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So I was told during a check-up at a Jiffy Lube oil change that their analysis turned up a failed alternator giving 12.1V. Given that Jiffy Lube doesn't replace parts and thus gains nothing if I do, I would be inclined to agree.

However, I want a second opinion on this. Is 12.1V for a Toyota Avalon '97 considered failing by spec? What is the spec range?

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I can't say how they did the test but the test shows a problem. At idle you should see somewhere around 14 volts. There are issues that can cause a good alternator to have a low output such as a slipping belt, questionable grounds, wiring or a bad ECM (engine control module). If you have a dash mounted voltmeter see if you notice any changes in the gauge with engine speeds. You have some options, you can remove the alternator bring it to your local auto parts store that does component testing at no charge. If it is bad you can pick one up and install it. I know Sears used to do charging system checks for free in the hope that you'll buy a battery.

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