Code P0171

Hello, yes, you're exactly right, this code is mostly caused by a vacuum leak. The intake manifold is most likely the cause, especially on this engine. The quickest, safest way to find a vacuum leak on these is to use a water bottle that you can spray some water around the lower section of the intake manifold. You'll hear the water being sucked right into the manifold. The guide below suggests using a flammable spray, but I recommend that second. The water will usually find a vacuum leak the safest way. Just spray down where the bottom of the intake manifold meets the engine block, and you'll hear it. If that way doesn't work, then follow the guide. I believe this engine is a straight 6-cylinder engine. So, spraying all the way around the engine intake manifold should reveal the problem.

I'm adding a Technical Service Bulletin about this problem on this specific model in the last three diagrams.

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Okay, so with the engine RPMs higher a vacuum leak will go away, and your fuel trims should return to less than %5.
So, the lean condition is being caused by something else. The Technical Service Bulletins are for Fuel Injector cleaning. What you should do next is watch the Oxygen Sensor activity. There are two sensors on this vehicle, one is in front of the Catalytic Converter and one behind it. They will both be in your live data stream on your scan tool. The front should be swinging from 0.2 volts to 0.8 volts back and forth. One swing per second is the rule of thumb. The rear Oxygen Sensor should be reading a steady 0.5-0.7 volts. Now this vehicle also has a Secondary Air injection system. If one of the Oxygen Sensors is reading a lower voltage, below 0.4 volts, that's a lean reading and that's what is causing the code. It can be a number of things, an Air Injection solenoid stuck open, Fuel Injectors clogged, Low Fuel pressure. Whatever the cause, it is extreme. Those numbers are very high fuel trim numbers. The ECM is adding a ton of fuel to the air/fuel mixture.
Start with watching the Oxygen Sensors and see what they are doing and clamp off the Air Injection rubber tubbing and see if the Fuel Trim numbers come down. It will take a minute or so. The Mass Air Flow sensor should be reading around four grams per second because it is around a 4-liter engine. Another rule of thumb for the Mass Air Flow sensor. Since there are 2 Technical Service Bulletins on this issue. I would consider a Fuel Injection cleaning. With Fuel Trim numbers that high and no other codes, Thats where I would start.
It's running good because the ECM is compensating so much for the fault.
It's good you have a scan tool that you can check the live data on, that's a great investment. I added some guides below for checking fuel pressure and such if you find nothing else wrong. But I mention starting with clamping off the air injection and watching the oxygen sensors because the fuel trims get a little better at higher RPMs.
One last thing, they sell Mass Air Flow sensor cleaner, but I don't think a dirty MAF is the problem because the fuel trim is so high.

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

https://www.2carpros.com/articles/mass-air-flow-service

https://www.2carpros.com/articles/how-to-test-an-oxygen-sensor-02-sensor

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

That front sensor looks strange, how long of a time base is that? It's average 0.55 volts and not making its upwards swing. If you raise the RPMs to 2,500, does it look the same?
I can see why you're getting the p0171. See those downward spikes, those are all lean voltage signals. I can see its minimum/maximum is 0.19 to 0.72 but it's not swing high long enough. Below are some examples I found of what to expect from correctly functioning 02 sensor. In the first picture, the waveform below the "Normal Operation" is what you're seeing there on yours. And because your fuel trims are so high, your engine is starving for fuel or has excessive air getting into the exhaust and fooling the oxygen sensor into going rich. Because an Oxygen sensor only reads oxygen or a lack of it, not fuel.
For example. A misfiring cylinder sends unburned fuel and oxygen into the exhaust, but because the Oxygen sensor only reads oxygen, it will actually enrich a mixture because of this. Thus, being fooled.
See what happens when you raise the throttle. And what type of brand sensor did you put in a couple years ago? That signal looks very biased. Is the wiring okay for it, it's not sitting on a hot exhaust pipe or something? Is the harness plug okay on it too?

You may want to take another look at that front 02 sensor again in your live data. That first waveform didn't look very active, that may be why the rear one came out with a lot of soot on it.
Looking back at that front 02 reading you can see where it was hanging up at more of a higher voltage, more towards 0.800v, it should be more active than that. It should have equal time rich as it does lean. Should be swing between 0.200-0.800mv. And also take another look at the fuel trim numbers, they will really give you the best idea of how the engine is running, or at least how the ECM is seeing the engine run. You really want your total fuel trim to be below % 10. Thats adding the short term and long term together.