You may be right about the LED tester, macdrone. I was thinking that if it had a check engine light--and I know some do not--it would also have an LED tester, up until the end of 1994, when they were discontinued. My old 93 400E and existing 1994 E420 have a CEL and LED tester. In any case, I agree that building the tester should be done to pull any of the codes on modules other than the DM.

For whatever reason, in Arizona, the state emissions inspection authority did not require me to show them an operable CEL (verify that the CEL has not been disconnected) when they tested my 1993 400E or my 1994 E420.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles