The alternator reads 0 on a voltage meter. The car is still running with a three year old battery.
I need to know where you’re testing the voltage and what problem you are having. A proper way to test the for voltage is at the battery terminals with the car running. When the car is running you should read 13.5 to 14.5 volts. If it is below that then you have a bad alternator if it above that or fluctuates then you have a bad alternator. A certified mechanic would be able to test if your alternator needs replacing.
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