OKI C5100 Error Code 540

[JerryF]

My yellow drum locked-up. So I obtained a 3rd party drum along with a toner unit. The printer (C5100) reported error code 540 (yellow toner sensor). So wondering if it were the yellow sensor, I swapped the black and yellow drum/toner units and the printer still reported error 540 (yellow),which I had temporary placed in the 'K' slot. So the 3rd party supplier sent a replacement drum and I moved the toner supply to their replacement drum and the same issue occurred, regardless of which slot it was placed. So they refunded everything and I purchased an OEM yellow drum, again moved the 3rd party toner supply onto the OEM drum and still get the same issue. The one thing that's remained constant is it's been the same toner supply unit I originally got from the 3rd party suppler, and it does have a window in the door-close lever. So before I spent more money for another toner supply, I thought I'd see if anyone here had any thoughts.