Hi everyone. Been trying to track this one down for awhile.

1997 Ford Ranger
2.3L 4cyl engine with 290k miles

I started out with simply a P1131 (sensor 1, bank 1 lean) code.

In the process of troubleshooting, I discovered my catalytic converter had a hole in it. I had that replaced, but it made no difference.

Then I replaced the fuel filter.

And then cleaned the MAF sensor and replaced the air filter.

Got a new upstream O2 sensor. No change.

Then I replaced all the spark plugs. After that, I started to receive an intermittent P0171 code.

I looked near the PCV valve for cracks, but found nothing. I replaced the valve itself because it was only $12.

I cleaned the IAC and re-seated all of the air intake tubing.

At this point, I was beginning to think it may be fuel pressure related. Checked the fuel pressure and it was at 32 psi. Chilton says acceptable range is 30-45 at idle.

Then today, I took it out on its first drive since cleaning the IAC.

I now have 3 codes: P1131 (sensor 1, bank 1 lean), P0171 (system too lean, bank 1), and P1443 (evap issue of some kind)

I've done a little bit of looking around, and don't see any vacuum hoses that are obviously cracked or loose.

Any thoughts? Thanks for any assistance.

Edit: I should also mention that it generally feels fine. No high/low idle. Doesn't die on me. Used to experience a slight loss of power when accelerating, but haven't experienced that since replacing the spark plugs.

I should also mention that the 3 DTC codes are currently in pending. The P1131 code has been confirmed multiple times, the P0171 code once or twice, and the P1443 is pending for its first time (as far as I know), and has never reached confirmed status.

 

After engine is warmed up and idling, unplug the IAC valve wires, IAC valve will close all the way.
RPMs should drop to 500 or engine may even stall either is good, it means no vacuum leaks
If RPMs stay higher then you do have a vacuum leak.

With IAC valve still unplugged and engine idling at the higher RPM unplug 1 vacuum hose at a time from the intake and plug that port with your finger, if RPMs drop then that hose has the leak somewhere.
Power brake booster hose and PCV valve hose are common leaks
EVAP system could be as well since you do have a code for that now.

O2 sensors read Oxygen levels in the exhaust not fuel, too much oxygen is Lean, too little Rich.
Engine is never running Lean when you get these codes, computer is adding more fuel to get O2 sensor voltage in the correct range.
The codes are to let the driver know that computers calculated air/fuel mix is incorrect, it is adding more fuel than it has calculated so something is wrong.
Because O2 sensors read only oxygen a leak in the exhaust manifold will pull in air from the outside, this will cause a False Lean because of the extra air, you can usually hear an exhaust leak.

 

Apologies for the late reply, but I have only recently had the time (and -- ahem -- desire) to tinker around. I unplugged the IAC valve, and the engine immediately dropped to around 500 RPM. Looking at my OBD reader, the RPMs were reading around 500 - 550. I also pulled the list of confirmed DTCs. P0135 - O2 Sensor Heater Circuit P1131 - Sensor 1, Bank 1 Lean P0171 - System Too Lean Any ideas where I can go from here? Thanks! EDIT: I am not sure why my post is being displayed as a single line! I had my post laid out nicely, but it's putting it all on a single line.

 

Check O2 heater fuse, it will be in engine fuse box

You have no vacuum leak with IAC test, good

Could be heater circuit getting power will help but could be low fuel pressure causing Lean code.

Again, the engine itself is not running Lean, Lean code means computer is having to add more fuel to get the O2 sensor at .4-.5volts than it calculated based on MAF sensor and RPM of the engine.

The computer "knows" it is running a 2.3liter engine, and it "knows" how much air a 2.3liter engine will use at every RPM, so it "knows" how much fuel to add to get 14.7:1 air:fuel ratio, this is easy to calculate.
MAF sensor and O2 sensor help fine tune that mixture

And it "knows" a 1997 Ranger fuel system has 35psi of Fuel Pressure at the fuel injectors.
So it "knows" how long to open each injector to get XXX amount of fuel to flow into the intake.
But what happens if fuel pressure is actually 20psi?
Yes, less fuel flows in, so computer has to open injectors longer to get O2s at .5volts and computer sets Lean code to let driver know, that what the computer "knows" seems to be wrong, lol.

Vehicle computers are large number calculators, that's it, they are given "known" values and calculate from there.
They actually shouldn't be called computers as that has a different meaning now a days.
Calculators is what they are, a vending machining from 1960's that could tell the difference from a nickle and penny was 100 times smarter than vehicle computer.

When you turn on the key the O2 heater fuse sends 12volts to O2 heater circuit which is Grounded in the computer, so computer "sees" 12volts on that wire, if not then it sets the O2 heater code, no smarts here, on/off thats all it "knows"

i.e. if this type of computer was helping you wire trailer lights and was watching the tail lights for you, and you were to ask it if right blinker on trailer was working the response would be:
"no it is not, yes it is, no it is not, yes it is, no it is not, ect............", while the response may be useful, obviously a flashing light, computer just doesn't "get it", lol.

And this brings up Long Term Fuel Trims
These stay around a week or two
So if you had a lean code and found a problem and fixed it you can still get the Lean code for the next few weeks until Long Term Fuel Trim(LTFT) comes back to normal.
You can clear LTFT, wouldn't be of much valve if you could

 

Thanks. I did check the fuel pressure once, and at idle, it seemed to be within the normal operating range. I will look into those things as well. Also considering replacing the downstream O2 sensor -- it's only around $30 on RockAuto.com and I already replaced the upstream sensor. The upstream O2 sensor looked pretty banged up when I pulled it out -- the tip was warped and blown out. At one point, my catalytic converter blew out. If it was getting that hot, who knows if it could have damaged the downstream O2 sensor as well. And $30 is within my "screw it, I' ll try that" price range.