Last year, the VMSpc paid for itself by showing me the lack of power the engine was experiencing was due to lack of turbocharger boost pressure. I confirmed this by hearing an exhaust leak. We were in Alaska at the time, so it was *GREAT* to know what the problem was. This year, we did all of the east coast, from Prince Edward Island all the way to Florida. While heading east from Las Vegas, our home base, I noitced the oil pressure reading on the Cummins ISB was jumping around on the VMSpc, from the normal 55 PSI to 103. Everything behaved normally, so I figured a sending unit was going south. FWIW, the dash gauge continued to be as worthless as it usually is, as it did not indicate any fluctuation at all. Eventually, the sending unit got worse, and the information it was sending to the engine computer was either -0- PSI or 103 PSI. When it drops to -0-, the computer cuts back on fuel delivery about 30%. I know this because of the VMSpc, of course! You can feel it on level ground, but it's not obvious. Now, after the computer has a -0- reading for more than about 5 seconds, the 'stop engine' light comes on (this is in addition to the 'check engine' light which comes on when the oil pressure first gets to -0-), and the fuel rate is reduced again. This time fuel delivery is about 1/3 of normal volume, so you have about half of regular boost, and effectively no ability to go up grades or maintain more than about 40 MPH into a 30 MPH head or crosswind. Again, thanks to the VMSpc, I know exactly what is happening. More importantly, I knew I did not need to pull over and pay giant $'s for a tow, lodging, etc. to have a sender replaced which was merely malfunctioning periodically and could be put off as long as I knew what was actually happening. Kelly '99 Bounder Diesel 39Z w/106K miles on it