Q: My 2001 expedition just failed the emissions test why with codes p0442,171,174? on 2001 Ford Expedition

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when emission test says a small leak detected with evaporative emission control system then bank 1-2 system too lean?
(1) Answer
you need a shop with a scan tool, smoke machine to find the failure in the evap system.
as far as the 171,174, it could be several things but once again you need a shop to check it for you and give you a bid. common issues are pcv valve, vacuum leaks, low fuel pressure and mass air flow failure.

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