My ML has had a check engine light on for some months and I don't know what is causing it

I am testing it without the air cleaner to see if that is the cause (dirty air cleaner)

The car is running perfectly, recent oil change; all fluids OK

What else could possibly cause that light ?? Need to eliminate it to pass TX inspection.

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Ben Harrison Carter
1999 Mercedes ML320 87K
1992 Mercedes 300CE 89K
1995 Corvette 29K -- Sold Dec 09
1989 Mercedes 420SEL 99K -- (Sold 4/08)
1968 Mercedes 230S (106K) (Sold 9/06))
1976 Mercedes 450SEL 130K (Just sold - 06)
1961 Mercedes 220Sb (sold years ago)