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2008 hino 268 J08E coolant temperature issue

Diesel tech94

New Recruit
Hoping someone has some insight. Taking job over for a client after previous techs

Have a 08 Hino 268. Driver stated temp went in red zone on gauge while driving. Customer brought it in and changed rad , thermostat, temp sensor, to no result. They swapped instrument cluster thinking that the issue , and swapped ecm, still the same. I went to the truck,started and ran. I held the accelerator to warm unit and temp went to red on gauge after about ten minutes at 1500rpm. When I let off accelerator , the temp went down to midway on temp gauge. When I checked coolant at this point , the coolant was warm at best. I then blocked off the rad to get temp to increase and when it was hot the fan came on. As far as I see it , it is reading temp as the fan engaged , but the temp gauge is showing overheat when it isn’t and responds with throttle.

any advice or suggestions much appreciated !