GM/Chevy 5.3 P0172 P0175 hard start and rich running

Diagnosing and repairing GM and Chevy vehicles with the 5.3 L flex fuel engine. Most common customer complaint is Hard start cold, no start, rough running and check engine light. This vehicle had a P0172 and a P0175 which is for rich running bank one and two. After observing the fuel trim and confirming that the vehicle is indeed running rich we check the fuel alcohol composition with the scan tool. The ECM is reporting 53% alcohol content. A physical test of the fuel indicates less than 10% alcohol content. The vehicle is improperly reporting alcohol content and the fuel composition needs to be reset. With a capable scan tool go into output test and reset fuel your composition and fuel trims And then also clear codes. Start up vehicle and monitor fuel trims and they should be +/-10%.

[@andrewk778]

Videos like this allow a DIY’er like myself to see repair trends as this. Thank you for sharing.

[@ThePracticalMechanic]

No problem Andrew.

[@vpjohnston]

Thank you so much for this information! Was pulling my already sparse hair out trying to figure why my 2013 Silverado (4.8) was running so rich. This worked immediately

[@tomspindler1459]

so exactly what was the problem.. mine is hard to start and useing a lot of fuel

[@gueroburban4989]

My truck has these codes and feels like is running by rough and waste a lot of fuel you think I need a reset?

[@vpjohnston]

@@gueroburban4989 it costs nothing to try it if you have access to a reader that will reset. My 2013 (4.8) runs great after one reset last year

[@gueroburban4989]

@@vpjohnston thanks just ordered it!!

[@SAAutoRepair]

It's funny how many shops like you and Eric see these. I get like 1 a year. Nice job sir

[@ThePracticalMechanic]

Seem to get more and more as winter progresses. I wonder if the life span of your average vehicle plays a part in it. Most of mine have been 2009-2013.

[@michealdoeden1738]

Thank you so much for posting this. I was lucky to find it. Exactly what was going wrong with my 09. Took care of my issues. Thank you very much.

[@weldingjunkie]

Great job chad that update looks a lot better now than the ones I’ve seen in the past.

[@weldingjunkie]

@Debbra Dobbyn nope don’t give a shit

[@creme757]

Worked for me on a 2009 suburban with an Autel MS908

[@jayyin88]

Hi, I have a 2003 Chevy Avalanche with 5.3 Vortec. Recently it is throwing P0172 and P0175 as well. I changed the MAF but it does not make any difference. Can it be caused by fuel composition as well?

[@mopira1]

Thank you so much 👍👍👍👍🍺🍺🍺

[@htownboy2k]

Had the exact same issue on a 09 5.3l, thanks for the post, hours of trying to figure it out, Alldata, TSB, scan tool and had no idea. However still don't understand the software update doesn't fix it, resetting it, it will just happen again, what's the solution

[@chrisburns143]

Any luck ??

[@Ftwandyou5997]

Thank you. This video helped a bunch.

[@ThePracticalMechanic]

Glad to hear it! An intermittent vacuum system leak can be the root cause of the miscalculation of these codes often set on perfect running vehicles as well.

[@bennyrod1135]

Thanks for this information

[@Valamilk]

great video...you forgot your tags....wear your seatbelt 😁

[@vaughnjolomi3332]

This seems to be the exact issues with my 2011 5.3L flex fuel, where are you located in Colorado?

[@andrewcavers1399]

Thanks for the information

[@taylorknack4940]

I have a 2003 silverado 5.3 with the codes p0172 and p0175 both running rich codes. But i do not have a flexfuel could it still be this problem you are showing here?

[@robquillan77]

Nice video I don't own a Snap-On scan tool and I'm on a budget so I'm trying to figure out what's the most affordable scan tool I could buy to do this myself also curious if it fixed the problem when you reset it on this vehicle I have a van 2013 with the same exact symptoms getting ready to have to take it to the dealership to have it reset if I can't find a scan tool to check it

[@MrAIM94]

Hi, I’m having the same issue with the same model truck in your video. Where is your shop located in Colorado? I would love to pay someone who knows what they are doing to fix this issue. Thank you great video

[@wyattoneable]

Nice.

