AC Capacitors and Temp Sensor Test Fine but Fail During UPS Operation—Why?

I am working in UPS industry. Met some issues where the AC CAPS AND temp sensor were faulty.
However, when I measure their resistance value they are all within the range. It is just when during operation time it will fail.

For example for the Temp sensor I rub the sensor the resistance value will go down and when I let it cool down the value will increase.

I just wondering if anyone met this issue where when you measure the component is totally fine however when on system operation it will have cause fault on the system. Any expert out there can explain to me. Thank you.

What you are describing are subjective measurements.
What sort of capacitors are you talking about, film capacitors operating at mains voltage? If so, they must be X and Y types, rated for that application, and tested at the voltage that they will be used at worst case.
Same for the temp sensor, what are its specifications, and expected values under operating conditions. You need to monitor the temperature it is operating at, and check that it reaches a value that is well within what is acceptable for correct operation in the circuitry it is designed for, that is, you need to know the worst case tolerances in the circuit.
Cheers
Richard