Have you looked at our exhaust flap? I don't have one, but I hear they rust closed...and there's some famous mod with a golf tee that I haven't heard about in a while...probably for a good reason! But, have 3 of 4 02 sensors with signal to ground problem seems odd...I wonder if it's because half your exhaust is closed...and maybe doesn't open up?
Worth a peak up them.
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