I have been in auto parts stores that have a "tester" on the counter. It is used to test key fobs. A simple tool that lights up when you press a button on your key fob. It checks to see if you have a output singnal.

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2009 CLS550
2005 X5
2004 ML350
2002 X5
1998 323ic (sold)
1996 S320 (sold)
1991 190 (sold)
1989 300SE (sold)
1984 SD (sold)
1984 300D (sold)