. Ensuring that the model plug is present and that its resistance value matches the specifications for your furnace is one of the most important steps in resolving this error. Keep in mind that on newer furnace designs, the traditional physical model plug may no longer be used; instead, the model information may be programmed directly into the control board. Because of this, verifying that the correct board and firmware version are installed becomes even more critical. By confirming these key components and settings, most Code 25 faults can be diagnosed and corrected efficiently.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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