[@oxygen454]

I have the old school Modis and didn't see a fuel alcohol reset.

[@gerardogarcia8364]

The pressure fuel pump it was the problem? Or did you found something else?

[@robertpope3619]

Hello, I'm in the heavy duty diesel industry so fuel trim is something I'm not to familiar with. Do u have a video on understanding fuel trims and what they should be?

[@ThePracticalMechanic]

I do not have a video currently on fuel trims. Fuel trims are broken up into short term and long term units. The short term fuel trim is adding and subtracting fuel very quickly to keep the fuel mixture correct. The long term fuel trims take a while to learn and are typically more stable. The fuel trims are a deviation from the factory programmed fuel map. Most vehicles will be within 10% of zero. Some vehicles will set a check engine light for Rich running or lean running if the value is above 10% rich or lean.

[@kyleteague3903]

Is this just a bandaid fix though, like what caused the alcohol content to become rich in the first place?

[@ThePracticalMechanic]

It’s often a vacuum leak when cold that causes it. When you fill up with fuel the engine monitors the fuel trims for a predetermined amount of time to see if you filled up with a different blend of fuel. If the learn doesn’t complete after fill up it will continue the next morning when the vacuum leak is present during cold start. The computer adds to the alcohol compensation thinking the blend changed. One the vehicle warms up and the gaskets seal up the vehicle will now run rich because of the alcohol composition being elevated.

[@stevend1384]

Do u think this could be same problem with my 2002 flex tahoe. I don't have any checking inexpensive lights. Replaced fuel pump.regulator and have done intake knock sensors. And such

[@ThePracticalMechanic]

Never know without reading the scan data. Some of the early flex fuel engines had an actual flex fuel sensor in the fuel line under the vehicle and they didn’t have this issue.

[@ygkpilot]

Thank you! Seriously helpful video. I have a 2010 Silverado 5.3L with this fault, was fixed by a mechanic a year ago. I only drive short trips. Is it possible if I go for a long drive, say half a tank of gas the computer might learn and correct these fuel/alcohol ratios? Please let me know, thanks again for this very helpful video.

[@ThePracticalMechanic]

The pcm is supposed to relearn every time the truck is filled up. I would recommend warming the truck up and then add at least 4 gallons of fuel before driving. It may relearn and may not. A long drive shouldn’t be required. 5-10 miles should be enough. Sometimes you just have to reset the system to get it to learn. Some vehicles also have a programming update to help but it may still have errors. Another option would be to find a shop that does tuning and they can turn off the flex fuel system so it doesn’t happen again.

[@ThePracticalMechanic]

Another option would be to buy a scan tool so you can reset it whenever it drifts. This scan tool is cellphone based and covers most vehicles. It is very affordable. amzn.to/3SW3PGz

[@ygkpilot]

@@ThePracticalMechanic thanks for getting back to me and thanks for the advice, I appreciate it.

[@gueroburban4989]

@@ThePracticalMechanicso i can reset my fuel trim with this scanner you say from Amazon ?

[@gueroburban4989]

@@ygkpilotdid you buy the scanner and did it work to reset fuel trim?

[@MichaelJohnson-ze4gk]

Is this for flex fuel I have this issue on a 2009 chevy avalanche. I've changed just about every part related to these codes. And I'm still running rich like -30/-14 is the highest the trims went.

[@ThePracticalMechanic]

Yes this reset is only an issue on flex fuel vehicles.

[@jmbrad1968]

Currently dealing with this issue myself on a low mileage 2011 GMC Sierra 1500. Hard starts on cold mornings. Sensor is sitting at 69.8%. I read where filling up with E85 will "shock" the system back to learning the fuel composition. Unfortunately I didn't have a scan tool to read the value before I filled up with E85, but since then the hard starts are much better. Just runs a little rough. Before I would have to press the accelerator to clear the flooded condition before it would start. So, my question is, is there any other way to reset that value without a scan tool? Battery disconnect, perhaps?

