I have a shower/bath that has low water pressure. I believe it might be in the shower bath control unit. ?

Question

How can I replace the control unit without going into the wall of the bathtub?

Answer 1

check any shut off valves leading to the shower...they may not be opened all the way. Most mixing valves have a replaceable valve body inside the handle but, normally when they fail it leaks.  

Answer 2

it is hard to answer this with out more details. usually the diverter control can be repaired by simply di-assembling the k**b. once this is done there is a valve that can be replaced. if you can get this far take these parts to a hardware or plumbing shop.  

Answer 3

Is there a wall behind the shower?  You can open that wall, to access the faucet, and piping. What brand faucet is it?  Is it a single handle?  Moen and newer Delta  faucets have a replaceable cartridge.  I would try that first, before opening any walls.