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Question for Bbobynski


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Recently my 96 STS has begun setting DTC P0171 and P0174 at the same time. Operating conditions are usually similar each time. The car has been running for at least 15-20 minutes - usually at 2000-3000 RPM (highway speed). When I come up to a stop light, or even let the car idle down a hill (engine speed less than 1000 RPM) - the SERVICE EMISSIONS SYSTEM message comes up on the DIC. According to the FSM, these codes indicate that both cylinder banks are having problems with a lean fuel trim condition. I've gone thru the diagnostic steps in the manual, but I haven't been able to find anything wrong - yet. The FSM tells me to do a fuel quality test - which I haven't done, but I did let the tank run almost empty and switched gas stations when I filled it back, so I fairly sure that I have run out any bad gas - if I ever had any in the first place. The car has been worked on quite a bit over the last few months. The fuel pump was replaced about 6 weeks ago and about 3 weeks ago, the fuel rail was replaced by my local dealership. Do you have any suggestions as to where I should look next? It seems to me, that whatever is wrong, is affecting both banks, so I don't think that the O2 sensors are bad. I would say that the chances of two O2 sensors going bad at the same time are extremely remote - unless that had been contaminated with silicone.



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