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I just finished reassembling after a head gasket job and am getting those codes. At first I had a p156 along with the 171/174 which is o2, bank 2, but after a day or two of driving around that code went away. At first I was thinking vacuum leak, however I am having no symptoms of a leak. The car does not stumble or bog down on acceleration, nor does it idle rough. I read that a bad O2 sensor can cause a 171/174 code, is this true? Should I still change out the O2 sensor even though the code disappeared? Any help is appreciated.
 

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171 and 174 means its reading lean on both sides of the engine .. most of the time this indicates a vacuum leak between the MAF and intake manifold or a vacuum line attached to the manifold. Its unlikely that the o2 sensor on both sides of the engine went bad at the same time.
 

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just because it runs doesn't mean you don't have a vaccume leak. Mine was throwing both and they would clear bt always come back after awhile. My leak was on my plastic tubing that goes from MAF to my richardm tube. Right where it clamped on to the richardm, took me lie 6 months to find that little leak. Fixed it and they never came back. Look at all your lines and couplers etc.
 

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No I haven't and quite honestly I don't know how to bench test them.
There are plenty of videos on youtube .. I could explain it, but really all you need is a propane torch and a multimeter. Just to verify the o2 sensors themselves are OK - they could have both been ruined by the coolant from the blown head gasket. Im not big on throwing parts at a problem just to gamble if that fixes it.

A small enough vac leak may not cause driveability issues. You might need to smoke test it to find one.

I just had a truck in here with a P0171 ( V6 engine went to one pipe that only has one set of air/fuel sensors, similar to o2, but not the same ) ... checking with a Snap on scan tool, the long term fuel trim was up to 35% and the short trim 3% .. thats a 38% increase in fuel to compensate for the erroneous false lean - faulty o2 can trick the computer into thinking its too lean and continue to add fuel until it reaches its threshold to light up the Check engine light.
 
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