extended warranty
A service contract that extends the coverage period beyond the standard manufacturer's warranty, often for electronics or appliances.
This computer comes with an extended warranty.
保証 is a general warranty or guarantee, while 拡大保証 specifically refers to an extended warranty that lengthens the coverage period.
Compound of 拡大 (かくだい, 'expansion') and 保証 (ほしょう, 'warranty').