I have a 2003 f150 with a 4.6 engine. It is showing a code p0171. The long term fuel trim on bank one is 29.7 at idle, 25 at 1500 rpm,24.2 at 2500 rpm. Bank 2 has -19.5 at idle, -19.5 at 1500, and -10.9 at 2500 rpm. The truck has 276,429 miles on it. I have replaced both updraft o2 sensors, plugs, idle control valve and three vacuum hoses. But it still shows lean on bank one. Can anyone help me with the fuel trim numbers?

 

Everytime I have seen trims like that the cat converter on the negative side was bad. Loosen the exhaust at the manifolds and see what the trims do or you can measure the backpressure if a gauge is handy.

 

People, the 171/174 code tells you the trim tables are shifted 'rich'.
His readings shows this. Dfference is one bank is shifted more than the other. Bank 1 has reached the limit and set the code but back 2 is not far behind.
What cause the shift?
As the Ox sensor detects to much oxygen in the exhaust, it adds counts to the LTFT tables until the limit is reached at which time the code/s is set.
Why did they shift? To much unaccounted for air into the intake tract.
Said another way....a vacuum leak.
Look at all the hoses for cracks, softness, disconnect etc.
Look on the back side of the 'vertical' intake manifold at the PCV hose connection. They go soft and get a hole sucked in.
Let everything else alone for this condition.
Good luck.

 

Haha I figured you would chime in. Reread what OP said, bank 1 is 25 positive and bank 2 is 19 NEGATIVE. A vacuum leak cant cause that. Also a vac leak will show up as a much higher trim at idle vs 2500 RPM.

 

You may be right.
I would use a Scanner to look at the Ox Sensor actions on each bank.
It should show an indication by carefully observing the action of the front vs the rear on each side.
Substrate blockage from breakup but still functioning enough not to set code 420 or 421 depending on which side is at fault.
Don't dismiss the air leak as possible until the trims become near equal close to zero.
Good luck.