P0455, P0420-Toyota sienna

Question

could both of these codes be from one cause, ie; loose fuel cap

Answer 1

P0420 Catalyst system efficiensy below threshold (bank 1 )

P0455  Evaporative emission control system leak detected  (gross leak)

P0455 can be a loose fuel cap,in fact that the first inspection, as for P0420 that will be a no, you would have a P0171 and a P0174 these are lean condition codes,