The batteries are the weak point, suseptable to moisture and cold, a good trick is to put the sensor in a ziplock bag and then put it in the fridge, I got two of the wireless thermometers from Bunnings at $9.95 each, 6mths on still going strong.
They are identical to the Companion/Engal jobbie, even use the same frequency, I have both, I put mine in a ziplock bag and both thermometers pick up the signal on the sensor in the fridge, having the sensor in the ziplock bag seems to have solved the early failure of the batteries..
At $9.95 you can't go wrong
Baz.