Lennox XP17 frozen, no fan, error E421

Discussion in 'HVAC Heating & Cooling' started by Seavlad, Dec 12, 2020.

  1. Seavlad

    Seavlad New Member

    Dec 12, 2020
    Lennox heat pump coils are frozen, turned off and they defrosted, powered on and they froze again. Ambient temp in Seattle is above freezing. Control board has been previously replaced. Previous lockout problems have been fixed by power cycling, but never had frozen coils.

    E421 error code - W outdoor wire, mis-wire fault.
    Fix - clears once voltage not sensed on output or power cycled.

    Have turned off breaker but unit is still powered to point where error led is on.

    How do I fully power cycle? Where LED goes off, assume some capacitor is powering..??..

    Other possible DIY fixes? Competent in measuring voltage across terminals, need direction.
  2. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    Freezing coils means one of two things, 1) restricted air flow or the fan is not working or 2) it's low on refrigerant. Since you stated heat pump, I assuming you're in heat mode and the outside coils are freezing. Is the outside fan running at all or very slow? Voltages have nothing to do with it unless a fan is not running. To fully power cycle turn off the breaker for the air handler and the compressor.

    Page 14 explains how to reset the error codes.

    For restricted air flow, the outside compressor cabinet uses louvers whereby you cannot see the actual coil. Therefore, rain cannot wash away dust, lint, or debris off the coils. If the outside fan is working shut it down and remove one or any of the side panels it possible. check for air flow restriction and if there is then backwash the coils with a garden hose.

    The W wiring error code is the white wire from the thermostat (w) signals the air handler to go into heat mode. The white wire causes the control board to run the fan at a slower speed. Another wire from the thermostat (AUX) turns on the electric heating elements if you have any. The O wire controls the reversing valve for heat or cooling mode. Usually no voltage is placed on the O wire for heat pump mode.

    This is the second post I found that had a timestamp of yesterday and was not on this forum early today.
    Last edited: Dec 12, 2020
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  4. fitter30

    fitter30 Well-Known Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Is indoor blower running or not. Not -unit could be in defrost mode and reversing valve not reversing. If blower is running outdoor fan might be bad or not getting power or have a bad capacitor.
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