Thermostats fail, so you'll have to get a new one if that is the problem. The thermostat itself is cheap, and usually easy to replace once the coolant is drained. I would check the thermostat first, because that's the most likely failure mode for the problem you're having. Obviously if the thermostat sticks open, the cooling system won't regulate temperature very well (too much coolant goes to the radiator) and often won't get up to temperature all the way. Usually that's an easy thing to diagnose though, because once the car is driven for a while and left running (sitting still) the temp will come up to where it should be around the time the fan will turn on.

If you can let the car idle for a long time in summer temps, and it isn't ever getting up to temp, it very well could be a coolant temp sensor issue. Coolant temp sensors can be tested pretty easily with a multimeter, because most of them are a thermistor type sensor meaning their resistance changes with temperature. So at ambient temperature (or close to it) the resistance can be verified against what a service manual says it should be.