I have a 2001 ford f150 4.2 6 cylinder... cylingder 3 Missfiring and system running to lean bank 1... Im so stumped the computer is throwing two codes P0171 & P0303.. I replaced coil pack, spark plugs, wires, passenger side cat, all oxy sensors, mass airflow, im about to do the fuel filter tomorrow... anybody have any sugestions??? please help

 

Mr Roberts,
Your off to a good start making the parts house rich..
For a single cylinder misfire code, why did you replace a Cat, MAF sensor and all Ox sensors?
The 303 code is specific to one cylinder.
All the replaced parts are common to all cylinders. The cat to all 3 cylinders that bank.
Fuel filter is not the trouble either being common to all cylinders. Leave it alone unless you want to change it for normal service.
The 171 code is fuel table shifted out of limits. This code is usually caused by an air leak the OX sensor detected. It's not the Ox sensor.
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Is the misfire constant at idle?
Have you inspected all the rubber parts and hoses?
These engines have a habit of leaking air from the intake manifold hold down bolts/gaskets.
Use Brake cleaner in a spray can to see of you can locate the source of the leak while the motor is idling for an idle speed reaction.
The misfire may or may not be related.
You have to get a lead on the issues first to sort them out.
Don't replace any more parts until proven they are needed.
Good luck.