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Kenmore Microwave Displays Error Code Reading "Prob"

Within 2 seconds of "Start" my Kenmore model 721.8943580, manufactured 8/90, beeps, stops, light goes out, and LED display reads "Prob."  "Clear", reentry and "Start" bring same error. If hours or a day go by before restart, the unit runs fine for up to a minute or more before shutdown and "Prob" indication reoccurs.  Then, an immediate attempt to run the unit again results in the same condition in 2 seconds.
I use this old unit, otherwise in top condition, only for heating water for tea.  Spending big money on repair doesn't make sense.  Could the fix be as simple as a cleaning?  I've replaced bulbs and belts in MW ovens and I'm an experienced do-it-yourselfer with complex appliance and other repairs.

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  • Fred3x3 Aug 10, 2008

    Thank you, MicrowaveSvc!

    The obvious professionalism, completeness and rapidity of your response are both unexpected and appreciated.

    Some days had passed before I planned to remove the unit from its high shelf.  Before doing so, I decided to try to heat a cup of water, again, and much to my surprise, I've been able to continue to heat my tea water in the microwave for two days, as of now.

    I'm sure that these are not the kind of problems that cure themselves permanently and, at some point in the future, I may have to followup with your suggestions.

    If I should have to open the unit, I'd also clean its innards well to make make sure the efficiency of its internal ventilation system is not compromised, etc.

    My problem had occurred during particularly humid weather, but the microwave is in a space that is air conditioned in such weather.  Would we expect to find a humidity probe as well as a temperature probe in a microwave that's 18 years old?

    If the problem recurs and I open the unit, I'll let you know the outcome.

    Thank you again.



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I would bet that the temperature probe socket has some accumulated grease, etc. in it which may be changing the resistance of the contacts, making the control board think the probe is installed but defective.

First, please unplug the oven before you investigate.

The best idea is to clean the probe socket contacts.

You'd need to remove the outer cover, and the probe socket should be located on the right side of the chassis.

If you can cut thin strips of fine sandpaper and work them back and forth between the contacts, that's the best way.

You can plug & unplug the probe while observing the socket to see how the contacts work.

Your socket may be sealed, and that would possibly require you to carefully take it apart. That's a bit of a gamble, as it may crack due to age.

Another idea is to take a wad of paper toweling, hold it with your right hand over the back of the probe socket, and spray contact cleaner or WD40, etc. into the opening of the probe jack from inside the oven cavity with your left hand.

Then plug & unplug the probe several times, wipe any spills and try the oven again.

If the probe unplugs form the control board, you could disconnect it and try the oven.

Let me know how it goes.

Posted on Aug 02, 2008

  • William Miller
    William Miller Aug 11, 2008

    The parts list does not show a humidity sensor.

    If you do decide to clean the probe, I would just focus on that. As they say, "If it ain't broke, don't fix it!"

    You can find exploded views and order parts by entering your model number here at the Sears parts site.

    The temperature probe is shown in the "cabinet" sectrion of the breakdown.



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