HELP! Weird F01 error on a Munchkin 80M

thehustla

Hello all,
Looking for guidance on a strange issue with an HTP Munchkin 80M.

Background / original issue:

• Boiler currently has a 925 control board
• Occasionally throws F18 (intermittent)
• Boiler otherwise runs, but F18 keeps coming back

What I did:

• Ordered and installed a 926 replacement kit/board hoping to eliminate the F18 issue
• After installing the 926 on this specific boiler, it immediately locks out with F01
• Startup behavior: blower starts, but it never attempts ignition (no spark/igniter action)

Key test / board is not bad:

• I have another identical Munchkin system in the house
• I swapped the 926 board into the other unit and it runs normally, so the 926 board is known good

What I’ve checked so far:

• Verified all harness plugs are seated / no obvious wiring damage
• Tried temporarily testing safeties to isolate a “pre-ignition prove” issue (no change)
• Still F01 on this one unit only with the 926 installed
• This same boiler will run on the old 925 board

Question:
What would cause a Munchkin 80M to run on a 925 but not accept a 926 upgrade, especially where the 926 board works on another identical unit?
Is there a known compatibility/config/jumper/sensor difference that would prevent the 926 from enabling ignition (F01 before spark)?

Info I can provide if needed:

• Serial number / revision tag on both boilers
• Photos of wiring at the control board + any jumpers
• Exact text definition of F01/F18 from manual (if needed)

Thanks in advance.

HomerJSmith

F01—Is the Safety Limit Flue indicator. First, inspect the flue for blockage. Second, check the condensate for blockage or frozen. Third, check the combustion chamber for signs of recirculation, or incomplete combustion (see F09, Fifth and Sixth)—-from the manual.

F18—Gas valve Fault. The gas valve wiring is not connected. Check wiring—-from the manual. My notes—Power company brown out-push reset S4 to reset boiler.

Although, the foregoing doesn't appear to apply to you, these are my thoughts.

I have had problems with the Molex connectors on the harness not conducting thru the jack to the pin. I have had to replace some of these jacks. Perhaps, the wire from the Safety Limit Flue sensor to the new board is failing. I would first check the sensor which is in the rear of the combustion chamber on the flue pipe. (Munchkin with spark ignition)—The pin on the X8 socket is pin 9 Block Vent Pressure Switch. Some- times the Molex jack doesn't make a tight contact with the board pin. You can follow the wire from the sensor on the flue to the nylon Molex plug. You can take a fine needle and push it between the nylon plug and the Molex jack on each side of the jack and close the jack opening a little. You can take a finger nail emery board and go over the pin lightly to increase the contact between the pin and Molex jack.