By Dennis Bandy
So your car's CEL (Check Engine Light) is on and you had the codes scanned at a local parts store. Your car has either a P0171, P0174 lean fault code or both stored in the computer, these codes are based on Oxygen Sensor (O-2) readings. A lean code or codes indicate that there's too much oxygen in the exhaust. Remember parts stores have employees that have good intentions but they may not have the experience necessary to interpret what the trouble codes really mean. These codes are based on oxygen measurements in the exhaust. A common mistake with lean codes is to replace the oxygen sensors. This could be a very expensive mistake that will not fix the problem. Especially if both codes are present, because the chance of both O-2 sensors failing at the same time is very unlikely.
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Most likely the cause is a vacuum leak. A vacuum leak can be caused from a vacuum hose, intake gasket or maybe even a leak in the air intake hose from the MAF (Mass Air Flow Sensor). Listen for a hissing sound that may lead you to the source of the problem. Some technicians will use a propane bottle with a hose attachment to help pinpoint vacuum leaks. With today's computers it's not quite as easy to check for vacuum leaks this way because the ECU (Electronic Control Unit) will compensate quickly for the added fuel and a change in idle is harder to notice. Oxygen sensor readings can be monitored with a scan tool while checking for leaks with propane, by looking for increased readings when enriching the mixture. Another way technicians can check for vacuum leaks is with a smoke test. By introducing smoke into a vacuum hose on the engine, the leak will be revealed when the smoke escapes from the problem area.
Aftermarket air filters that use oil on the element can sometimes damage the MAF. Over oiling the air filter may allow some excess to get on the MAF sensor wire or element. This can alter the reading, fooling the ECU into seeing more or less air flow therefore changing the air/fuel mixture incorrectly. I once worked on a car that would not start that had a problem with a MAF. When looking at the wire in the MAF, there was a burned piece of trash that made it's way past the air filter. After cleaning the sensor the car ran perfectly. The ash that was on the MAF sensor wire was altering the reading by enriching the mixture so much that the car could not run. After talking with the customer, he said the air filter was just changed. This was obviously when some trash got into the air intake hose that settled on the hot wire of the MAF.
Fuel Pressure could also cause a lean condition. If the fuel filter is clogged or the fuel pump pressure is low, there could be higher level of oxygen in the exhaust also. Most of the time though, the ECU will compensate for the reduced fuel volume. So this is one of the least likely causes of a lean code.
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About the Author. Hi I'm Dennis Bandy, thanks for reading. I've been an ASE Master Auto Technician for over 20 years and have also sold professional auto tools since 2005. I am in a unique position to help readers with automotive repair related topics since I've had personal hands on experience with many auto repairs and automotive specialty tools. This is not a hobby for me, this is my business. Your feedback is really appreciated, if you thought this article was helpful or informative, please rate it below (thank you in advance).
Half the battle when it comes to auto repair is having the right tools, that's why we have supplied name-brand automotive tools (like OTC, Makita, Ingersoll), shop supplies and automotive shop equipment at discount prices, at DenLors Tools since 2005, many readers put us on their favorites list and visit our site when auto specialty tool needs arise, we hope you do the same.
21 comments:
THANKS A MILLION FOR THE INFO, IDID LEARN SOMETHING NEW WITH YOU TODAY, YOU ARE REALLY GOOD EXPLAINING, THANKS AGAIN.
Excellent, Excellent writeup, I'm experiencing these exact codes (read free at Auto Zone) and idle problems on my 2002 Toyota Sequoia. I will be taking my vehicle to my nearest honest mechanic to get a smoke test done for possible vaccume line or manifold leak. I have ruled out MAF sensor putting in two new ones. Once my Check Engine Light came on (took nearly 2 months of stalling at idle and a few even while driving high speeds.. Scary!) computer/engine compensated fuel/ air mixture. Im now having no stalling problems but Truck is running a little rough and fuel economy has been terrible. I will follow up with how I finally solve my p0171/ p0174 mystery. Thanks!
Hello Dennis,
I have a Mercedes C230 W203 with a V6 engine (M272). 2006 year.
The faults are the that you named: p0171 and 174. I have clean injectors, and throttle body, change MAF sensor and change spark plugs.
And the DTC's have came back.
