My D3 2005 SE TDV6, 155k, recently started to exhibit error messages, such as "Special programs not available". Then the radio stopped working! The bi-xenon headlamp shutters would intermittently stick open - on high beam. The cruise control would intermittently fail to engage ... but no other obvious problems!

My heart sank. I thought there must be a ground fault or other connection problem somewhere and I prepared for the long hunt. I started at the battery - terminals clean and tight. Thought I would check the battery - 12.5 volts, sight glass was green, charging rate 14.5 volts, so all seemed ok.

Ran out of time for more than those steps.

Next, today, on one of the first cool mornings for the season, the car would not start. No problem, jump started from the auxiliary battery (Traxide set-up).
Drove to various meetings - started OK during the day. Tonight checked the battery - still reading 12.5 volts, and the sight glass still showed green. Even though everything tested out OK, as the battery was 3 1/2 years old, I arranged to replace it.

The very helpful RACV mobile battery service technician quickly established that the battery had "surface charge" of 12.2 volts, but could produce only 325 cold cranking amps rather than the expected 900 CCA.

.... so new battery installed ... and all the faults have miraculously gone.

My experience suggests that if your car develops niggling electrical faults and your battery is not quite new then replacing or at least properly testing the battery might be a good place to start.

(PS The new battery was rated at 900, but tested as producing 1328 CCA!)