Do you mean a problem with THAT particular Pioneer HU?

What's it doing that makes you think it's defective?  Does it come on at
all?  Did you install it yourself (if not, DEFINITELY take it back to where
you had it installed)?

If we are going to have any chance of helping you, we will need more info.
It's possible that it is not installed properly.  Pioneer tends to make
pretty reliable HU's.

MOSFET