Thank to all for your responses.

This question came from my experience trying to remove bolts holding a catalytic converter to an exhaust manifold on a Mercedes. I was having quite a time getting them loosened. I did eventually get them off after applying penetrating oil an letting them sit for 2 days. I just used a regular ratchet as I was concerned I might break the bolt with an impact wrench or twist it off with a 3/8" air ratchet.

My ½" drive Snap On impact is the IM 6500 HP which has a lot of adjustability as far as power goes; like 5 setting in the low power mode and 5 in the high power mode.

It sounds like I could have tried the impact at very low power setting to see if it would break the bolt loose. I think I'll try the impact in the future with small fasteners, although starting on a low power setting. Incidently when I said 13mm bolt I meant the bolt head was 13mm not the actual bolt.