A bad electrical ground (earth) as can cause all manner of CELs so as your car ages make sure and check these. Pull them, sand them down and put them back on with a dab of low or medium strength Loctite so they never come loose.

From the Bentley manual:



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The one's I found in the engine bay, the ones most likely to rust, are:

In front of battery to engine a braided strap. Need to remove the headlight to access the other end. Mine looked ok....but showing age.






On the front valve cover for the spark plug coils.



3 of them on the driver's side in front of firewall...you need to remove the ECM and the fuse box needs to be unfastened so you can pull it out of the way. Its 4 10mm and one 8 I believe. Very easy.



Just behind the motor/trans mount for the 3 brown wires.





Also here is a TSB on a bad electrical ground related to Airbag, ABS and DSC; electric windows or other systems are not working; and there is no communication with various control modules. This might correspond to the two right next to each other in the pic above but I'm snot sure. The drawing below makes it look like they are under the wheel well cover.

November 2009
Technical Service

This Service Information bulletin supersedes SI M61 05 07 dated September 2007.

[NEW] designates changes to this revision

SUBJECT
Bad Ground Causes Various Electrical Problems

MODEL
[NEW] R55 (Cooper Clubman, Cooper S Clubman)
R56 (Cooper, Cooper S)
[NEW] R57 (Cooper Convertible, Cooper S Convertible)

SITUATION

Various warning lights are illuminated, e.g., Airbag, ABS and DSC; electric windows or other systems are not working; and there is no communication with various control modules.

CAUSE

Ground points X13004 and/or X4 are loose (not connected correctly).


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PROCEDURE

WARRANTY INFORMATION
Covered under the terms of the MINI New Vehicle Limited Warranty.

Please refer to the latest KSD for all applicable labor operations and allowances.

If the appropriate labor operation is not contained in KSD, then a work time labor operation should be used.

Defect Code Refer to KSD for the code that best represents the problem concerned.

If you know of any others in the engine bay let me know!