171 means the passenger side of engine has too much oxygen in the exhaust, Bank 1
174 means the drivers side of engine has too much oxygen in the exhaust, Bank 2
Most common issue for these two codes at the same time is a dirty MAF sensor.
MAF sensor is on the air cleaner box and what the big air tube is connected to, you need to use "MAF Cleaner" no brake or carb cleaner.
MAF sensor uses a heated wire inside, as air passes over the wire it is cooled off, the amount of cooling tells the computer how much air is coming into the engine, computer uses that information to calculate the amount of fuel it needs to add via the fuel injectors.
If this wire gets a coating on it(dirty) then it doesn't cool down as much, so computer adds less fuel, as it should; since the amount of air hasn't really changed there is too much air so you get a lean warning.
Since it is effecting both sides of the engine the MAF sensor would be the first stop.
Now that you know ALL the air going into the engine MUST pass thru the MAF sensor it also makes sense that a leak in the big air tube or a vacuum leak would also cause the Lean codes.
P1443 - EVAP fault is not related to the other 2 codes, unless it has a vacuum leak.
The EVAP system sucks fuel vapors from fuel tank, it uses engine vacuum to do this.
If EVAP system has a vacuum leak then it won't work so you get 1443, and you could also get Lean situation because of the leak.
The other side of the Lean "coin" is the fuel pressure.
On a '97 Ranger the Computer expects 40psi of pressure at the fuel injectors, it doesn't have a pressure sensor for this.
The computer calculates the fuel amount being added by the "dwell time" of the injectors, dwell is the amount of time the computer opens the injector.
Computer calculates for XXX amount of air it will need to open injector for XX time at 40psi pressure.
If O2 sensor is showing lean at XX then computer will increase dwell to XX +1, then XX+2 then XX+3, ect......until it hits it's limit which is about +20, then it will set the 171/174 codes, because something is wrong.
Low fuel pressure would be a long shot from your description, that is first noticed at highway speed but idle is fine.
My money is on MAF sensor.
Last edited by RonD; 12-14-2014 at 02:55 PM.