He should disconnect everything from the amp. No speaker wires, no cables except the power cord going in the mains.
1) If the error code 11 appears again when the amplifier is turned on or after 1h of idling, the amp has a failing component. Could be a simple fix for an experienced tech.
- measure DC offset on the terminals. Is it bigger than 25mV DC?
2) If no error appears, then it could mean the problem is elsewhere.
- there's a problem how the speakers are connected, arangement of terminals, out of phase, or bits of wire touching.
- the speakers that are connected are missmatched in impedance (never combine 4ohms and 8ohms).
- some of the speaker drivers have blown.
- speaker crossover component failed.
- also I'd test the amp with everything disconected, but attach headphones.