Po171 po174

I have been fighting this code for weeks. I installed a new fuel filter early on because I got a 171 and 174. Now it is just 174 I have replaced the plugs, O2 sensor and cleaned the mass air sensor.
The code hit today after only being started for 1 min. Am I wrong the O2 sensor doesent work until 600-800 deg so it cant be reading O2.
Could it be fuel pressure ie junk in the tank or pump.:screwy:


3.8 V6 Mustang 2000


Thanks for any help
Jim