I had an Oxygen Sensor Back 1 OBD code (I do not remember the code) and a P0172 (System Too Rich Bank 1) code on my 2006 MCS GP with 80,000 miles. I replaced both oxygen sensors with NTK sensors. Both original sensors had bare wire showing where the wire cover and insulation had worn through due to rubbing on the vehicle near wire clamps. I purchased NTK sensors and replaced the sensors myself because the dealer said it would not be covered by extended oxygen sensor warranty if the wire was damaged in any way. I reset the codes and all seemed well. A week later I get a P0172 code (System Too Rich Bank 1) again but no P0132. I reset the code to see if it will come back. A week later I get a P0172 code again. I then suspect the new oxygen sensor or one of the MAP sensors. I have read that the NTK sensors work well on the MCS so I guess that I might have a leak at a MAP sensor because I had recently replaced (before getting any codes) the thermostat housing and gasket so I had unplugged the MAP near the thermostat. I then removed the MAP sensor and lubed the two O-rings. I reset the code. A week later I get a P0172 again. This time I replace the MAP sensor and o-rings (near the thermostat) with an OEM sensor and I reset the code. I did not replace the line going to the bottom of the MAP sensor but it looks good. A week or so later I get a P0172 again. This time I replace the T-MAP sensor at the intake manifold with an OEM sensor. It was a little oily so I think, oh that was the problem. That was not the fix. The P0172 code returned. The car is running fine but I can tell it is running rich (smell and more exhaust residue). All vacuum hoses appear to be good, at least where I can see. There does not appear to be any vacuum leaks but I did not do any type of smoke test. There does not appear to be any exhaust leaks prior to the Cat. So now I am suspecting that the car does not like one of the two new NTK oxygen sensors. Has anyone had problems with the NTK sensors? Do I need to try an OEM sensor (which I thought was NTK but maybe not)? Which sensor would be the likely suspect pre or post Cat? Any thoughts on what else to check/replace such as the fuel pressure regulator?
I had the same problem before trying to save a few bucks with the Amazon NTK O2. Long story short, I replaced my coding NTK front O2 with a real OEM O2 and all my error codes went away.
Update: I think I finally traced down my P0172 problem. Over the past two months I have swapped O2 sensors, MAP sensors, checked the fuel pressure, and looked at various vacuum lines. I found a loose vacuum line where the Fuel Pressure Regulator (FPR) vacuum line connects to the manifold. Must have been leaking slightly. I replaced the short hose and all has been good so far. History: About 5 years ago I added a boost gauge which required adding a tee fitting to the FPR vacuum line. To add the tee fitting a used a 2 inch long piece of 3/16" vacuum line to connect to the intake manifold where the FPR line previously connected. Well that short piece of hose was now slightly loose fitting at the manifold barb.
Sounds like mine may be there someday, as I had done that same/similar boost gauge install a couple years back. Thanks for the info Mike. Also, did you ever drop by and see Justin to get his or his Dad's opinion during your diagnostic chase?
I spoke a few hours too soon. P0172 came back last night. It is like a bad dream. Looks like a little more investigation for me. I might even have to break down and take it to a shop.
Did you ever find the fix for this problem? I'm trying to fix the same issues, do you think the non oem o2 sensor was the problem?
I fixed my issue by replacing my front NON OEM O2 with an OEM O2 and I found an issue with a failed exhaust gasket between my header and my Milltek exhaust.
Sorry for the slow reply but I have been continuing to troubleshoot. I first replaced the front NTK sensor with a BMW OEM sensor. That did not solve the problem. I did find and fix a loose vacuum line for the boost gauge and that did not fix the problem. Last week I replaced the rear NTK sensor with a BMW OEM sensor and thought that corrected the problem but after about 100 miles the P0172 code came back on. Thus it was not the NTK sensors causing the code. I suspect there is a slight vacuum or exhaust leak somewhere that I cannot find. Next I will look more closely at the exhaust gaskets.
I believe I finally solved my P0172 problem. I always suspected a small vacuum leak somewhere but could not find a leak. I finally replaced the plastic hose that goes from the Manifold Pressure Sensor (near the thermostat housing) to the manifold because the code started not too long after I replaced the thermostat gasket. I also replaced the intercooler boots. No code since with hundreds of miles driven in various conditions, highway, city, and Auto-X.