Code P0171, Troubleshooting Lean Problem?

Tiny
LSROWE9
  • MEMBER
  • 2014 HONDA CRV
  • 2.4L
  • 4 CYL
  • 4WD
  • AUTOMATIC
  • 149,000 MILES
I drive my car ~35 miles a day round trip during the week. I got a CEL with P0171 code while driving; the console light (green while cruising) immediately switched to white while driving after CEL activated. A day later CEL goes away a console light becomes green again while cruising but the issue repeats soon afterwards. The economy worsened from 24-25 mpg to 22-23 mph over the next few days. Idle was rough and low 400-700 rpm. On rare occasions I smell a trace of fuel but thought it was smoke from another car. I might notice some trace white smoke after cold start but I don't notice smoke from back while driving. 4 days later, I ran a few live scans. Sort Term Trim was 15-30% and drops towards 0% while increasing rpm to 3000. Long Term Trim steady at near 26%. I thought vacuum leak. As additional fyi, I live scanned the MAF and upstream A/F ratio 02 sensor while driving. Lamba was ~1 but spiked to 2 whenever the rpm drops. MAF was 0.2 lb/min while idle, about 1.6 at 3000 rpm steady and as high as 7 lb/min at 3000 rpm peak while driving. I did a smoke test, feeding the intake tube post-maf sensor. I noticed 2 missing clips for air bleed tube (after MAF) and a smaller B12 tube, thanks to my mechanic. Smoke leaked from those points but didn't appear significant. Hard to say. I looked inside the throttle body had black soot ring around choke; so, I cleaned around it with carb cleaner (I might have sprayed a little in there by mistake). Removed PCV valve and cleaned inside with carb cleaner; I saw a stopper on bottom side and spring on the tapered side. Not sure if this is normal for a PCV valve. I reinstalled PCV valve. Engine has been idling high 1000 to 1500 rpm ever since. Engine sometimes feels a little rough sometimes while foot on gas near 30-50 mph. As soon as I take foot off the gas while cruising 10-35 mph, the car hesitated before decel. I now have a permanent P0171 and new code P0507. 5 days later, I live scanned the O2 sensors while idle. Upstream A/F ratio O2 sensor is ~1 lambda and 0-20 mA current; Downstream O2 sensor is 0.6 V average, no spikes while idle. Only thing I haven't checked yet were the fuel injectors and fuel filter. I hope it's not those. Any ideas? I plan to clip the affected tubes and replace the original PCV valve, MAF, and MAP as soon as parts arrive. The only thing I haven't looked at yet was the fuel injectors and fuel filter. Any ideas?
Wednesday, September 18th, 2024 AT 10:21 PM

15 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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You are on the right track, the PCV valve and MAF sensor replacement should fix the problem, can you confirm no vacuum leak? If so, then proceed.

P0171: System Too Lean (Bank 1)
P0507: Idle Control System RPM Higher Than Expected

Also, with that many miles I would remove the throttle body and clean it thoroughly. Here is a guide to help with instructions for your car in the images below:

https://www.2carpros.com/articles/throttle-actuator-service

This guide can help as well:

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

the MAF sensor once misbehaving must be replaced and cannot be cleaned to fix the problem. Check out the images (below). Please upload pictures or videos in your response of any problems so we can see what to help you with.

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Thursday, September 19th, 2024 AT 10:41 AM
Tiny
LSROWE9
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Thank you for helping me on this. So, 5 days ago I replaced the MAF (and MAP). I also ended up replacing the throttle body and tb gasket. I experienced issues reconnecting the outlet coolant hose to the throttle body [1/8" short] and I will need to replace that coolant hose later. I also had issues replacing the original PCV valve and will try again shortly with a new part. I wasn't sure how to do the TP Position check. However, I performed the relearn procedure. Car is idling much better and both P0507 and P0171 codes have not yet returned (no codes). Things appear to have improved, but the economy still is not that great ( 22 mpg avg); it's like the actual mpg is always much higher than the avg mpg but doesn't raise the avg mpg much. Even on highway. Power on highway feels a little weak. While engine idles with radiator fan going on/off, the short-term trim sawtooths between 25-40% while O2 downstream inverse-sawtooths between 0.1 and 0.8 V which is much different than before; upstream O2 lambda seems OK (same as before). Could the O2 sensor be bad? I recently noticed a small cut by the purge hose where the missing b12 clip was supposed to be and will also replace that hose soon (it's taking a while to get these parts).
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Thursday, September 26th, 2024 AT 9:39 PM
Tiny
KEN L
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  • 54,137 POSTS
It sounds like you still may have a vacuum leak, so check for that. This guide can help:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

You may have a weak oxygen sensor as well, but I would check for vacuum leaks first. Let me know, it looks like we are getting closer to a 100% fix. Also, the fuel milage you need to give time to relearn before you get the true readings.
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Friday, September 27th, 2024 AT 10:52 AM
Tiny
LSROWE9
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So, I first changed the PCV valve and the car drove fine for about 2 weeks before CEL P0171 returned. Avg mpg was as good as 25-26 mpg for the most part and as low as 22 mpg. Depending on how I drive (city/hwy), CEL P0171 appeared for a few car starts and then self-cleared for a few days before repeating itself.

