Upon arrival generator was faulted for High Engine Temp. Tried to bypass the sensor but generator still faulted. While diagnosing the High Engine Temp alarm I noticed the generator controller displayed Low Battery. Battery was tested and even though the date is 2017, battery tested good. Customer declined new battery. It was determined the controller was faulty causing the Temp Alarm and the charging issue. Once controller was replaced we simulated an outage. Generator is now operating properly.