The actual sensor is built into the cpu. I'm not saying the sensor isn't damaged, but if it is, so is the cpu.
I'm thinking the the base of the heatsink and maybe the cpu heatspreader, got really hot and warped a little. Hopefully the heatspreader is ok. You can check by removing the heatsink and checking the base with a straight edge, corner to corner both ways. If you can see a little gap it's warped and it can warp in or out. You check the cpu heatspreader the same way.
This would be a good heatsink for your cpu. It's large but not huge like some of the newer ones.
Newegg.com - XIGMATEK HDT-S1283 120mm Rifle CPU Cooler - CPU Fans & Heatsinks