I think what he meant here was that since the oxygen sensor is in the exhaust system, which is after the cats it will only tell you if the exhaust gas is rich with fuel. The coil could have been damaged internally from the high heat and is no longer producing a powerful and 'complete' spark to burn all of the fuel/air mix in the cylinders before it is extracted out by the opening of the exhaust valve. A weak coil would leave an engine producing less power and give the impression to the oxygen sensor that the engine is running quite rich.

Other than that...

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)