2003 international secondary tank gauge issue

What model of International?
The instrument panel air gauges get their signal from the system controller. The system controller gets its data from two transducers that are probably located above the throttle pedal up in the dash.
I would suspect a connectivity issue or a failed transducer or a failed instrument cluster. You need to scan it. Diamond Logic Builder or aftermarket software should be able to point you as to where to start looking.
Before you start troubleshooting disconnect the batteries for a few seconds and see if that helps.
If you give me the last eight of the VIN I can look up the wiring diagram and tell you the wire numbers
for the transducers. Swap connectors on the transducers and see if the problem moves to the
other gauge. There are 3 transducers 2 air for air pressure gauges and one for the brake lights.

 

You may have to cut a few tie straps to get the transducers down where you can work on them. They should be tied up with the dash harness above the throttle pedal. They are about 3 1/2" long 3/4" in diameter with a 1/4" plastic push in connector on one end and a three pin metri-pack connector on the other end. There will probably be three of them one for the brake light/cruise circuit two for the pressure gauges. If you can't find them you may have to remove the instrument cluster.
Identify the transducers buy the wire numbers and swap the primary and secondary connectors and see if the problem moves from the secondary gauge to the primary gauge.
If it does then you have a failed transducer. If it doesn't then you need to find someone that can scan it for you as the next likely problem is the circuit board in the instrument panel needs to be replaced.
The circuit board is a bit pricey and you don't want to be replacing it if its not the problem.

 

The circuit board is available as a rebuilt exchange item from international. They are pricey up here in Canada. Is there a wrecking yard near you that might have a complete instrument panel?