Albert Barnes Commentary 2 Chronicles 17:6

"And his heart was lifted up in the ways of Jehovah: and furthermore he took away the high places and the Asherim out of Judah." — 2 Chronicles 17:6 (ASV)

His heart was lifted up - This expression generally occurs in a negative sense (Deuteronomy 8:14; 2 Chronicles 26:16; Psalms 131:1; Proverbs 18:12), but here it must be understood differently. The marginal note, “was encouraged,” accurately expresses the true meaning. He first set an example of faithfulness to Yahweh. He then proceeded to use his best endeavors to extirpate idolatry.

He took away the high places - Compare this with 2 Chronicles 20:33 and the note on 2 Chronicles 15:17.