looks like I'm sending it back! thanks for all the suggestion, hopefully the replacement will work

sadly not

I dont have another camera, no. this is the only lens not working correctly, and I have cleaned the contacts already. I suspect it is beyond repair. just taken it out in bright light and it was shifting wildly between iso 200 and 1600, f4 and f6 and shutter for 1/20 to 1/400, it doesnt seem to know how much light its getting. and the one shot i managed to get it to take was almost completely dark at iso 800 and 1/60. its also constantly switching for af.s to MF on the display

I have the PLM version. I have updated to the latest firmware and that didn't make any difference.
it is the motor in the lens that twitches constantly but wont focus. switching the camera to MF doesnt stop it

My 18-135 works perfectly, but the DA 55-300 will not focus with any settings, i have tried multiple combinations and a factory reset. The in lens motor works constantly but will not focus at any length in auto or manual. when the camera is set to manual the lens continues to flicker between manual and af.a or af.s. the manual focus ring has no effect. This is a used lens which worked perfectly on the camera in the shop. all other lenses work on my camera. it does not work with either view finder. am i missing something?