Re: P0171 code
How are the plugs on that side? The wasted spark ignition runs one bank of plugs backward and quickly burns off the center electrode on that bank, especially if you used something like a Bosch fine wire - you might get a couple of thousand miles out of those if you're lucky.
That side will misfire under load, and the ECU doesn't always see the misfire. The O2 sensors don't detect unburned fuel, only excess oxygen so they see the misfire as a lean condition. If it gets bad enough the other bank may begin to misfire too.
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