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The flap lock mechanism is at least in my Polo a simple pneumatic pump. Remove the side of the trunk liner behind the fuel flap. It should be an pneumatic pump there with an air hose to the lock (the lock is a pin which is driven thru a wedge on the flap's inside.)


The engine ECU uses the readings from these sensors to get the fuel to burn as weak as possible while still being efficient and keeping the burn temperatures safe for the engine. If the MAF and O2 sensors are not giving valid signals, the ECU will have to err on the side of caution and make the fuel mixture rich to ensure that the engine cannot be damaged ...


It's the pre-cat sensors that are used to determine the mixture departing the engine, the post-cat ones are there to monitor cat function. Since the fuel trims adapt to maintain the value at the pre-cat sensor, it'll look good, if it's producing a fairly rational but off-calibrated to the lean side signal, even though the real mix is too rich, per the plug ...


Sorry for the late update, and thanks for all the help. We finally got the issue fixed: injectors. The old ones had basically no flow at all, except for one, which had way too much flow. I think this weird combination was what contributed to it being so hard to diagnose from the logs. In hindsight the massive boost to fuel was a big sign, but I only ...

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