Take out the smoke unit, disassemble it and scrape out any old, hardened pellet material. Clean the entire unit with Naphtha.
Make sure the air hole through the top of the cylinder into the bottom of the heater bowl is clear.
Make sure the heater element is intact with no broken wires, and no wires touching each other. Install a new smoke unit liner.
Reassemble the unit and assemble it to the loco. Apply power to just the heater, add a pellet or liquid smoke fluid, and move the smoke lever by hand to see if the unit smokes.
Larry