Fuel Trim Lean

Hey all,

Threw a CEL about 1 month back and our mechanic is still not able to track the problem down. Was wondering what you all would suggest.

Car - 2005 Impala Police Package

DTC = P0171 Fuel Trim System Lean Bank 1

Freeze Frame Data:

Fuel System 1 Status - CL
Calculated Load - 52.55%
STFT Bank 1 = 0%
STFT Bank 2 = n/a
STFT Bank 3 = -100.78%
STFT Bank 4 = n/a
Intake Manifold Pressure = 8.84 inHg / 29.94 kPa
Vehicle Speed = 9.38mph / 15.09kph
Fuel System 2 Status - n/a
Engine Coolant Temp - 131F / 55C
LTFT Bank 1 = 12.5%
LFTF Bank 2 = n/a
LFTF Bank 3 = -100.78%
LFTF Bank 4 = n/a
Engine RPM = 1866.5rpm
Spark Advance = n/a

Our mechanic says he did a thorough vacuum leak check, with negative results. We have a pretty ok scan test program for our in-car laptops which shows live data. I ran it a little bit yesterday to see the O2 sensors in action, seemed that the voltage on both stayed between .55 and 1.0V but sometimes the second sensor would go up to 2.0V or a little above, but I think that may have been because the windshield defroster was on.

Anyone have a course of action to suggest. We have been nickel and dimed by the GM dealer on this car and our department is tired of it.

Thanks in advance for any suggestions.

Our mechanic says he did a thorough vacuum leak check, with negative results. We have a pretty ok scan test program for our in-car laptops which shows live data. I ran it a little bit yesterday to see the O2 sensors in action, seemed that the voltage on both stayed between .55 and 1.0V but sometimes the second sensor would go up to 2.0V or a little above, but I think that may have been because the windshield defroster was on.

Anyone have a course of action to suggest. We have been nickel and dimed by the GM dealer on this car and our department is tired of it.

Thanks in advance for any suggestions.

If your FT is leaning it that much, I would suspect an O2 sensor or it's circuit is the problem. When that O2 sensor went to 2V, I think you found your problem. The O2S is telling the PCM that the fuel ratio is way rich, so the PCM is assigning a way lean fuel trim.

Agreed or you have fuel entering from a bad press. regulator or a leaking injector.