2000 ford escort ZX2 with P0171 trouble code. Cleaned MAF, replaced MAF, replaced vacuum lines, replaced upstream o2 sensor. No change. Any ideas?

I am working on the same problem. This 2000 ZX2 was idling rough and throwing a p171 code (bank 1 too lean). I hooked up a scanner to the OBDII port and you could see the O2 sensor at 0 or very low voltage and not switching and the fuel compensation maxed out which indicates a lean condition. Using a flammable Carb cleaner and the engine running I sprayed around the vacuum hoses until the idle picked up. I found a rubber elbow that connects to the bottom of the intake maniflold and feeds vacuum to the PCV valve. Replacing the the elbow with a suitable replacment from the auto part misc parts cards display had it running smoothly and the O2 sensor switching properly. Since this was happening at idle it was not the fuel filter which can cause a lean condition in upper rpms. It is not the MAF or the O2 sensor since it switches when not at idle. Also check the smaller vacuum lines that go to the evap control from the intake vacuum ports (back of manifold just below the throttlebody, most will be cracking if your car is anything like mine. I have also seen this problem with a bad PCV valve.

P0171 simply means that bank one is lean!
It does not mean the 02 is bad
I'm guessing the auto parts sold you the O2 after they pulled the code... I hate that

So if you want to fix first, first thing we have to understand is what parameters the PCM is looking for in order to set the code... basically the PCM has some extra give/take it can use to control the fuel/air mix

in your case it has commanded the injectors and extra 25% and still cannot see, what it assumes is the correct switch rate of the O2 sensor

So what can cause this...

• Fuel system
• Induction System (air intake)
• EGR
• Base engine

• weak fuel pump
• dirty injectors
• clogged fuel filter

1. measure fuel pressure and volume
2. run a power balance test (you'll need a pro scan tool)
3. do a compression test

P0171 OBD-II Trouble Code - System Too Lean (Bank 1)

What does that mean?
Basically this means that an oxygen sensor in bank 1 detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1.

Note: This DTC is very similar to P0174, and in fact your vehicle may show both codes at the same time.

Symptoms
You will more than likely not notice any drivability problems, although there may be symptoms such as a lack of power, detonation (spark knock), and/or a hesitation/surge on acceleration.

Causes
A code P0171 may mean that one or more of the following has happened:
* The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
* There could be a vacuum leak downstream of the MAF sensor.

Possible Solutions include:
1.- In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
2.- Inspect all vacuum and PCV hoses, replace if necessary
3.- Check for a dirty fuel filter and proper fuel pressure

Are you mek the items 1 and 2; try the item 3. I'd say first check the intake manifold gaskets (two of them) for leaks.

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Check your battery conenection on the back of your altanater and chang O conector out the wire sometimes gets hot and curent is not getting to the eng systom

Hi! One thing that can resolve this is by replacing your fuel filter. The po171 code is related to a faulty MAF and vaccum lines but youre done replacing this and thats a great move. The last thing to do is to replace the fuel filter to resolve the problem. Hope this helps and have a good one!