Tunze ATO not working

[Coralgurl]

I noticed yesterday the water level in my sump dropping, checked for leaks, nothing. My ATO shows the water level low but isn't pumping water.

What should I be doing to ensure its working ok? Over the last few weeks it's been pumping too much water, setting off the alarm, now it just seems to have quit. I bought it used, was told it had only been used for 3 months prior and it's been on my tank for about 5 months.