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Discussion Starter · #1 ·
Was driving my car last Friday Sept 9th on my in town news deliveries, when the check engine light came on. I finished my routes, and ran the code when I got home. The only code it showed, was P0171 Bank 1 system too lean.When I bought the car a week prior on Sept 1st, it did not come on during the 45 mile drive home. Between the 6th and 8th it did not come on as I drove in town. I cleared the code, and it seems to come on after about 90 min of the stop/go driving I do during my routes.
It's not experiencing any driveability issues such as stalling,rough idle,hesitation etc.It runs and idles smoothly with very good acceleration and power. What could the issue be? MAF sensor looked clean, I installed new air filter this past weekend
 

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Dont get to concerned.Alcohol content in gas has been increased under Captain Crackhead by executive order.the CARI index for alcohol(how much oxygen it takes for combustion) is differtent than gasoline.I would suggest a tank of high quality fuel and see if the check engine light comes back.
 

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The E15 change is a temp one that mirrors the same thing done in 2019, and is regularly pushed by Iowa reps to get more corn sales (unnecessary, cuz it's already subsidized, but that's another story). My SAE engineering/racing competitions in college were sponsored by this same thing 20yrs ago, so nothing new.

You can try finding some non-ethanol gas in your area thru this old site:
www.pure-gas.org
There's also additives sold in the parts store near the octane booster, meant to reduce impact of E in fuels that you can try.
If that doesn't change the situation, you may have to start looking for possible vacuum/exhaust leaks that could cause the O2 readings to be low.

O2 sensors are meant to monitor hydrocarbon emissions from burning gasoline, but ethanol increases NOx emissions, which don't reflect well in the sensor. Our older cars have less flexibility in adjusting, and getting to the edge of that range can make then nervous. It may or may not be the cause, but
 

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Discussion Starter · #5 ·
The E15 change is a temp one that mirrors the same thing done in 2019, and is regularly pushed by Iowa reps to get more corn sales (unnecessary, cuz it's already subsidized, but that's another story). My SAE engineering/racing competitions in college were sponsored by this same thing 20yrs ago, so nothing new.

You can try finding some non-ethanol gas in your area thru this old site:
www.pure-gas.org
There's also additives sold in the parts store near the octane booster, meant to reduce impact of E in fuels that you can try.
If that doesn't change the situation, you may have to start looking for possible vacuum/exhaust leaks that could cause the O2 readings to be low.

O2 sensors are meant to monitor hydrocarbon emissions from burning gasoline, but ethanol increases NOx emissions, which don't reflect well in the sensor. Our older cars have less flexibility in adjusting, and getting to the edge of that range can make then nervous. It may or may not be the cause, but
There's no vacuum leaks,I inspected all the hoses etc. The exhaust is quiet, so no issues there. What I can do, is when it gets to 3/4 tank I can top it off with 87 octane 10% ethanol, which I have never had issues with in any car (Has tank full of E15 now) Has to be something stupid, as it runs so damn good!
 

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Discussion Starter · #6 ·
Looked at the tip of my old sensor, did not like how it looked, so I have a new ACDelco one arriving in the morning.Anything special I have to do after installing it, like disconnecting battery to clear the ECU? Car will be driven on my in town delivery routes during the afternoon hours
 

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The fuel trims will adjust themselves after a few minutes of driving, as it sees the new readings throughout the range of RPM and airflows. If you want to unplug the battery for an hour while doing the swap, that can expedite it, but it'll still have to make tweaks +/- from that zero point once you get started.
 

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Discussion Starter · #8 ·
The fuel trims will adjust themselves after a few minutes of driving, as it sees the new readings throughout the range of RPM and airflows. If you want to unplug the battery for an hour while doing the swap, that can expedite it, but it'll still have to make tweaks +/- from that zero point once you get started.
Ok thanks I'll just clear the code with my scan tool, and drive it. I drive about a mile and a half first to wait for my bundles, then 90 min of in town driving. Should be enough to get things reset
 

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Discussion Starter · #9 ·
Put in new O2 sensor this morning. First drive cycle was an hour of paper deliveries with several stops. During this time the check engine light popped on. I finished the deliveries and went home for a break. Code was the same P0171. I cleared it with my scan tool.
After about a 90 min break I went out again to finish my work, this drive cycle was about 40 minutes of same kind of driving. This time the light did not come on. After supper I decided to take it out on the highway, so I went on a 6 mile round trip avg. speed of 55 mph,and returned home. No check engine light on this trip. So I'm assuming the more I drive/run it the sensor and PCM will get "in sync" with the fuel trims?? Yesterday morning I pulled a spark plug to get a look, here's a couple photos.(sorry if they are a tad fuzzy). Plug I pulled was an NGK Iridium
Textile Sleeve Pink Red Creative arts
Hand Product Textile Sleeve Pink
 

