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Our toyota corolla suddenly had its alternator light show up on the dashboard. What is the most common cause of this?

First check is to make sure the alternator drive belt is still in place or not slipping.

Failing that, it sounds very much like the alternator itself has a problem.

There could be a number of reasons an alternator stops charging the battery, and most good auto electricians will want to dismantle the whole unit and repair/replace all worn parts.

In some cases they may even offer a complete exchange alternator as the most cost effective repair.