My power supply tester says -5 volt is bad

-5v is no longer part of the ATX spec and was optional for quite a few years, take a look at your psu label, does it claim to have a -5v rail ?
Look at the 20/24 pin connector, is there a pin missing ?

January, 2002 is when the -5V specification was removed from the ATX12V Power Supply Design Guide.

The -5V rail was used for old-school floppy disk drive controllers and some ISA bus cards.