Troubleshooting why your heater won't heat the pool can be absolutely frustrating. If you are lucky enough to find out it's because of a bad water temperature sensor then you are in luck, because it was extremely easy to fix and you can save a ton of money by replacing it yourself. If you know how to cut the power to your system, use a screw driver to open your system panel, use a micro screw driver to release and replace the sensor connectors, use a screw driver to release the clamp over the existing installed sensor, use a drill to slightly increase the existing opening in the piping, then you should have no trouble doing this on your own. For reference, I am a 30 year-old male with only 3 seasons of experience working on his own pool.