The imaging unit need to be changed but it was just changed

When a new Imaging Unit is installed there is a fuse contained within the new unit that blows and resets an electronic counter in the machine. If you installed or had installed a new imaging unit, then the only way the machine would be asking for a new imaging unit now is if either you ran about 70,000 copies or somehow the new unit did not reset the counter. I have never heard of one not resetting the counter as at the same time the counter is reset, the machine initializes the new unit and adjust the ATDC ratio. If this had not occured the mixture of toner to starter (developer) would not be correct and the copies would either become light (undertoned) or have a dark background (overtoned).
If you had the new unit installed then I would contact the service company that installed it and ask for a replacment. If you installed it yourself then there may not be much you can do. You can order a new reset fuse for around $5 and replace the fuse in your imaging unit so the machine thinks there has been another unit installed. precisionroller.com has them. Just go there and enter "Di350 fuse" in the search box. In case you were not aware, Your NEC is made by Minolta and is indenticle to a Di350/351.
Those would be your options.
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WheatlandServices