once turned on, my photosmarts starts clicking and won't operate!

I have the same problem. I removed the glass so I could see what was happening. The scanner head moves OK but when first turned on drives in fits and starts until it reaches home but it doesn't know it is home so it continues to drive making the clicking sound as the gear driving the toothed belt slips over the belt teeth. If left it will probably will wear off the belt teeth. I don't see a limit switch nor anything that resembles a "I'm home" position sensor. Good opportunity to clean the glass inside but it won't he;p if I can't get the scanner to know where it is. Any offers or suggestions?

Found the answer on another website.  Take the light assembly off like you were going to replace it.  Turn it over and clean two strip mirrors on the bottom.  Also clean the underside of the copy glass where you see a white strip.  The mirrors and strip are how the light assembly finds “home”.  This fixed my printer.

The printer head may be stuck and trying to return back to its start position. Check it for obstructions and see if you can tell if it is trying to move and causing the clicking and keeping printer from being able to do anything else until it is where it should be.

I disconnect the mii engine and running the printer again, but the mini engine keeps on turning left and right.

That's why i,am thinking the mini engine is broken and need to be replaced. If the engine is still to buy/

There is either a timing belt or a timing wheel that sets the limits of the print head motion. If it is a timing belt, it would be made of metal with a lot of little slots, one or more may have dirt or debris that interrupts its timing. The same with the timing wheel, usually black and mounted next to the stepper motor that moves the print head.