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Ya, just keep an eye on it and maybe freshen the plugs once they get 4-5 or so oil changes on them, ballpark
 

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Discussion Starter · #11 ·
Ya, just keep an eye on it and maybe freshen the plugs once they get 4-5 or so oil changes on them, ballpark
Will do, I was gonna replace the wires before cold weather sets in, so will go ahead and give it fresh plugs then too. I'm stumped as to why that same code popped up after replacing it. Will see what it does tomorrow
 

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Here's a screenshot of what the GM manual says for the code. (open image in new tab to zoom, and see "Conditions for Setting the DTC" section)

The fuel trims may have remained at the high value (+20%) until seeing lower readings for a bit longer than 6sec, to not need to add that much fuel with the new sensor's readings.


Font Screenshot Rectangle Parallel Number
 

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Discussion Starter · #13 ·
Here's a screenshot of what the GM manual says for the code. (open image in new tab to zoom, and see Conditions section) The fuel trims may have remained at the high value (+20%) until seeing lower readings for a bit longer than 6sec, to not need to add that much fuel with the new sensor's readings.

View attachment 11849
So the PCM still had the trims from the old faulty sensor, and had to adapt to the new one?? I didn't do the 5 minute negative battery disconnect method to erase things, I just used my scan tool (an older unit) to clear the code.
 

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Correct. Clearing the code just tells the dash to shut off the light, but the PCM still had long term fuel trims adding a bunch based on the most recent driving on the old sensor. It takes a bit of driving under each condition (various rpm and airflow ranges) for the trims to re-adjust to new inputs being monitored. They adjust slower on purpose, figuring that your vehicle's condition isn't changing immediately. However, short term fuel trims have a more instant reaction to O2 values, and were slowly starting to tell the long term values to come down to normal... this probably just didn't happen quick enough in a few ranges to beat the 6sec clock.

A cheap way to monitor is with an ELM bluetooth OBD2 scanner and a phone app (Torque, ChevroSys, etc.). These go alot deeper than a code reader scantool to show live sensor values, but also have the code/etc. functions, too.
 

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Discussion Starter · #15 ·
Correct. Clearing the code just tells the dash to shut off the light, but the PCM still had long term fuel trims adding a bunch based on the most recent driving on the old sensor. It takes a bit of driving under each condition (various rpm and airflow ranges) for the trims to re-adjust to new inputs being monitored. They adjust slower on purpose, figuring that your vehicle's condition isn't changing immediately. However, short term fuel trims have a more instant reaction to O2 values, and were slowly starting to tell the long term values to come down to normal... this probably just didn't happen quick enough in a few ranges to beat the 6sec clock.

A cheap way to monitor is with an ELM bluetooth OBD2 scanner and a phone app (Torque, ChevroSys, etc.). These go alot deeper than a code reader scantool to show live sensor values, but also have the code/etc. functions, too.
Ah ok, so just give it time to get the new values into memory? I've put maybe 12 miles on the car yesterday on the car since I put it in. half was in town,half highway miles
 

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Discussion Starter · #16 ·
Well.... it came on AGAIN, same damn code. Running perfectly smooth, very good acceleration and power. No hesitating or rough idle, stalling,hard starting or other poor driveability issues. Should I now do the negative cable disconnect, to wipe things clean? Sounding like the old trims aren't clearing from the PCM memory. Or should I just drive it daily, maybe it'll clear itself ?
 

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Only way to know for sure is to spend the $15 on a scantool to see the actual info from the car. I'd do that for peace of mind, and a more convenient way to deal with codes, too.
 

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Discussion Starter · #18 ·
This is what I've been using, was gifted to me by a car dealer friend many years ago. Actron CP 9580. Based on the fact that it's not experiencing any issues with the way it runs, maybe just drive it for now? Car stays in town used for my news deliveries
Motor vehicle Gadget Measuring instrument Gauge Communication Device
 

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Ya, those are just overpriced relics of the 90's at this point. I'd just drive normal, and if it keeps coming back, there's probably a little more troubleshooting to do.

Things like the ELM is $12-15 and does 10x what those do, via an app on your phone.

Rectangle Font Electric blue Magenta Brand

Toruqe App (free lite version, or Pro version is ~$5)
Light Font Gauge Technology Measuring instrument


ChevroSys free app
Font Screenshot Parallel Number Electric blue


Rectangle Font Screenshot Parallel Number


Font Parallel Rectangle Screenshot Electric blue
 

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