My wife took the car to do a alternator check and the guy said the alternator diode is bad. Then he suggested to replace the alternator.

What kind of problem will a bad alternator diode cause?
Do I need to replace the alternator right away?

thanks

A bad diode (or multiple bad diodes) will effectively reduce the alternator's output and will also make the electrical system more "noisy" because there will be a higher-than-normal AC component in the alternator output due to the missing diode(s). In addition to possible insufficient charge to the battery, there can be strange behavior of various devices in the car, such as dash warning lights that go on and off, poor engine performance etc.
I'd suggest replacing it as soon as possible.

sounds like the problem I've experienced with it.
I'll try to get it fix as soon as I have enough $$$ for it.

Thanks