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Munchkin F03 error

MunchkinG Member Posts: 3

I am receiving an F03 error on a Munchkin M140 Rev. 1 boiler.  This is supposed to indicate a thermistor problem.  I checked the Ohms on both inlet & outlet thermistors; both in the normal range per water temp.  I checked wiring continuity; no problem there.  I checked the DCV, which reads 6.3 DCV on each thermistor.  So, in theory, according to the service chart, everything checks out so I need to talk to the factory.  Unfortunately, HT Products is closed due to Hurricane Irene.   I'm stuck.  Any thoughts?

One note:  After reconnecting the leads to the thermistors, the boiler fired correctly, satisfied demand for DHW (SuperStor).  Previously, through the weekend, I could reset the boiler and it would fire, but then the F03 error code would appear and the boiler shut down, usually as it was going up in temperature.

Thanks for your help.


  • clammy
    clammy Member Posts: 3,114

    i have ran into a few of these and on all of them the  main compartment  of the boiler was quite dirty  mostly onstruction dust .After removing all componets ,wire harness i use compressed air to remove any dust or debrie from the connetions.Also the 3 that i have had these issues with where not regularly serviced and had very dirty heat exchanger and where running mostly hi temp .I had also checked the thermister resistance and they fell with the manafacture specs.Check all the connetions and  make sure they are making good contact .if your unit has not been serviced and cleaned regularly then get it done and make sure that the burner is set up with a combustion anlyzer.You can switch the thermister (supply and return ) to narrow it down .If the promblem continues and you are sure the thermister is good then be prepared you may have to order a new wire harrness been there done that .Hope this helps a little bit .Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • MunchkinG
    MunchkinG Member Posts: 3
    Munchkin F03 error

    Thanks for the advice.  So far, I'm waiting to see if just by having disconnected the wires, the problem will have been resolved.  Perhaps it was a matter of bad connections.  The boiler was serviced fairly recently, and the high/low flame are rectified correctly (so far as I know).  Probably worth scheduling another service before winter, when things are put to a real test.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669

    How is your boiler vented. Concentric vented???  Are the intake and exhaust separated at least 18 inches vertically???

    Reversion of exhaust gases will cause corrosion on the electrical connections and cause error codes.
  • MunchkinG
    MunchkinG Member Posts: 3
    Munchkin F03 error / venting

    I'll inspect the venting tomorrow.  I appreciate the tip.  I don't think the intake is 18" apart or above the outflow.  I suspect corrosion may be the culprit since having now disconnected and reconnected the leads to both thermistors, the boiler has not again failed.  If it remains in normal working order, I'm going to chalk this up to slight build-up on the connectors and thoroughly re-check them all.

    Is there an anti-corrosion spray anyone recommends?

    Thanks again for those who've contributed their help.
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