Hi guys:I have been getting the cel coming on with 171/174 codes,I have replaced the 3 oxygen sensors maf sensor pcv valve, checked for vacuum leaks I reset it and it still comes back seems to happen when its below 20 degrees .any help would be appreciated.Thanks Mike
BTW its a 97 ranger 4wd 4.0L

 

Those codes are related to problems with the fuel air mixture, so anything that could affect mixture, could cause those codes.
When you replaced those sensors, did you inspect the wiring for each, to make sure they were all good from the sensor to the computer?
Have you checked fuel delivery: fuel pressure, fuel filter, fuel pressure regulator, fuel injectors, etc.
Since it seems temperature related, maybe the intake air temperature sensor isn't working right, not allowing the computer to make adjustments to mixture, or the engine coolant temperature sensor.
If you still have trouble finding the problem, this might be a case where it's worth it to pay a shop $50-70 to put it on a good scope and tell you what the problem is, before putting more money into parts that we hope will fix the problem. I once started the process of replacing sensors, hoping to fix a similar problem, decided to take it to a shop instead to get a good diagnosis, and it turned out that the computer itself was bad. The money for the diagnosis was well spent in that case.

 

Ka1cu, I know you've already checked for vacuum leaks, but it still sounds like one to me. There was a TSB on the 4.0 about loose intake manifold bolts. They are torqued in three stages; 36 INCH lbs, 72 INCH lbs, and the final torque should be 144 INCH lbs.

That's not a whole lot of torque, so I can see how they can loosen up. If you find that some are loose, I would use Lock-Tite on them. Good luck.

 

Lower intake manifold gaskets are rubber o-rings ,in that cold of weather and over time they shrink and leak. trying to torque them with out putting new gasket/o-rings in could crack your plastic intake.
I just did my 98 explorer with same codes there is a tsb out on it

Rich