If the K&N filter was over oiled it can kill the MAF, you can try cleaning it with MAF cleaner and see if that helps... but I am not a fan of K&N filters, I have seen them do more harm then good unless oiled correctly.
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When working with GEMS vehicles. Never try to locate a misfire by pulling plug leads off - pull the injector connector instead. Otherwise you can blow up the ECU.
Secondly never swap ECUs between vehicles to test them, because both vehicles and ECUs may stop working, and you'll have to have them all reset by a franchised LR dealer (ouch!).