Volvo EC240B High Voltage Error Code 21

Recently my 240 started displaying ER21 which I have learned is a "Voltage" Error. The LED readout on the panel says that the voltage is 38v (give or take a volt). Obviously well above the 24-28v range that it normally is. The Volvo dealer has told me that it could be a major problem and high voltage like that could burn out one of the computers (Ecu) and not to run it. I had the guys swap out the batteries because that is pretty simple and inexpensive, but it didn't change. Could this be a bad alternator? What would cause such high voltage? Thanks! -Matt