If you are lucky you will see a leak above the alternator right where the thermostat housing attaches to the engine block. I needed to replace the housing and thermostat. The total cost was under $50 for all of the parts and it took less than an hour to replace it. Mine leaked very slowly, when cold only, so it will dribble a few drops out overnight, enough, apparently to work its way onto the pulley and cause it to slip when the car was started up.

Like I said, it's been over a year and no warning lights since I did that work...hopefully you'll be as lucky as me.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz