Maybe she will, maybe she won't

I have a 2003 Mercury Sable, 80k, that has an intermittent electrical issue. Sometimes when I try to start the car there is apparently no current in the system at all- not even enough to burn the dome light dimly. No clicks or noises. Just dead silence. After anywhere from a few minutes to a few hours the power is restored and will start and operate normally. I have owned it for less than three months. Early on it did it once for only a couple minutes, but in the last two days it has done it twice and was several hours before it came back on. When it came back on it did so instantly. The only clues I have are that the light socket seems to have a very small amount of current- a cellphone charger light glows during the failure, will get brighter and dimmer and then go out and this happens over and over and may be reactive to turning of the ignition switch. Also, the theft light on the dash still operates normally, going on and off at intervals to indicate the system is functional but not activated. The battery shows 13.5V at rest but indicates 66 percent under load. My mechanic suspects the battery but I am hoping to get a positive diagnosis so I can have enough confidence in the car to use it normally again. A lengthyt process of alternately being stranded and trying one expensive part after another and never knowing if it’s fixed is not optimal. How can the issue be identified?

Did your mechanic actually do load tests on the battery and alternator? This is a very routine thing to do and there is no guessing involved. In fact, many corporate chain-type auto parts stores do this testing for free and will install a new battery for you.

The first think I would do though is clean and inspect the battery cables - you have to hit both ends - at the battery and then everyplace it goes. You’ll also want to get a look underneath of the cable insulation to look for corrosion on the cables.

Yes, they did a load test on the battery. I seriously doubt that it’s the battery as even a weak battery will deliver partial power, even if insufficient to start the car, but there is virtually no power-not even enough to illuminate the dome light-when this malfunction occurs. Then it corrects itself instantly-not typical of a battery. Also, this occurred momentarily once while operating the car, during which time it would be generating its own power and not reliant on the battery. I think there is either a short, or more likely some type of malfunctioning relay, in the system due to the randomness of the occurrences and the instantaneous resolution without anything having been done to account for the reversal.

I agree about the battery. But one of the most common causes of unpredictable and intermittent power is actually just corroded / loose / damaged connections. And cleaning it all up certainly won’t hurt and if you just do it yourself it also only costs time.