ERROR with SDT-420 Level Transmitter!

Hi Everyone, Someone have experience, please help me to slove this problem.
The problem: I have a level transmitter SDT-420, i set a range 0-400mm for it. And when it is in environment without water, i do zero trim for it. Now i can see some values on HMI of transmitter like:
level: 0 mm
current: 4 mA
LRV: 0mm
URV: 400mm
however: output current (AO) only 2.89mA so I can’t connect to HART 475 (by current under 3.5mA), and to connect to HART 475, I must reverse current from 4-20mA to 20-4mA and i saw 16mA show on display HMI of transmitter, after that i try to connect to HART 475 to do D/A trim. but i can’t because when i do, Hart 475 show alarm Disconnect device.
Anyone have any ideas or have experience to solve this problem, please help me by comment under.
Thank you so much.

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