Avoiding Good Faith Violation

A good faith violation occurs when you buy a security and sell it before paying for the initial purchase in full with settled funds. Only cash or the sales proceeds of fully paid for securities qualify as "settled funds."

Liquidating a position before it was ever paid for with settled funds is considered a "good faith violation" because no good faith effort was made to deposit additional cash into the account prior to settlement date.

It's not clear how you paid for the initial position. What were your settled funds prior to buying stock A on Monday?

It sounds like your settled funds on Monday were insufficient and you incurred a violation because you sold stock A before additional funds were deposited or prior sales were settled.

You sold stock A on Monday.

Therefore the cash will be settled by end of the day Tuesday (T+1)

You bought stock B on Monday with the unsettled cash from A, and then sold stock B on Tuesday, before the unsettled cash from A had settled (as noted above, A settles at end of the day Tuesday)

That constitutes a good faith violation, because you bought AND SOLD another stock using cash from a previous trade that had not yet settled.