Here is the full list of error codes for the SR-447HPC2, they are visible on the channel display.
E1：VOLTAGE LOW - Test the voltage at the power cord, it should be no less than 12v DC
E2：VOLTAGE HIGH - Your voltage should be no higher than 15v DC.
E3：BAND not valid
E4：TX SWR High - Check your antenna system
The radio below is showing an SWR of 1.3, if you are seeing E4 when you transmit, your SWR is too high.