It could be the harness, but I would see what a different gauge says about the actual pressure you have. If a second opinion mechanical sensor says you have good pressure, it's sounding electrical. Of course I would want to test with the secondary gauge under a variety of circumstances to see if you can make it throw low pressure again.

You didn't happen to test the previous sensors did you? Your sensors might not be the problem, but what they hook to instead.

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2001 E53 3.0 5pd