Thank you.

I have a motor that stopped working. The capacitor was replaced, and all is fine now.

It's a 50uF 440 VAC

I replaced with the same.

The old one reads 42.1uF

The new one reads 45.3uF

Will this difference cause the motor to not work?

 

Not a problem as long as you matched your voltage spec.

 

Maybe the old capacitor failed because its equivalent series resistance (ESR) became too high but you did not measure it.

The 9V battery in my smoke detector failed. I measured 8.5V with no load and 3.5V whith a 10k load. It failed because its ESR became too high.

A brand new 50uF electrolytic capacitor has a wide tolerance of from maybe 25uF to 100uF. If your new capacitor has a low ESR then it will work fine.

 

The old one reads 42.1uF

The new one reads 45.3uF

Reads with what? I have found it impossible to get a valid reading on an oil filled motor capacitor with a self powered tester. The only method that works for me is to put the capacitor directly across a 120 VAC or 240 VAC power line and measure the current. Then I can calculate the capacitance under (close to) the conditions it was built for.