[@mrpipefitter9963]

I hope someone can answer this I have the same problem

[@jmbrad1968]

@@mrpipefitter9963 I was able to get the alcohol reading down to 6%. I ran down to fuel light came on and refilled with E85. That forced the reading up to 80%. Ran down to empty again and refilled with Ethanol free. It dropped to 6%. Make sure to drive at least 10 miles after fill up and don't fill up on a cold engine either.

[@mrpipefitter9963]

@@jmbrad1968 thanks only problem I’d there’s no e85 near me

[@mrpipefitter9963]

Is*

[@jmbrad1968]

@@mrpipefitter9963 time for a road trip!

[@movieuser6336]

Are any of those capable scanners under $600???

[@ThePracticalMechanic]

there are many capable scanner for around $600 and some for less. If you want to scan multiple brands and access most of the modules on the vehicle you may want to go with a launch diagun or other launch scantool(the cheaper ones may not have the reset function). You can often find 5yr old snapon scantools with nearly current software for that price. I choose to use what I use because that is what I'm used to using and I rarely have issues.

[@chotobigboy9365]

Just a little research

[@angeld1278]

Please help 2015 chevy tahoeI have this code po175 for the last 13 days and I've been wasting a lot of cash trying to fix this problem so you mean that this is the only way to raise the problem with that's

[@gerardogarcia8364]

Did you found the problem in your truck ?

[@lifeshare9074]

This works for a 2005 also??

[@ThePracticalMechanic]

I’m not sure if the 2005 has the same issue or not but if the alcohol content is elevated in scan data then this should work.

[@moecapitolnetwork2916]

2008 suburban 5.3 flex fuel was reading misfire codes and catalytic converter codes and upstream o2 sensors, so we replaced plugs wires catalytic converters and o2 sensors, reset codes ran great, no codes. For some reason i rest the fuel alcohol content because it was %80! After i reset it it was at %3. Now the truck acts up hard start , and lean codes p0171 and p0174 and p1153. I need help. Thanks

[@ThePracticalMechanic]

you may have to check the actual alcohol content. If the fuel has a high alcohol content then it will run poorly after reset. If the actual alcohol content is less than 10% you have other issues causing the lean condition.

[@moecapitolnetwork2916]

@@ThePracticalMechanic makes sense thank you

[@fuckwitpeoria]

@@ThePracticalMechanic will that work a for 2011 Cadillac with the 6.2 flex fuel motor looking to purchase just want to make sure it covers my make model and year thanks

[@ThePracticalMechanic]

@@fuckwitpeoria yes the scan tool will also work on that model. I reset one a few weeks ago.

[@fisherguy]

What year was that truck? My 08 has had this problem, had the alcohol levels reset but I still get the code now and then and when I do I use my tuner to clear them. One symptom I've noticed is the tranny shifts hard when I have the code. What can I do to cure it permanently?

[@ThePracticalMechanic]

Depending on the tuner you can turn off flex fuel entirely. Some have an option to turn it off and some require you to edit the maps from the ecm. You might be able to request a custom tune from the device manufacture. You may have something else causing the lean condition that is the root cause of the flex numbers rising. Check for Vacuum leaks when cold, clean mass air flow sensor and throttle body and see if the problem is resolved. Toyota has the same issue from the tundra and sequoia and it is caused by a weak fuel pump. The pump may deliver too little volume on cold starts which leads to falsely assuming the presence of flex fuel.

[@fisherguy]

@@ThePracticalMechanic, interesting. I cleaned the MAF anf throttle body the 1st time I had the codes again, didn't help. I had the alcohol levels rested about 18 months ago and since then I've had them come back, first for 1 bank then if I leave it for both. I've just been clearing them with my 'superchips' which clears the code and fixes the hard shifts. Frustrating, hope it's not the fuel pump, PITA to change.