Because you chassis is less than 20 years old info on testing should be available from Freightliner's free help line .
No reading from the gauge would point me towards the gauge its self but if you don't get a low air alarm ; someone may have disconnected it .
I have diagnostic info for the VDC and LBCU systems, in use before and after, but not the MMDC.
Have them send you an air system diagram too.
Have your serial number handy and call ;
Freightliner help line .
Ask about joining DTNA Connect so you can get wiring diagrams and schematics , and part number info linked to your serial number .
THEN keep detailed notes , as F/L is slowly discontinuing support for the older chassis . My DTNA Connect access was terminated in May.
If you do get any diagnostic info , I'd like to have a copy .