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When this light comes on, most commonly this is a sign of a failing alternator or battery. You can check this yourself with the use of a hand-held voltage reader. With the engine running, take a voltage reading at the battery terminals. If the voltage is not between 13.4-14.4v, or is fluctuation in and out of this range, the alternator will need to be replaced. If the alternator passes this test, shut the engine off, and let it sit for 15 minutes. With the engine off, take another voltage reading at the battery terminals. If the voltage is anything less than12v, the battery is not accepting a charge, and will need to be replaced.
If you do not have access to a voltage meter, most local "big chain" parts stores will run these tests for you, for Free, right in their parking lot.
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Inspect the serpentine belt as well, with the engine running. Make sure it is not "slipping" over the alternator pulley. This will also cause an intermittent charging fault.