I would look at the O2 sensor as the primary cause. Replace it, as well as the other sensor on the same side, clear the codes and see what happens. If the plugs and wires are old, replace those as well.P0154 O2 Sensor Circuit No Activity Detected
P0174 System Too Lean
P0304 Cylinder 4 Misfire Detected
Just got the code reader and pulled these codes as I had the engine light on any help would be appreciated!
Make sure the plug wire is seated correctly on the plug and on the coil pack. You could double check the gap as well.I'll order sensors and replace. Maybe try cleaning wiring in meant time.
Regarding the miss fire. Just replaced the wires and plugs so does that mean #4 is a bad plug or not gapped correctly or should I just pull it and reseat?