31a Do not use a comma with a coordinating conjunction that joins only two words, phrases, or subordinate clauses.

The examples read,

Ron discovered a leak (strikethrough) comma (end strikethrough) and came back to fix it.

The coordinating conjunction ‘and’ links two verbs in a compound predicate: ‘discovered’ and ‘came.’

We knew that she had won (strikethrough) comma (end strikethrough) but that the election was close.

The coordinating conjunction ‘but’ links two subordinate clauses, each beginning with ‘that.’