Hello,

I'm chasing a long-time (1.5 years) intermittent P0171 code (lean, sensor 1). The car runs fine, so haven't been real motivated. This year however its timing interfered with emissions check, so...I finally got into the world of code reading, etc, which is fun. Using the Torque app with a cheap bluetooth reader off Amazon.

Oh, this is a 2005 MCS, stock except a 15% pulley and K&N drop-in filter.

Current thoughts on the P0171 - classic cause for that is a vacuum leak, but I don't think that applies to MAP engines, only MAF engines. I don't have a smoke tester, but sprayed stuff all around the intake track while idling and didn't hear the idle change. Well...a couple times there seemed to be a tiny change, not repeatable, maybe imagined, maybe not... Checked fuel pressure this morning, normal results, posted here:
https://www.northamericanmotoring.co...-question.html

I also did some live parameter recording for O2 and Fuel Trim, attached. LTFT is higher than it should be. Weird how it comes down with the motor revved. First, I didn't expect the Long Term to react that fast, kinda makes me wonder if the stored LTFT is not a single value, but rather is mapped to RPM? I haven't read that anywhere. Also, it's weird how at idle the STFT is negative, constantly negating some of the LTFT. Doesn't seem to behave the way the gurus say it will. When I rev the motor and let off, the LTFT spikes briefly, sometimes into the low 20's (again, didn't think the Long Term was supposed to move that fast). I figure it hits 25% at times and sets the 0171 code. Don't know what's wrong.

Upstream O2 sensor (1) seems to oscillate the way it should. Not sure I understand the behavior of the Downstream sensor (2) - maybe the cat gets too cool at idle to work well, and warms up at 2500? But my code is related to sensor #1.

One thing I tried is dispensing propane in the intake while watching the Fuel Trims. I don't have a pic, but the STFT dropped ~10% while doing that, so the O2 sensor was definitely reading the rich and the computer was responding. LTFT did not respond much to a few seconds of propane.

Now oddly enough, I cleared the codes a few days ago and the car was ready for a successful emissions run today, except that after all this testing this morning, the next thing I knew, I had a new code - P0130 (O2 sensor problem, sensor 1). After a few minutes and a couple restarts, I was able to clear that and it is now again code-free, but not yet ready for emissions.

So what's up? This second code, related to the same O2 sensor, may help in the diagnosis. It's seemingly normal oscillations make it hard to believe the sensor is bad (it is original AFAIK, so 121,000 miles...old, I know). I'm not averse to replacement, but I greatly dislike throwing parts at a problem without diagnosis. Some people report rubbing of the O2 sensor wires on metal clips which can then cause an intermittent short or open on an otherwise functional O2 sensor...that could be it, I'll look for that. I looked up both codes on the web so I know what the "usual suspects" are, but there are a number of them. The O2 sensor itself, and it's wiring, is one of the common denominators.

What do you y'all think? Have any of you run across this scenario before?

Thanks, Jeremy

 

Nobody?

 

Honestly, all though I hate to just throw parts at a problem without really knowing for sure, I'd change the O2 sensor.
If it was simply on its way out, that would explain both of your codes. Plus anywhere where the wiring could have been compromised on the front sensor, would be easy to see while your under there, it's a pretty straight forward run into the engine compartment.

 

Jeremy,
My '04 MCS threw the same codes starting at ~95k. In my case:

0171 can be small leaks in the intercooler boots. If you haven't changed them it might be worth a try. I had intermitted P0171's and new (OEM) boots at just over 110k eliminated the issue.

I read somewhere that P0130 can be caused by the heater in the O2 sensor failing. The sensor works but fails a preheat temperature check which might explain your data. I changed the pre-cat O2 sensor and the problem went away.

-Jim

 

OK guys, you have convinced me. I just ordered the authorized NGK/NTK O2 sensor. The symptoms seem to be converging.

This morning I checked the sensor wiring thru the clips, etc, and it was OK, a little wear on the outer protective sheath, but nothing too bad. Connector was fine. Wiggled the wire and connector while idling and watching O2 signals, no response to the wiggling. However, Sensor 1, while mostly doing its cross-count thing, did seem to drop out for a few seconds once or twice randomly (not while wire-wiggling). And the LTFT was running a little higher this morning. It is cleared for emissions testing but produced a pending P0171. I'm hoping - wait, stop the press, my bride just texted and it passed emissions today before the codes came back, woohoo! But hopefully the new sensor will fix it for good.

Jim, my IC boots were nice and supple and seem to clamp down fine. I've not replaced them. How can you check for leaks? It should be under blower pressure there, right? I sprayed soapy water on the connections while idling and didn't see any bubbles.

Thanks! Jeremy

 

A P0171 can also be caused by a number of other small leaks:
Fuel pressure regulator vacuum source fallen off the intake manifold.
Failed intake manifold gasket
Cracked single layer steel gasket (fairly uncommon)
Green gasket between the SC and intake duct
PCV valve
Exhaust leak at the collector of the 4 cylinders on the header
I've even seen the fuel tank vent reed valve by the engine mount cause problems due to the quick connects being blown out and not keeping proper retention.