Now, at this moment my car is on Mercedes Benz specialist dealer service, and after a day of diagnosis, they told me the is the Intake Manifold. Few weeks ago one spark plug appears broke on electrode part. Then I replaced them. But the mechanic blames the intake manifold because that trouble with spark plug. One super small part inside the intake manifold brake and this part goes directly inside the cylinder causing the problems.
Although, the alarms back after all I do trying repair and/ or change all parts implicated.
So, what do you think about this solution that my local service told me and priced?
Thank you and sorry for my English!
Greetings.
Thanks, after reading this i when out to the car and just listened and could hear the vacuum leak, just a intake manifold fitting that was plugged and the rubber plug was deteriorating. 2000 Malibu
could this code be caused by an exhaust leak?
Very well written! I just learned some new things again!
Keep up the good work and thanks!
Hello Dennis
I have the same problem with Hyundai Tuscon 2005-2.7 engine. I have had a smoke test which was negative for leaks, replaced the MAF which did not solve the problem, replaced the o2 sensor -Bank 2 sensor 1. Only to have the code come right back. I am at a loss, do you have any further suggestions.
thanks
amin
I have a 2005 acura rl sh-awd and have a code PO174. I know it's fuel to lean but wondering if I have a vacuum leak. Can I detect this using propane or is a smoke test better
I have a 2005 acura rl sh-awd and have a code PO174. I know it's fuel to lean but wondering if I have a vacuum leak. Can I detect this using propane or is a smoke test better
You are the ONLY one to get it right, I hear vacuum leak etc everywhere but you are the ONLY one to mention the AIR FILTER. Sure enough it was an aftermarket "oiled" air filter that was supposed to be so great that was causing these codes. Replaced the filter and just drove it a bit, codes went bye bye, been living with those codes for years LOL! :(
THANK YOU!
Oh and by the way the "Stealership" wanted me to pay them 800 bucks to fix that. ROFLMAO.
I have a 2005 dodge 1500 Hemi, and in the last year ive replaced the EGR valve, replaced a EVAP hose (truck has a 12" on 39's and a bolt caused the hole), then system too lean bank 1&2 poped up. Cleaned MAF sysnsor, and replaced both upstream O2 sensors. It pasted smog finally but still getting too lean bank 2 nonstop. I dont think its injectors or anything with fuel cause the truck runs perfect, ZERO hasitation (and due to lift/tires i would deff notice a hasitation).... it runs perfect, but fuel mpg is down due to CPU trying to add fuel nonstop. Any suggestions? If possible reply to my email with any ideas, thank you. Nielsentoddjohn@yahoo.com
So, you were getting the p0171 and p9174 codes from just a dirty intake air filter?
NO. The oil from the filter was getting on the sensor. There were other problems though and in the end, I replaced the Computer with a preprogrammed new one off of ebay. Runs like a DREAM now!
kani,
i had a same issue with holden commodore and found vacuum switch was leaking it self and replaced with new problem fixed
The smoke test may save you money and enlighten you as well, I am a woman with a little mechanic experience thanks to my Dad, and used his logic and won by saving money using the smoke test to diagnose and rule out vacuum leaks prior to purchasing other parts. Unfortunately i have seen people go through a lot of grief due to small things like that. Not knowing where to turn they go to a mechanic and unfortunately some of them were self proclaimed and didn't know crap.
If you ever have these codes just replace your computer immediately. Buy a pre programmed computer off of Ebay for less than 100 bucks. Problem solved.
Just because a computer fixed one vehicle issue with "the lean codes" doesn't mean it will fix all vehicles with P0171/174 codes. Codes don't tell you exactly what's wrong, it's a starting point to begin troubleshooting. You have to know how the system works and how the interrelated parts function. These codes can be from a clogged fuel filter, low fuel pressure caused by numerous issues, a bad or dirty MAF sensor, vacuum leaks, intake manifold gasket leaks and other issues. Going by a code and changing the "most likely cause" might work but more than likely you will be making the parts stores richer and finding yourself with much less beer money and free time!
What did u find out on that exhaust leak and if that could cause this code dusty kolar??
Hi thank you for your information. I own a Freelander 2 Petrol with those two codes appearing all the time. My mechanic advise me to change the MAF sensor what I did and didn’t fix the issue. Since a week my car is smoking a lot a white smoke. Can you please help me to fix this issue?
The article written is really great and informative. will be looking further for these type of posts. Thanks.
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