P0171 and P0174
Thread Starter
Heel & Toe
Joined: Jan 2019
Posts: 18
Likes: 0
From: Portland, OR
P0171 and P0174
I recently bought a 2010 GS, which is almost entirely stock. The car has 80k. It's throwing P0171 and P0174, bank 1 and 2 too lean.
I plugged a scanner in and looked at fuel trims. Long term fuel on bank 1 was 23, and bank 2 was 24. I've seen a few people say that if you increase throttle and hold it, and the trims decrease, that it indicates a vacuum leak. I did this, and saw them dip to 20, then 19, and I stopped at that point. Is that dip sufficient to indicate a leak? Any other diagnosis I should consider?
I plugged a scanner in and looked at fuel trims. Long term fuel on bank 1 was 23, and bank 2 was 24. I've seen a few people say that if you increase throttle and hold it, and the trims decrease, that it indicates a vacuum leak. I did this, and saw them dip to 20, then 19, and I stopped at that point. Is that dip sufficient to indicate a leak? Any other diagnosis I should consider?
Drifting
Joined: Apr 2021
Posts: 1,934
Likes: 1,126
From: Peoria, AZ
2023 C6 of the Year Finalist - Modified
Fortunately, there are a few steps you can take to rectify a trouble code P0170. The most common malfunctions occur with the MAF sensor. American vehicles tend to return other similar codes instead, including P0171, P0172, P0174 or P0175. Keep in mind that your OBD-II scanner tool may detect other concurring error codes. In those cases, you should ideally troubleshoot and diagnose the root causes of those other errors first.
Possible places to start troubleshooting include observing your vehicle’s MAF sensor readings and replacing the sensor if those readings are not within manufacturer specifications. Next, check for oil intrusions at the connector, measure fuel pressure, rule out external or internal regulator leaks. Inspect and replace any cracked, missing or faulty vacuum hoses. You should also look for vacuum leaks at the intake manifold gaskets or rips in the air supply hose.
I would tend to think the first thing I would look at is the MAF sensor.....Good luck, let us know what you find out!
Possible places to start troubleshooting include observing your vehicle’s MAF sensor readings and replacing the sensor if those readings are not within manufacturer specifications. Next, check for oil intrusions at the connector, measure fuel pressure, rule out external or internal regulator leaks. Inspect and replace any cracked, missing or faulty vacuum hoses. You should also look for vacuum leaks at the intake manifold gaskets or rips in the air supply hose.
I would tend to think the first thing I would look at is the MAF sensor.....Good luck, let us know what you find out!
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
I recently bought a 2010 GS, which is almost entirely stock. The car has 80k. It's throwing P0171 and P0174, bank 1 and 2 too lean.
I plugged a scanner in and looked at fuel trims. Long term fuel on bank 1 was 23, and bank 2 was 24. I've seen a few people say that if you increase throttle and hold it, and the trims decrease, that it indicates a vacuum leak. I did this, and saw them dip to 20, then 19, and I stopped at that point. Is that dip sufficient to indicate a leak? Any other diagnosis I should consider?
I plugged a scanner in and looked at fuel trims. Long term fuel on bank 1 was 23, and bank 2 was 24. I've seen a few people say that if you increase throttle and hold it, and the trims decrease, that it indicates a vacuum leak. I did this, and saw them dip to 20, then 19, and I stopped at that point. Is that dip sufficient to indicate a leak? Any other diagnosis I should consider?
Last edited by C5 Diag; Feb 14, 2024 at 08:10 AM.
Thread Starter
Heel & Toe
Joined: Jan 2019
Posts: 18
Likes: 0
From: Portland, OR
Just a quick update on this. I had a friend get me a new OEM MAF at cost, and given that they're so cheap, I just went ahead and installed it. The long term fuel trims on my scanner dropped to 14/15 at idle, and at cruise, in the 0-4 range. I am unsure what the ideal range is, but the values I see at idle seem high to me.
Thread Starter
Heel & Toe
Joined: Jan 2019
Posts: 18
Likes: 0
From: Portland, OR
You should hold the throttle to around 3000 RPM for a few minutes and see what happens with the long term fuel trims...has any maintenance been done on the car ??...can your scan tool look at "freeze frame data" ??...that would help in a diagnosis...did it happen under load, idle etc...
When I picked the car up from the truck, there were no codes present. The car tripped these codes immediately on startup later in the day. At idle, LTFT was sitting at 25 on both banks.
It is perhaps worth noting that the car has a Corsa intake, but the powertrain is otherwise stock. If 14-16 is what you'd expect at idle for a car with an open intake like that, then I may have fixed the problem with the new MAF. Trims at cruise/accel are 0-4.
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Thread Starter
Heel & Toe
Joined: Jan 2019
Posts: 18
Likes: 0
From: Portland, OR
Thanks for the info you've given me as I've troubleshot this. Given that the fuel trims while driving seem to be well within acceptable limits, and that the trims at idle seem to be lower than the threshold that would trigger a code, is it reasonable to assume that getting a tune isn't an absolutely urgent/immediate requirement? I just bought the car, and I'd like to add a couple more things to it before I tune (rather than tuning multiple times)