2006 VW Jetta 2.5L P0016 Code: Is It Serious? Causes, Symptoms & Fuel System Cleaner Safety

Hi everyone, I'm looking for clarity on the seriousness of the P0016 code in my 2006 VW Jetta 2.5L and whether it should return after being cleared. I live in northern Canada, where access to VW specialists is limited. The car has been sitting for three years, with only 4-6 week drives during work breaks. For most of that time, the check engine light was on. Initially, I suspected a faulty coolant temperature sensor (which I replaced) or disconnected cooling fans — both of which were common in cold climates. While idle, the coolant temp reads high, but stays stable when driving. This worked fine for some time until it started running weak at idle and during acceleration. A scan revealed codes P0016 and P0480. I reconnected the cooling fans (though they don’t engage under normal conditions) and cleared the codes by disconnecting the battery for 10 minutes. After that, the car still runs poorly — with severe idle vibration, hesitation, and backfiring when revved. It shifts fine into gear but feels extremely jumpy during driving. I’ve checked spark plugs and coils; all appear normal. I suspect water contamination in the fuel system due to long-term storage. My main questions are: - Should a serious issue like a camshaft or crankshaft position fault have triggered the P0016 code again after clearing? - Is it safe to run a fuel system cleaner at idle mixed with high-octane fuel? Or should I prioritize addressing the original P0016 issue, even though it no longer appears in diagnostics? Any advice or insights would be greatly appreciated. Thanks, James