During the speaker test the speaker being tested is the only one that's on. If your speaker wire termination is simple bare wire a strand could be touching the wrong speaker terminal on the rear of the receiver. Use a flashlight and look closely.
This would be a good time to unplug the power cable and reseat every connection on the back of the receiver.
This would be a good time to unplug the power cable and reseat every connection on the back of the receiver.