One month later, I was finally able to change out the purge hose that was cut at one of the ends. I also changed both inlet and outlet coolant lines to the new Throttle Body as well. I smoke tested airflow tube post-maf sensor [again] and did not see smoke escaping any vacuum lines. The only suspect areas I found were a few points highlighted on the air flow tube between MAF and TB (see image). Hard to say if the airflow tube and clamps are getting old.

Also, I ran the scan tool and saw much different behavior. While idling, the Short trim was 46.9% flat and O2 downstream 0.065 V flat. Sometimes when radiator fan toggles on/off, the O2 downstream toggles between 0.045 V and 0.7V while Short trim stays 46.9% flat. The Short trim is lower and much closer to 15% while driving but after stopping the short trim ramps back up to near 46.9% Tightening the clamps at both ends of airflow tube didn't seem to make a difference.

I will be checking some areas with carb cleaner shortly.
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Sunday, October 27th, 2024 AT 1:25 PM
Tiny
KEN L
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The lean trim is trying to richen up the system due to the long term trim being lean. To confirm you did replace the MAF sensor with a new one correct? We should do a fuel pressure test because the fuel pump can be lean, here is a guide to help show you how and I have included instructions on how to check the pressure in your car in the images below.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Fuel pressure should b: 47-54 PSI.

Check out the images (below). Please let us know what happens.

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Monday, October 28th, 2024 AT 10:01 AM
Tiny
LSROWE9
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During the pressure relief, I did not have the HDS but removed the PGM-FI main relay #2 instead. The sooner I turned off the engine and then removed the relay (like right away) the more the engine idles until stall after starting engine (about 1-2 seconds from idle to stall). If I waited a couple minutes after removing the relay, then the starter would turn but the car wouldn't start which was interesting.

After setting up the fuel pressure gauge, I turned the key to II, the pressure was 50 psi for about 2 seconds then 46 psi. After starting the car, the pressure was 50 psi during idle. After turning off the car, the psi dropped to 46 psi and then within 5 minutes dropped to between 38 and 40 psi.
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Sunday, November 10th, 2024 AT 9:34 PM
Tiny
KEN L
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Nice work, the fuel pressure looks okay. Can I ask if the exhaust system is leaking? This can draw outside air into the system and give a lean code.
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Monday, November 11th, 2024 AT 8:58 AM
Tiny
LSROWE9
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I placed a smoke tester into the exhaust and did not see any smoke coming out from anywhere else under the car.
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Wednesday, November 13th, 2024 AT 4:32 AM
Tiny
KEN L
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You can try the primary oxygen sensor is the only other thing I can think of besides a bad PCM.
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Wednesday, November 13th, 2024 AT 8:31 AM
Tiny
LSROWE9
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Thanks for your help on this. I swapped the primary oxygen sensor and it did not appear to make a difference on the ST Trim response. After starting, ST Trim around 17% for a few seconds and then started climbing to 46.9% as soon as the radiator kicked in.

Here are some pictures on what the original sensor and new sensor looks like. Looks like silicone poisoning but I am not sure what that means overall. I'll consider switching the sensor again with one that has the shield (OEM-like). I am not sure why some compatible sensors come with the shield and others don't.

Otherwise, I will be preparing for a PCM swap. This P0171 code is proving to be a dreaded one.
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Saturday, November 16th, 2024 AT 3:49 PM
Tiny
KEN L
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Lean condition can only be so many things, vacuum leak, not enough fuel, exhaust leak. MAF sensor. We are running out of items to check besides the PCM.
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Monday, November 18th, 2024 AT 10:23 AM
Tiny
LSROWE9
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It took a while but I think I finally found the potential vacuum leak source. I smoke tested the intake again from the brake booster hose and pcv hose (after PCV valve). This time I see smoke reeking out from behind the left, center, and right of the intake manifold. I must have missed it during previous smoke test, thinking it was just the lower connection of the airflow duct tube with the throttle body.

The P0171 error code has remained intermittent throughout. I do remember seeing the check oil light flicker just once for a spilt second while in traffic a few days ago when the CEL reactivated. Same day, I ended up adding engine oil (2 qts) after checking the oil indicator (below minimum mark).
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Sunday, November 24th, 2024 AT 4:04 PM
Tiny
KEN L
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It sounds like you have some gaskets or a hose leaking down at the intake. I would remove the intake manifold for inspection. I have seen these intakes crack and leak as well so be on the looking for that. Check out the images (below). Please upload pictures or videos in your response of any problems so we can see what to help you with.
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Monday, November 25th, 2024 AT 9:14 AM
Tiny
LSROWE9
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After removing the manifold, I found a crack by the elbow of PCV Hose behind the manifold connector, seems like a common issue with the OEM part. The manifold did not appear to show any signs of cracking and was reused. After replacing the PCV hose and surrounding gaskets (upper manifold and TB rings), the Short-Term trim is now well below 10% (4 to 7%) during idle. The issue appears to be fixed, finally. Many thanks for your help with this.
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Saturday, November 30th, 2024 AT 10:34 PM
Tiny
KEN L
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Good find, that will cause the problem, thanks for getting back to us and uploading the images, this will help people that visit this post.
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Sunday, December 1st, 2024 AT 10:30 AM

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