System too lean can be low fuel pressure, fuel filter dirty, fuel pump weak or dirty pump sock/filter, dirty/restricted fuel injectors, MAF contaminated, exhaust leaks near O2 sensor, EGR gasket or diaphragm leaking . The P0500 speed sensor code could be a wiring/connector issue or the sensor itself. Check and clean the connections first.
SOURCE: 1999 ford taurus emission reading p0171 and p0174
just tune up ur car periodicly, clean filter. clean ur sparkplug.
SOURCE: p0171
I would assume that it has no vac leaks since you took off and put back on the manifolds. I would check the mass air flow sensor to see it is dirty. If it is clean it.
SOURCE: poor idle dtc P1151, P1131,P0174, P0171. runs ok
Change plugs, and Mass Air Flow sensor. This should do the trick
SOURCE: codes p0171 and p0174.help please.what is the most
The fuel filter is restricted,or the fuel pump is weak.
SOURCE: check eng light on codes p0171 and p0174 what do
If you do not find an obvious problem in the vacuum hoses or PCV connection, I would sadly suggest you prepare yourself for a new lower intake manifold gasket. Those two codes combined indicate that the leak is not in one spot, but across the rear of the intake. This 4.2 V6 is known for this failure, and many TSB's have been issued regarding it. If you don't want to spend hundreds replacing sensors and getting your ECM rebuilt just to have a truck you cannot get inspected, spend the $90 for a smoke test on the vacuum system. I wish I had done that two weeks ago. I will be changing the gasket this weekend.
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