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Ford Thunderbird P0171 - P0174

Q. On a 1996 Ford Thunderbird with a 3.8 liter engine, I am getting a Check Engine Codes P0171 and P0174, which are Bank 1 and Bank 2 too lean. I was told to replace both sides upstream oxygen sensors, which I did with Bosch replacement.

Ford Thunderbird P0171 - P0174

However, upon driving, the vehicle performed the same, a little roughish under load and stumbling when first driven cold, and when I went up about the third expressway ramp, the Check Engine Light came back on. What are some other probable causes of this? The car has 99,000 miles.

Thanks...

A. Again I remind people to read the diagnostic codes carefully!!

Code P0171 and P0174 is: "Indicates the air/fuel mixture is too lean. The fuel adaptive system is at the full rich correction limit". In other words, it is telling you the air/fuel mixture is so lean that both HO2S are at the full rich limit of their capabilities. It does not indicate a fault with the HO2S.

Now that we know the system is lean, it should be fairly easy to troubleshoot from here. In the fuel system it could be a bad fuel pressure regulator, low fuel pressure of a bad injector or injector circuit. First thing I would do is replace the fuel filter, that's a very good possibility..

You could have a vacuum leak in a vacuum line, PCV system or even an oil dipstick or loose oil cap could do it. You could also have an EGR valve not closing completely.

See also Ford Mass Air Flow Sensors.

Additional Information provided courtesy of ALLDATA

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