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P0171....System too Lean

This code can be tough to chase down. Your best bet would be to get a scan tool that can show live data and look at short and long term fuel trims. Positive numbers mean the PCM is adding fuel to compensate for a lean condition (which is highly likely with this code). Numbers in the teens or higher indicate a big vacuum leak or injector issue, single digits normally mean a cracked vacuum hose or small leak somewhere.

Have you done any work around the battery? There is a hard plastic vacuum line that runs from the intake, along the fender near the battery tray, then back to the firewall to supply the HVAC system. It is common for this to get bumped/cracked when moving the battery around, and it can cause a lean code if the crack is small. My best suggestion is to inspect all the vacuum hoses/lines/connections under the hood first before changing any more parts.