Sure you can just buy the temp sensor alone which is far cheaper.
Get an OBD in and see what temp the engine is running at.
When in traffic on a hot hot day the temp will get to 90c, anything over and the fan will kick in till it drops to around 86c / 87c... That is what mine and many others do.
If you have an iphone, you can download an app "engine link" and then get a bluetooth OBD II from ebay. I got one and in a single use it paid for its self. Better than paying a garage £30 / £40 for the obligation.