P0171 Mercury Code - Fuel Injection System Too Lean Bank 1

Quote:"If the problem persistss the front oxygen(O2) sensor may need to be replaced."

Front may be misleading, as P0171 refers to BANK 1, which is the passenger side for RWD, or the firewall side for FWD models.
tom

If the problem persistss the front oxygen (O2) sensor may need to be replaced.

Read more: https://www.autocodes.com/p...
. If the problem persistss the front oxygen (O2) sensor may need to be replaced.

Read more: https://www.autocodes.com/p...
. If the problem persistss the front oxygen (O2) sensor may need to be replaced.

Read more: https://www.autocodes.com/p...
If the problem persistss the front oxygen (O2) sensor may need to be replaced.

Read more: https://www.autocodes.com/p...

Could it be the air filter on 2000 grand marquis.. Odb code is po171 and po174

Yes, it is possible that the air filter is setting the code. Also, make sure the Mass Air Flow (MAF) sensor is clean as well.

2000 Mercury Sable LS wagon. I was getting p0171 and p0174 codes. Replaced MAF, no fix. Then noticed broken 90 degree vent connector on front cam cover (DOHC) and replaced that. Got only p0171 then. Well, there's another one on the rear cam cover. Broken also. Replaced that and all is well. Dorman 46027 PCV Valve Elbow for Select Ford/Mercury Models
These vent tubes go up to the intake tube just downstream from the MAF.

Thank you for sharing!!

2006 Mercury Monterey with 4.2L V6. Got P0171 codes off and on for a month and then both P0171 and P0172 codes with flashing CEL so initially we thought O2 sensor went bad. We replaced all O2 sensors because we had to drop the exhaust and it was cheaper than coming back a second time. Replaced plugs, wires, coil and fuel filter since this is the first tune up and the car has 70k miles. Started getting some P0302 codes after tune up, but P0302 went away after resetting computer by removing ground for ten minutes, but the engine continues to run rough at low speeds with P0171. Trim values on Bank 1 are 5 - 19 on short term, and 25 on long term. Not great, and sometimes throws a P0171 at 900 rpm. On Bank 2, trim is 0 - 5 on short term, and 0 - 7 on long term. Trim on bank 1 gets better at higher rpms. Did leak test with smoke since Trim improvement with rpm would indicate vaccum leak; but found no leaks. MAF sensor looked good and would affect both banks; so since bank 2 is good and we are only getting error on Bank 1, I am thinking the problem needs to be something unique to Bank 1. The only thing I can figure is an injector or intake manifold seal problem on bank 1. Any suggestion before I open up the top?

As you have experience diagnosing the P0171 code can sometimes be difficult. Unfortunately, there are many things that can set the code and you have already checked and/or replaced the most common components. From my experience, with other vehicles (I haven't had that problem with the model of your vehicle), the intake plenum gasket is the most frequent problem.
I checked a national database of repairs for the P0171 on the 2006 Mercury Monterey and here is a list of repairs. I hope that helps.

I got the car back working today. I took the intake plenum chamber off to replace the gaskets and at the same time I decided it was easier to go ahead and change out all the fuel injectors since I was able to find a complete set of six new injectors for $50 on Ebay. The six plenum gaskets did not look bad, but since they are rubber and they are 16 years old; time to change. Got the car back together and it is now running good with no roughness at low speeds and no check engine light coming on.
When all was said and done, I think the problem was a weak fuel injector on the number two cylinder which is the middle cylinder on bank One, the side closest to the firewall. Once I got all the injectors off, I tested the resistance of each injector and found that the number 2 injector had a lower resistance than all the others. Each injector contains an electric coil which creates a magnetic field whenever power is applied. When the coil is energized, the magnetic field pulls a plunger inside the injector and allows fuel to squirt. When power is off, a spring pushes the plunger back into its original position and stops fuel from squirting into the cylinder. Using a simple multimeter which can measure resistance, I measured the resistance on the two pins inside each injector connector where the two pins are connected to the two ends of the wire that makes up the coil inside of each injector. One normally checks the resistance on these pins to make sure the coils are good and the injector will work, but I also think that all of the injectors should measure with similar resistance so they will function similarly. On my car, five of the six injectors measured at 12-16 ohms while the sixth one, which was the injector on the #2 cylinder, measured at 2-4 ohms; which is why I think it was a weak injector on #2 which was causing that cylinder to run lean and throw the P0171 code. This whole process was difficult because I was ever only getting a lean code on one side of the engine which seemed to indicate that whatever was wrong was something that was unique to only one side of the engine and having a discrepancy on the #2 injector would meet that criteria. For those who are having similar problems, I would strongly recommend that you purchase a wireless OBDII code reader. I got one off Amazon for $18 and it can connect to my tablet or smart phone with a Google Play app and you can read most of the sensors that you need to. I was able to see the voltage readings from the O2 sensors to see that they were actually working or not. Later I was able to read the fast and slow trim reading, which indicate how much the computer is having to adjust the fuel input to achieve a good air-fuel ratio. Having access to these readings was great and a real help in figuring out not only where the problem was, but also seeing when i got the problem fixed.