temp gauge sits at 70?

[stuburg]

B6 2.0 tdi 140

Hi , please can someone help - as above my temp gauge only gets to 70, it starts at o and gets to 70 after a few miles and heaters seem hot, my mech has had a look and done some checks and says it is the dashboard gauge that won't go past 70,

can I swap out the dash gauge from another passat or will it need coding?

thanks for any help
Stu