Passat (B5) :: Temp / O2 Sensor Came On - Battery Is Damaged?

Man, do I long for the days of carburated engines and distributor caps. Problems were obvious and so were the fixes. About 2 months ago, my engine light came on. There were no symptoms of any problems. Plugged in VAG COM and got the codes. They pointed to a possible faulty temp sensor and a possible bad O2 sensor. I grabbed my multimeter and tested both. They checked out withing specs. I read about a problem with the OEM temp sensor and replaced it for $4.50. A new O2 sensor is about $125. I didn't feel like wasting $125 on a perfectly good O2 sensor. So I waited until I could afford to take it to VW to let it be their problem at $90/hour.

Last week, the car was hard-starting from what seemed to be a low battery. All the voltages tested fine, so I called a friend with a load tester to test it on the weekend. Before the weekend came, I had to get jumped once and pop-start a couple other times. I needed a new battery. After all, it was the original, oem battery.

48 battery for my 2001.5 Passat. This battery does not fit. It needs a group 91. This was a pain in the a&$ to locate and one expensive battery at $119 before tax and core charge.

The silver lining, however, is that replacing the battery seems to have made my engine problems disappear. I have dome the requisite amount of driving to reproduce an engine light, but it has not re-appeared. VAG COM reported that everything is A-OK. I would never have guessed to replace the battery to fix the codes I was getting.

Anyway, take note, I am not sure what lesson I leared here, if any, but it's worth having this knowledge if you ever run into this elusive problem.
BTW, I also undestand that you can fix a faulty headlight by kicking your Passat in the tailpipe.