My original, 10-year-old sensor threw an error (flashing LED on the controller) so I purchased both a thermal sensor as well as a high-limit sensor since I read that the two should be replaced as a pair. They were easy enough to replace without draining the tub -- kind of a "finger-in-the-dike" sort of maneuver. I had purchased one o-ring with my order but the one for the other sensor (different diameter) was out-of-stock so I merely reused the old one which was still in good shape. The replacement sensors appear to be working perfectly and I have not had the problem reoccur. Be aware that these sensors might be a newer revision than the part you're replacing. I found that the photo of the high-limit thermistor was an old photo (all black plastic, without the longer, metallic sensor), but the part was compatible and I experienced no adverse issues. I'm very pleased with my Hot Tub Warehouse experience.