45 MBCA Ductless Unit and the “CP” Error Code
June 12, 2025
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Most ductless products have all of the error codes listed in the installation and service manuals. Error codes listed in
When you are servicing a furnace whether it is for seasoning cleaning or diagnosing a no heat, sometimes the furnace will stay off due to a “time delay” Code 12. Although this off time is only 90 seconds, it feels like an eternity!
What is Code 12, and how do we prevent it?
Every modern furnace control board has Logic built into them; this means they are capable of processing inputs to decide the needed output. These logic centers need 24vac to power up. If the board voltage drops below 19vac OR if the voltage is gone (zero), then upon the voltage rising above 19vac, the logic center will reboot.
When the logic center reboots (while the furnace has a 24vac call on W) it does not know if the burners were burning prior to the voltage drop. So, to be safe, the logic will run the blower only for 90 seconds instead of firing the burners and entering the blower off delay (as a normal call for heat would do). THIS is what a code 12 is (blower on after powering up). Without this safety, in the event a blower delay was initiated while the heat exchanger was already hot, then the chances of burning a hole in the heat exchanger would be high.
Code 12 prevention:
***This should only be done if you know (for a fact) that the heat exchanger is not hot***
When you power down the furnace during troubleshooting, remove the wire off the W terminal, apply power back to the furnace, THEN put the W wire back on the furnace board terminal. This will make the logic see 24vac on W AFTER the power turns on.
Since the code 12 is only initiated when the logic sees 24vac on W at the same time power is applied to furnace logic, this will allow you to bypass the 90-second delays that slow you down.
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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