I’m betting the low command is for demonstration only but it will turn your c.0 pin into an output. Port C pins start as inputs on powerup. You’ll need a DIRSC command before your PULSIN to set it as an input again.
Also, I’m not sure which sonar you’re using but a 50us activation pulse is pretty long. Most require around 10us. The following “pause 200” is not necessary. That’s almost a quarter second delay. You really want the PULSIN command to be fired up as soon as possible to catch close range objects.