Whip the cover plate off the injection pump body and check that all pumps are moving. Then give it a workout and see if it clears up. Pressure in middle of range is not a bad condition, it just means that it's time to check filters etc. Mine barely registers but still runs ok. My understanding is that Cat intended for the machine to continue to run fine as the filter clogged but the gauge would enable the operator or maintenance guy to easily see when it's time to replace the filter. Of course, a worn transfer pump etc. would also be a source of low(er) pressure readings so I'd imagine that the serviceman's reference would also make the suggestion to check that component and repair / replace if found lacking.