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2004 996tt Oxygen sensing left bank

Engine

Vehicle Information: Year: 2004; Engine (Size, Modifications): 996Turbo, K24, exhaust, GIAC; Total Mileage: 10,100;

Car has typical k24 mods, including awe exhaust, diverters, and GIAC programming. Car/Mods have been working perfectly without any CEL issues for a couple of years now. No further mods have been done. After a routine oil change the CEL started to light up after about two cycles. It may be cleared but always comes back on after two cycles with the "oxygen sensing left bank" fault. Car has been extensively checked and no leaks, damaged hoses, sensor wiring damage, etc. have been found. The MAF was changed also and made no difference. The CEL always comes on at idle after normal highway driving for about 20 minutes like when stopping at a traffic light; never has initially appeared with the engine under load. My question is if such a fault results in any power (mixture, boost, ignition, etc) adjustments by the computer. The car has not been dynoed to determine differences before and after the fault but I don't feel a change while driving except for the typical "mental" concerns and "I think I feel..." that bother all car enthusiasts. The only performance variation I can comment is sometimes slight hesitation when going WOT from mid-range rpm but I cannot assure is due to the CEL (spark plugs are still factory originals...). What specific sensor should trigger the "oxygen sensing left bank" fault. Can you direct us to any specific device or check.If the fault doesn't directly affect performance, I'm about ready to give up on the issue before continued "troubleshooting" of the car results in new problems being created by the techs... the car really feels great aside from the CEL remaining lit.

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