Another Fix for 4x4 Owners with 0171 & 0174 Codes

[ksandman]

Have a 99 ranger w/4.0 OHV and have been chasing these 2 codes (on & off) for 3 years. When checking for vacuum leaks, I never had it in 4H and so never discovered that the vacuum lines attached to the front hub assemblies had dry-rotted. Thus, when I would put it in 4H for the 10 seconds it takes to get out of my driveway, I would get the CEL about 4 minutes later (just as engine is warming up) (Light must come after vacuum leak "event" according to some pre-programmed computer list of conditions/time-Once I figured it out I had it happen 3 times in same day in the same way). Been 5 days now, passed inspection yesterday-so far so good. Now, I get to take it back to mechanic (great guy-honest and reasonable) to see which of new hub assemblies won't hold vacuum. (he thinks he may have o-ring problem ). Hope this helps some of you and thanks for all the help you've given me.
Update: Eureka- He didn't replace o-rings on both sides when installing them, because aftermarkets didn't come with any and there was no mention of them in the instructions. No codes (Oh happy day!!!) so far and mileage is better than ever.