Reads code 171 lean mix but runs very rich - 2000 Dodge Neon

no. (but the isc valves loves to stick but will not cause black smoke)
the 4.0L is one of 5 million made, it's well known,
the black smoke don't lie. it's rich and the cat seems burned up too.
as with black smoke the cat will try to catalyze all that and glow red hot. is your cat missing?>
so why not believe the code 171, its real and is catastrophic failure. there are 2 engines here, FED and California.(exhaust)
(2 different exhausts and sensors there.
as they say, fix what is OBVIOUSLY bad, 171
(find why using tools or inspections)
Fuel System Lean (Bank 1)in most cases the exhaust is cracked near B1S1 0xy sensor
this causes it to stick at 0volts, (lies) and the PCM sees that obvious lie then goes rich. (limphome rich)
if the pcm cant measure oxygen, it must now guess at fuel trim.(bad that.)
it goes rich, so you dont melt pistons running lean.
(by design)

bad front oxygen sensor (aka , up stream bank 1)
in calif cars, it can have a split exhaust on the I6 and have 3 sensors. (hope not yours, do look..)
most have 2 sensor, front and the useless behind cat sensor(B1s2)
S2 is useless for running mix, the rear is for smog test the cat.

so it rich, and now misfires. and the spark tips are black.
and it misfires to bad it can even idle, sure. (Drops)
the cause, is too rich.
the 171 means the PCM sees it stuck at 0v forEVER.

there are in fact other causes.
but these 2 are top cause.

on all 3 of my 4.0L engines
the exhaust manifold cracked on the under side.
side facing block. (i use a hose to ear to hear the putt putt puts, (with FULL PPE protection from burns from a dead cold engine.) ask how.
even the SST exh. man. did that.

or the header flange gasket is TOAST. (that too putts when bad.)

Since you have replaced the sensor, my first guess would be low fuel pressure or a vac leak.
Code 171 means the computer can not make the fuel mixture rich enough to make the sensor switch from lean to rich and back.

Code 171 and 174 are for bank 1 and 2 running too lean. The engine has compensated as rich as it can go, and it is still to lean. This does not necessarily mean your mixture is bad, but something (a sensor) is telling the ECM to correct lean. The oxygen sensors alert the ECM to how lean/rich the exhaust is, and then the ECM trips a code. Maybe you have a bad oxy. sensor, but since there are 4 of them (2 per bank), I don't think they could all go at once.

PO 171 Bank 1 too lean
PO 174 Bank 2 too lean
Why ?
The front O2 Sensors need to be replaced at
100,000 miles or sooner, if they have actually failed
They are reading too lean because the system is too
rich & the computer is driving the fuel trim or injector
pulse lean, thus the codes or failed sensors, yes both
Have to check fuel trim with a scan tool
Have to check fuel pressure
Have to check there are no leaking injectors
Have to check for Vacuum Leaks from the intake
all the way to the exhaust converter
Your working a rich condition keeping in mind the system
is driven lean
You look at fuel trim short & long,sort of a past history
of events
May want to look at converter temps also
The converters won't last long if you have or has
a rich condition from fuel or cylinder misfires
Now the spark plugs can't be over 3 years old
& no mifires,get that out of the way before working
the O2 issues

The check engine light is on to tell you the computer has found a problem that will affect performance, gas mileage, or emissions.
Such as the code 171 you asked about.
Code 171 is for the fuel mixture on bank one being too lean. The O2 sensor is telling the computer the fuel mixture is too lean and the computer can not make it rich enough.

You did not list a vehicle, so here are the generic codes.
Code 171 is system lean bank one and code 174 is system lean bank two.
The computer is getting a signal from the O2 sensors saying the fuel system is too lean, and the computer can not make it rich.
Possible causes are low fuel pressure, vac leaks, EGR valve problems, and so on.

Try some injector cleaner in order to see if the lean condition improves. Lean conditions can sometimes be caused by o2 sensors or other sensors that are not within specifications.

what should we check when we have those pesky rich or lean codes? The most common issues for lean codes are:

1. Vacuum leaks - check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
2. Restricted fuel filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
4. Engine misfire – Yes I know this one may seem weird. You might think that if there is a misfire then you will have all that unburned fuel and it should read rich; right? Well the O2 sensors read only oxygen content in the exhaust, so if you have all that unburned fuel from incomplete combustion then, you guessed it, you also have all that unburned oxygen. High O2 content in exhaust equals a lean reading! There are also some other possibilities such as an internally leaking EGR system, (but this will typically set a separate code). A leak in the exhaust system before the O2 sensor will also cause incorrect readings. And always check for after- market modifications. These can throw a wrench into the works! The only other possibilities (however unlikely), are wiring issues, computer concerns or a bad O2 sensor! .

P171 Lean to bank one
P172 Rich to bank one
P174 Lean to bank two
P175 Rich to bank two.

These are conflicting codes. I would suspect that you have a bad MAF sensor. take it out and try and clean it before you replace it. It could also be a bad O2 sensor. But unlikely that both sides went bad at the same time.

I looked up code 171 and found 'system lean bank one' Sounds like a fuel injection sensor. countrycurt0