Rule out settings and contacts first
Check the AF/MF switch on the lens (and the body) is set to autofocus — it's easy to knock to manual. Make sure you're not too close to your subject; every lens has a minimum focus distance and will refuse to lock inside it.
Power off, remove the lens, and gently clean the gold electrical contacts on both the lens and the mount with a dry cloth. Dirty or worn contacts are a very common cause of intermittent or failed autofocus, and of 'lens not recognised' style errors.
Hunting, grinding or no movement at all
If the lens hunts back and forth and never locks, try it in good light on high-contrast subjects — AF systems struggle in low light and on flat, textureless surfaces. If it still fails across different scenes, the AF motor or a flex cable inside the lens may be at fault.
A grinding noise, or a lens that won't move at all, usually points to a mechanical or motor fault (USM/STM and similar). This is an internal lens repair, not a home fix.
Front/back focus and stabilisation
If focus locks but images are consistently soft — slightly in front of or behind the subject — that can be a calibration issue between lens and body rather than a fault. Stabilisation rattle or an 'IS/VR/OSS' error is a separate internal fault we can assess.
Quick checks before a lens repair:
- AF/MF switch set to AF on both lens and body
- Subject beyond the lens's minimum focus distance
- Lens and mount contacts cleaned
- Tested in good light on a high-contrast subject
- Note any grinding noise, error message or soft-focus pattern