However, therefore, and moreover do not join two sentences the way and or but do. They need a semicolon, or they start a new sentence. A comma in front of however between two full thoughts is a comma splice. Semicolons are covered in semicolons on ACT English.
The pattern
The bus was late; however, we started on time. Semicolon before the adverb, comma after it. The same sentence with only a comma before however is wrong. You can also split it: The bus was late. However, we started on time.
Words in this group include however, therefore, thus, moreover, consequently, and meanwhile. They comment on the relationship. They are not conjunctions.
Do not treat them like FANBOYS
And, but, so, and the rest of FANBOYS can join two independent clauses with a comma. That rule is in FANBOYS on the ACT. However cannot. If you are unsure, check whether both sides are complete sentences. The comma rules overall are in ACT comma rules, and clause types are in independent and dependent clauses.
The bottom line
Comma plus however between two sentences is the trap. Use a semicolon, or make it two sentences.
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