It turned out to be the ECU!! If I had to "guess"...I'd say a solder joint must have cracked in the ECU went it was exposed to 4 degrees F. My car has always been in the sun belt, and usually only driven when it temp is above freezing. But it could just have been a coincidence.

PLEASE NOTE THAT THE O2 SENSOR LIGHT WENT OUT. 560SLs O2 sensor (or check engine light on '88 and '89 models) go off when the issue is resolved. 1985 and earlier models' O2 sensor light is illuminated when the car reaches a certain mileage (from a sensor on the speedometer).

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All the best,

J. P. Mose
1968 250SL
1970 280SE 3.5 Cabrio
1987 560SL
1990 560SEL