I have a 96 F150 4.9l throwing P0171

It was throwing more codes, and I have replaced both upstream O2 sensors and the canister purge valve. Now it only shows P0171 (lean bank 1) but not P0174 (lean bank 2).

I've cleaned the MAF and the air filter is good. Could the downstream O2 sensor be the cause? If not, what else would cause this?

If anymore info is needed, let me know. And thanks in advance.

 

Assuming this is a V8, the 302. You could have an intake gasket leaking, or a clogged/dead fuel injector. Do you have access to a live datastream on a scantool to watch short term fuel trims?

And no, a bad downstream O2 will not cause this. It's only purpose is to monitor catalyst efficiency.

 

A crack or leak in the exhaust manifold can cause a lean code. The physical location of the leak can trigger whether or not it shows up as Bank 1 or Bank 2 or both.

 

It is the 300 I6. I've just been running up the the parts store to have them scan it.
I'm getting both codes now, so I'll check for any leaks.

Thank you both.