I have a 02 Malibu ls With 180xxx and notice my temp gauge in the summer is either above half or in the winter under half I was wondering if there is a sensor that could cause this to happen, it rarely stays at a constant middle temp range, or is it common for the temp gauge to be over the half way mark

I would've thought the PCM would display a constant temperature gauge needle as long as the coolant temperature was within the normal range. It may be either the ECT sensor/circuit or general cooling system efficiency issue including the thermostat and water pump. You can sanity-check the coolant temperature with an OBD scanner. The factory thermostat is a 195-degree one.

Well I change the water pump and the thermostat within the last year, could it have malfunction within the last year.

The ect sensor is located where and is it easy to replace?

It's located near the thermostat housing. The connector should have two wires colored ORG/BLK and YEL. It needs some thread sealant and torqued to 17 lb ft. That said, I don't recommend throwing parts at a problem. The ECT sensor problem is more often a wiring problem to boot.