Transmission wouldn't shift. Fluid low added to full mark per manual.

What is your question?

Few days later service engine light. Code said 28. Transmission pressure sensor. Had to add more fluid again. Where is fluid going. No leak noted or fluid on garage floor. 1994 3500 Chevy 454 engine.

How long has your 1994 Chevrolet C3500 had this problem?

A few weeks.