Charles Ellicott Commentary 2 Kings 16:12

"And when the king was come from Damascus, the king saw the altar: and the king drew near unto the altar, and offered thereon." — 2 Kings 16:12 (ASV)

The king approached the altar and offered on it. —So the Targum renders. But all the other versions: The king approached the altar, and went up on it (Compare 1 Kings 12:32–33). It thus appears that Ahaz, like Uzziah, personally exercised the priestly function of sacrifice.