I'm having the same issue, I've ordered a lower intake gasket, but I'm not sure which one it is, if it's upper or lower, is there one in particular that it typically occurs in? And how hard is it to change the gaskets? and about how long does it take? I kinda have an idea about the process, but I need to know if I'll have enough time to do it or not.

 

I had the codes 0171 and 0174 on 1998 explorer 4.0 liter ohc. Tested MAF, changed pcv and hose, changed egr valve, codes kept coming back. Engine ran pretty good, nothing really noticable as far as idle. But after reading this forum decided must either be fuel pressure low or intake manifold leaking. Tried spraying wd40 around intake manifold gasket and didn't notice any change in rpm or sound, tried once more and found a place where spraying made a difference. Behind the EGR valve ther is a HOLE in the bottom of the plastic throttle body! Anyone have any idea why this would be here? I patched the hole with Bellzona (industrial strength 2 part metal patch) and will see if the code disappears after this. I have photos I would like to attach if I am allowed.
Dave

 

I agree that both codes you got aren't due to your assumed Ford catalytic converter problem. Basically this means that an oxygen sensor in bank 1 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. You will more than likely not notice any drivability problems, although there may be symptoms such as a lack of power, detonation (spark knock), and/or a hesitation/surge on acceleration. Both code P0171 and P0174 may mean that one or more of your MAF (Mass Air Flow) Sensor is dirty or faulty... Take note that the use of "oiled" air filters (K&N, etc.) can cause the MAF to become dirty if the filter is over-oiled, another cause could be a vacuum leak downstream of the MAF sensor...

In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling, also inspect all vacuum and PCV hoses, replace if necessary or check for a dirty fuel filter and proper fuel pressure... Good luck!!!