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F26 is a "Door Switch Error". These machines will display this error under a couple of different circumstances.
If the door has not been opened for 3 consecutive cycles or the door
switch is open while the door is locked for more than 5 seconds.
Possible solutions are as follows -
Open and close the door.
Verify CCU operating by running a Diagnostic test or any cycle.
Door switch open while door is locked.
Push the door and check if it completely closed.
Verify CCU operation by running a Diagnostic Test or any cycle.
Check door switch contacts.
The resistance of both door coils (between pin 1 and 3) should be approximately 320 ohms.
You can remove the lower kick panel and open the cabinet up and see if you can locate the tech sheet inside. There is one in a plastic bag with more specifics and diagnostics to the left side usually.
Let me know what you find and if you would like more information.