The thermistor is bad. Believe me - I know this too well. Bought 2 of these and both died w/ this same issue right after 1 yr warranty expired. The thermistor is located on bottom-right side of evaporator coil. It has a chrome metal clip that holds it against the evap coil copper tubing. It also has a 2-wire harness that runs out of it up to the front-panel control board. Beware that if you had a big ice-up like this, there is potential damage to the power control board too. It will get a puddle of water under it when the ice thaws and this will cause this circuit board to corrode. If your power control board isn't ruined (power switch works and no other power strangeness like can't turn power on/off unless you pull the cord), you could just order another front panel control board (the one w/ all the LEDs/controls/7-segment displays on it). It comes w/ all the sensors (including the thermistor). It was ~$38 when I purchased it at RepairClinic.com (twice...).
You need to have the r22 charged... As stated above, if its low on r22 then it will ice up... no dehumidifier should ice up.... call a repair guy or better yet, buy another one... good luck.
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