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Hi, I'm Mike G and I'll be assisting you. I suggest testing the breaker pole to pole for 240V and each pole to the neutral bar for 120V each. Let me know your readings.
One more thing. Turn off all other breakers but not the main and retest. That is to see if the low voltage is a feedback.
Test the terminations of the incoming service wire with only the main breaker on. Then test the bus's after the main, pole to pole.
Bad breaker or bad connection. You'll need to check the service main if the reading are low at the sub feed main. Check before and after the main the same way.
Need to take it a step at a time. You're welcome.
Very good. You now know about feed back on a 2 pole when one leg is dead.