If I have both sensors saying they’re not getting correct 02, could it be they’re both bad? Not sure how long this code has been active. Truck has a K&N filter, I am going to see if cleaning the MAF might help? It’s running a bit rough for my taste. 1997 ranger 4.0 4wd POS edition

 

Its good to post the definitions with the code numbers as no one has them ALL memorized, lol
P0171 System too Lean (Bank 1)
P1151 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2

So lean on both banks of a V engine
No, not likely to be O2 sensor issue, extreme longshot for two O2s to fail at the same time

But O2s should be replaced every 100-130k miles, they do run out of chemicals to detect oxygen in the exhaust, this also gets back better MPG

Yes, when its Both Banks its likely to be something common to both banks, so MAF is a good place to start, could also be upper intake vacuum leak

To test for vacuum leak warm up the engine first
Let it idle
Unplug the 2 wire connector on the IAC Valve
RPMs should drop to 500 or engine may even stall, either is GOOD, it means no vacuum leaks

If RPMs stay high there is a leak, you can unhook each vacuum line and plug that port with finger, when RPMs drop thats the line/hose with the leak

A failing/dirty PCV Valve can let in too much air, these should be changed every few years, and its hose cleaned

Fuel pressure can also cause Lean on both banks
In 1997 Computer expects 30-35psi fuel pressure, there is no fuel pressure sensor
If fuel pressure is 20psi then computers calculations will be off

Lean codes mean computers calculations are off, either too much unreported air is coming in or not enough fuel is coming in
There is a test port on the engine to test fuel pressure

When was Fuel Filter last changed?
Should be every 5 years or so, $15, a dirty filter can lower pressure

If injectors are getting dirty then less fuel flows out, try running a can of Seafoam or similar injector cleaner in the gas tank, run 1/2 tank then reset codes and see if they come back

 

almost brand new fuel filter (I change everything when I buy a vehicle. Call it peace of mind?) I cleaned MAF, tightened all of the clamps and the codes aren’t appearing. Seems to run smoother.

in other words, at 204k miles, I should probably change the 02 sensors just to be safe and hopefully regain some lost mpg? My gas mileage seems to currently be pretty lacking.

 

Yes, change the upstream O2 sensors, the downstream O2s seem to last twice as long but up to you

Yes, MPG will improve, older O2s will show Leaner burn as chemicals run out, so computer is running engine Richer than it should, which is safer but more expensive, lol

 

got them replaced thanks to my trusty can of kroil, but the 0171 code is still hanging around. I’ve cleaned the MAF already, could it be associated with a high idle/vacuum code?

 

After engine is warmed up and idling

Unplug the 2 wires connector on the IAC Valve
RPMs should drop to 500 or so, engine may even die, either is good it means no vacuum leaks

If idle doesn't drop then there is a vacuum leak

 

ive ordered a new PCV valve and hose, and lengths of the various vacuum hose sizes on it already.

I’m sure the pcv has never been changed, nor has it been connected properly I assume.

My EGR valve is looking pretty rough, along with the AC compressor that’s shot out. I can’t catch a break with this thing! At least it’s still getting me to and from work 100 miles a day... (knock on wood!)

 

back to the codes, I’m still getting a po171 and 174, which is both oxygen sensors I just replaced. What else could it be? Vacuum leak?

idle doesnt drop, or at least it didn’t. EGR is about rusted completely through, it may be leaking there. I tried the brake clean trick but no success.

All the vacuum hoses have good ends on them, and don’t seem to be broken anywhere. My idle is high (~1500RPM) and only goes down to about 1k sometimes at random.

If idling for a while the CEL flashes, and I get a p0300, which is random misfire. New coil pack, plugs/wires. This thing is a basketcase....

 

You have a vacuum leak, not sure what else to tell you?

We can't find it for you

You can google: engine smoke test

Smoke is blown into the intake manifold of the engine(engine is OFF) and you will often see the smoke come out of places that are leaking air into the engine when its running

You can DIY it or call a shop and ask how much to do a smoke test

O2 sensors and coil pack, plugs/wires are ALL things that needed to be changed on ALL vehicles over time, so not a waste of money, but none of those solves the problem of a vacuum leak or several vacuum leaks, these are mechanical issues so can't be solved with electrical fixes

 

I'll look into this leak some more. Thanks for all your input! I'll let y'all know what I find.