In the upper 30s and low 40s notice frost on the outside unit and the fan will be off and the unit will hum all day and night with the fan kicking in once in awhile. The system used to make a noise of sh when going into defrost but now doesn't.

If the system spends more than 15 minutes in what seems like defrost you may have a problem. Can your tstat indicate defrost?

A tech was out here and replaced the defrost sensor for a 3 straight year and has ordered a control board. I told the tech that the unit just continually runs with a hum and never shuts off unless we get into the upper 40s. He had to jump to make the unit make the thursting sound when it goes into defrost but once in stays and never comes out. I had pass to tech that first year in the house unit froze outside and was discovered that the defrost was not wired in. Same year defrost problems, second year defrost problems sensor replaced, control board outside replaced and thermostat. Here we are in the third year and the defrost sensor replaced twice and digital thermostat installed. At least the heat turns off when temp is reach but the outside unit continues to hum. Tech is supposed to be out here tomorrow to replace the control board again. Isn't this thing under some kind of warranty? To have all these problems is a pain and time off work.

I do not know what a TSAT is unless it's the sensor?

Tstat is short for thermostat. Depending on age the unit could be under warranty. Only way to find out is contact the manufacturer.

Your problem description is not clear but it sounds like the unit is stuck in defrost. Correct? If so, I would likewise suspect a sensor or the defrost board.

Your right it sounds like its stuck in defrost and I call and found that parts are still under warranty 5 yrs the unit is 3 yrs. I am hoping when the tech arrives tomorrow to replace the control board on the out side unit that corrects the defrost problem since the sensor was just replaced 1 week ago. The tech stated that after he replaced the defrost sensor if it continued the control board needed to be replaced. That the unit shouldn't be humming all day. The funny thing I guess was another tech thought it was an old unit and I stated the MFR 6/2005. I guess Trane doesn't make XB10 anymore.

BTW I replaced the old mercury Thermostat with a Honeywell 7400 series digital.

If as you mentioned this is not the first board replaced in 3 years I would have the tech check power to the unit. Specifically the 24VAC transformer that is powering the boards. I would be suspicious you are somehow slowly frying parts.

Tech came out and everything check out OK with the exception of some sort of external temp for the defrost I guess. It was reading 42 when the actuall outside temp was 28. That was do to the sensor being to close to the hot copper line. Once relocated the system seems to be operating normal and I have been checking it and the frost and ice has stopped. Tks for all your help.