Bubbles in the coolant reservoir usually indicate air or gas entering the cooling system. This can be caused by:

Air trapped after a coolant change – Often harmless and disappears after driving.

A leaking head gasket – Exhaust gases enter the coolant system; a serious issue.

Cracked engine block or cylinder head – Allows combustion gases to escape into the cooling system.

Faulty radiator cap – May not maintain correct pressure, allowing air in.

Coolant boiling – Due to overheating or low pressure in the system.

If bubbles persist or increase when the engine revs, it’s best to have the system tested for exhaust gases (CO₂ test) to rule out a blown head gasket.