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Hi, I am a Land Rover Technician, Sounds like you do some off-roading, good for you just remember to "tread lightly". We have had problems with aftermarket O2 sensors robbing power, If you have these installed you may want to have them replaced with OEM. They will not set a code but you will get reduced power and fuel economy. I do not believe it to be an electrical problem because you would hear the alternator whine if it was running full field and supporting a large drain. I noticed that you were using and aftermarket air delivery system (snorkel ect) Another problem we have seen is when people use K&N filters (or a similar oil base filter) The oil used on these reusable filters contaminates the mass air flow meter and caused a multitude of drivability concerns. It will kill a new sensor and as you know these sensors are very expensive. These also must be factory, these vehicles do not like aftermarket parts. These are really the only thing I can think of that will rob power but not set a fault code in the system.
2003 Jetta left rear window blue plastic part of the