The brief
Mr Wong's Jetta was flashing a battery fault code, so we told him to drive straight in before the battery ran flat. That warning is the car telling you the charging system isn't keeping up, and if the alternator has failed, the car runs off the battery alone until it's dead and won't restart, so it's not one to leave. The alternator is the belt-driven generator that keeps the battery charged and powers the car while it's running. They normally last eight to ten years and need no maintenance, but they can fail early if something gets into them, and on this Jetta an oil leak above the alternator had been dripping onto it and soaked it, which finished it off. A soaked, failed alternator doesn't recover, so it gets replaced, and the leak that ruined it has to be dealt with so it doesn't ruin the new one.
The diagnosis
The battery fault code and a charging test pointed at the alternator, which had failed, and the eagle-eyed look found why, excessive oil leakage from above had been dripping into the alternator and soaked it, which is what caused it to fail. The battery, the belt and the wiring were otherwise fine. That's an alternator replacement plus addressing the oil leak above it, rather than fitting a new alternator under the same drip.
The work
The drive belt was released, the oil-soaked failed alternator unbolted and a new genuine VW-spec alternator fitted, the belt refitted and the tension set to spec, the connections checked clean. The oil leak above it was traced and dealt with so the new alternator stays dry, the area cleaned up. The charging voltage was checked good with the engine running, the fault cleared, and the battery's state of charge confirmed. A road test confirmed the battery fault gone, the charging voltage steady, and the electrics behaving normally.
The outcome
No battery fault code, the charging voltage steady, the battery holding charge, the new alternator clean and dry, and the electrics behaving as they should. Mr Wong got the Jetta back with the charging sorted and the leak that caused it dealt with. Replacing the alternator without fixing the drip above it would only ruin the new one, so doing both is what makes the fix last.
