I have a 2001 Ranger. The "check engine" light has been on for five or six years now. I'm sure it's just a bad sensor, but the interesting thing is, when I travel to elevations above 5,000 feet, the light goes off and my mileage/gallon increases by 3 to 5 miles/gallon. I don't notice anything else that affects my motor. Is mileage generally better at higher altitudes?

 

Welcome to the forum

Yes, there is less Oxygen in the air at higher altitudes so less gasoline is needed for the 14.7:1 air:fuel ratio, but there is also less power available.

Any auto parts store will read the Codes for free, just have paper and pen ready to write them down, EXACTLY, "I think it was.........", is worse than no code at all.
Post them here.
Do NOT buy anything after getting the codes, they don't always mean what the auto parts guys think they do

CEL(check engine light) can be very important in some situations, the fact that it is on all the time with no problems means it will probably be an easy fix.
But because it is on now you will have no warning if something does happen that could strand you if not addressed.

For $20-$30 you can buy a Bluetooth OBD2 reader and get the codes on your smartphone or tablet with Free APP
You can also reset to clear the codes to see if they come back after a "fix"
These work on ANY vehicle sold in the US or Canada after 1995, so not a Ford thing, it's a Law thing, lol, so not a bad long term investment if you intend to have vehicles over then next few years

 

You really should always check codes when the light comes on, then decide if you can safely ignore it for 6 years.......


Anyway, do as RonD suggests to make sure, but i'm going to suggest (based off your description) that the codes are related to the upstream O2 sensors.