If the system uses an IVR. Use a voltmeter to test for regulated voltage at a point common to all the gauges. If the voltage is out of specifications, check the ground circuit of the IVR. If that is good, replace the IVR. If there is no voltage present at the common point, check for voltage on the battery side of the IVR. If voltage is present at this point, replace the IVR. If regulated voltage is within specifications, test the circuit from the IVR to the gauges.