When the SUN2000 inverter has an alarm, the alarm ID is 400-21 (system failure – grid relay failure), which of the following troubleshooting suggestions are correct? (Multiple choice)

When the SUN2000 inverter has an alarm, the alarm ID is 400-21 (system failure – grid relay failure), which of the following troubleshooting suggestions are correct? (Multiple choice)
A . Disconnect DC input switch, AC output switch, close after 5 minutes
B . Try to upgrade the software to solve
C . Check if the AC line has insufficient insulation to the ground.
D . If the above method still cannot be restored, replace the whole machine.

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Answer: ABCD