I bought a cheap OBDII code reader from Amazon for less than $20 to read SES. It's the Maxi-CAN 3000. For more involved diagnostic stuffs, I pull out my INPA/DIS laptop.
You can swap the coil like how the dealer did to test it.

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00 E39 DINAN S1 //M5 (82k miles)
06 E53 DINAN 4.8iS, born 2/18/2006 (126k miles) http://www.xoutpost.com/bmw-sav-foru...s-my-dslr.html
90 Straman Z32 TwinTurbo Convertible, (1 of 44 ever made) 5 speed, 444rwhp/451rwtq
01 360 Novitec Spider F1 (26k miles)