I was also getting an occaisional idiot light on my '98 at startup but found that a small leak at the t-stat housing was causing the belt to slip. I replaced the t-stat, seal, updated housing and, while I was at it put a brand new belt and tensioner spring on and it's been over a year with no lights. You may want to make sure your belt is in good shape, properly tensioned and most importantly that you don't have a leak at the thermostat housing.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz