No on the O2's, it would mean both O2 sensors on each exhaust manifold would have to fail at the same time, so very very unlikely.
171 and 174 means the computer is opening the fuel injectors on each bank longer than it should have to based on the air volume figure from MAF(mass air flow) sensor.
In other words if the computers calculation was used it would be a "Lean" mix, so computer ignores its calculation and opens injectors longer until the O2 sensors confirm a "normal" burn.
So engine never runs Lean, the codes are just giving driver a heads up that the calculations are wrong.
That could be because of a vacuum leak, but it would have to be from an upper intake manifold leak to effect Both Banks.
It could also be from low fuel pressure, computer can't check fuel pressure, it bases its calculations on factory preset pressure, 1998 Ranger 3.0l should have 65psi pressure at rail.
MAF sensor is where it all starts, these need to be cleaned every 5 years or so depending on driving conditions.
Computer reads the MAF data and bases how much fuel should be added to the available air to get 14:1 air:fuel ratio
Computer opens each fuel injector for specific length of time to add the calculated amount of fuel to the available air
Air:fuel is then burned in the cylinder
O2 sensors can only detect Oxygen in the exhaust, not fuel, if there is not enough oxygen then engine is running Rich and computer reduces the open time of injectors.
If there is too much oxygen then it was a Lean burn so computer opens injectors longer.
This is an instantaneous change in injector open time, so engine never runs rich or lean.
If the open time for the injectors fall outside the preset window for a 3.0l engine with 65psi fuel pressure then computer will set a Lean or Rich code.
The rear O2 sensors are to determine if cat converter is working, they are also used for long term fuel trims, but wouldn't be involved with 171/174 codes.
Last edited by RonD; 10-07-2015 at 05:02 PM.