The TA-29 thermister can fail due to either mechanical or chemical damage

Mechnical failure is often due to 'crunching' of the thermister or, more commonly, breaking of the leads from repeated flexing. Chemical failure can result, over time, from exposure to strong electrolytes (e.g. potassium solutions) found in most biological media. We've not found a thermister material that's totally impervious to these ions and while maintaining good thermal characteristics. 

Once a thermister fails, your options are to buy a new TA-29 cable assembly (order number 64-0107), or to soldier on a replacement TS-70B thermister (order number 64-0270).

The latter option requires a slight amount of work, but is less expensive.