An auxiliary gauge may be required for testing older Chrysler vehicles with Evaporator Pressure Regulator (EPR) valves or Fords with Pilot Operated Absolute (POA) valves. It can be either a separate gauge used in conjunction with a two-gauge set or an integral gauge in a three-gauge set. The 0 to 150 psi reading provided by the gauge will be within the same range as shown on a low side gauge. Therefore, the auxiliary gauge should be the same type as used for low side testing. On older model Chryslers, the gauge is attached to the additional cylinder head inlet fitting on the compressor. On Fords, it’s attached to the suction fitting.