Why does my HTC One X+ work only while charging?

My HTC One X+, now 18 months old developed a problem yesterday - unless the phone is connected to an AC power source (for charging), it switches off.

The battery indicator shows full battery power, and though the battery was showing signs of weakness, I'm sure the battery isn't completely useless as yet.

Has anyone faced this problem? Please suggest a remedy.

I'm sure the battery isn't holding enough of a charge to keep the phone on. Do you usually discharge it past 40%? If so, 18 months is a lot longer than it should have survived. (If you always discharge it until the phone tells you to charge it, 6 months is a long life. The longest life is charging at 50% discharge.)

Welcome to Android Central! I agree with Rukbat. Since the One X+ doesn't have a user-removable battery, your options are to get the battery replaced by a cellphone repair shop, or try do to it yourself, if you can find a YouTube video that shows you how.