My 98 ML320 would not start at first crank when already warmed up after driving ~100 miles but started on the second try immediately after the first try. This happened two days ago and did not re-occur untl today. The engine started fine when cold but would not restart after a short 10 minute drive. After spending an hour walking to get a spare key from my wife (her key is only 2 months old), the vehicle started perfectly. I drove two hours non-stop. After turning off the engine I immediately tried the old key...it did not start the engine even though it cranked as usual. With the new key, the vehicle started just fine.

Perhaps the old key has gone "bad"? Perhaps it has nothing to do with the crankshaft position sensor?

Any thoughts?

Thanks in advance.