P0171 – Meaning, Causes, Symptoms, & Fixes

Code P0171 Definition

System too lean (Bank 1).

What Does P0171 Mean?

Combustion engines run most efficiently when they maintain an air-fuel mixture ratio of 14.7 parts air to 1 part fuel. When there are more than 14.7 parts air to 1 part fuel in the air-fuel mixture, a lean condition exists and code P0171 is triggered. The lean condition can be caused by a vacuum leak, which introduces more air into the air-fuel mixture, or by a weak fuel system, which does not input enough fuel into the air-fuel mixture. To keep the engine running properly, the powertrain control module (PCM) tries to compensate for the lean condition by injecting more fuel to the mixture in an effort to maintain the proper 14.7:1 ratio. When these adjustments become too large, code P0171 is triggered.

P0171 Symptoms

  • Check Engine Light is on
  • Lack of power from the engine
  • Rough idle
  • Engine coughing
  • Engine misfiring

P0171 Causes

  • Dirty or faulty mass air flow sensor
  • Vacuum leaks – PCV hose, vacuum hoses, intake manifold gasket.
  • Weak fuel pump
  • Clogged or dirty fuel injectors
  • Exhaust leak
  • Faulty oxygen sensor

Code P0171 Severity – Moderate    

It is okay to drive a vehicle with P0171 for a short period of time, but driving with this code for an extended period of time can overheat the engine and cause internal engine damage.

Code P0171 Common Diagnosis Mistakes

It is important to complete the entire diagnostic process when diagnosing P0171. Many people will replace the air fuel sensor or O2 sensor as soon as they get a bad reading, but the root cause is often a dirty or faulty mass air flow sensor or vacuum leak, thus causing the O2 or A/F sensor to read differently to compensate. Reading and analyzing fuel trims and the freeze frame data is the key to properly diagnosing P0171.

P0171 Diagnosis

  1. Use FIXD to scan your vehicle to verify P0171 is the only code present. If other codes are present, they must be addressed first.
  2. Inspect all vacuum lines and hoses for leaks, and make sure they are properly connected. If a leak is present, you will hear a hissing sound, though it may be difficult to hear with the engine running. If you suspect a vacuum leak but are having trouble finding it, check out this detailed guide to finding vacuum leaks.
    • Reconnect any disconnected vacuum lines and replace any damaged vacuum lines that are leaking, then use FIXD to clear check engine light code P0171. If check engine light code P0171 is triggered again, check the rest of the vacuum lines for leaks and continue to the rest of the diagnostic process.
  3. Remove the mass air flow sensor and clean the sensor using mass air flow cleaner or contact cleaner.
    • Reinstall the mass air flow sensor and clear the check engine light using FIXD. If the check engine light comes back on with code P0171 continue the diagnostic process.
  4. If check engine light code P0171 persists after you have reconnected and replaced all damaged vacuum lines and cleaned the mass air flow sensor, perform a fuel pressure test. If any components in the fuel system are failing, replace them as necessary.
  5. If you confirm there are no vacuum leaks, have a clean mass air flow sensor, and your fuel system is functioning properly, it is time to replace the oxygen sensors.
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Comments 17

Dennis smythe

April 24, 2018

I have a 08 ford escape v 6 when ever my gas tank goes down to empty on i full it up it wouldn’t start

Thomas Padgett

August 31, 2018

I have a 2001 Lincoln last v8 with P0171 I replace the plugs coil packs oxygen sensor check the intake manifold and tried to check for exhaust leak still have rough idle what to do


December 14, 2018

bad fuel pump

David Marquard

June 9, 2018

I’ve cleaned the mass airflow sensor I’ve replaced the distributor cap all of the wires to the spark plugs I’ve replaced with the oxygen sensors both oxygen sensors I replace the crankshaft sensor I checked the vacuum lines I’ve replace the fuel pump I’ve replace the evaporative control system I’ve cleaned the fuel injectors I’ve replace the fuel cap and I still have a p0171. Any suggestions?

FIXD Marketing

June 25, 2018

Hi David,

You might need to take it to a mechanic and check to see what the problem could be. There may be internal circuitry issues.


August 4, 2018

You might have a cracked exhaust manifold, with the engine off and cold, push air into the tail pipe with a shop vac, while spraying soapy water around and on the exhaust manifold.

Erik Griffith

August 7, 2018

Intake manifold gaskets


August 29, 2018

Did you fixed? What was the problem i have the same problem? Pleadr answer me


October 24, 2018

Just junk the car

Kevin Davis

July 30, 2018

I have 2007 Chrysler Sebring. 4 cyl 4 door sedan. Ive change my upstream sensor and now im getting better mpg but its still not like before. My car has been jumping when I turn the ignition on and im starying to think its my fuel injectors. What do you think?

FIXD Marketing

July 31, 2018

Hi Kevin,

There are multiple causes to trouble code P0171. Please follow our diagnosis steps to make sure that you are checking all probable causes. Let us know what you find!

The FIXD Team

Steve Rohman

August 13, 2018

I had a check engine light and reduced accelerator response on my 2006 Cadillac Xlr-v a course le of weeks ago. Once I let it warm up for about 15 minutes the accelerator issue went away but the check engine light stayed on. I ordered the FIXD unit last Saturday and checked the car again (before I received the FIXD unit) and the ch Co engine was off and no throttle issues. This car sets s lot and is on s trickle charger. I received the fixed unit today and received a P0171 and a P2127 code but the check engine light was still off. Can these issues just go away?

Bryan knapp

September 3, 2018

I have a 2000 GMC Sierra 5.3 that runs lean on bank 1 I’ve done everything I had a tapping lifter I even pulled the heads and and replace them new fuel injectors new oxygen sensors the valves look great went over all the vacuum lines many of times and I know it’s not the massive air flow sensor I’ve replaced twice it only doesn’t on bank one it would mess up the entire engine like a lot of other parts this truck just acts up at random I went over every ground attached to the frame motor I’ve been reading on a lot of issues over the past few years and never came up with anyting except for one not the guy online had to reset are tuna Bank one from a diagnostic tool which I’m not going to go out and buy a $400 tool to try it I had it in the shop 5 years ago and they couldn’t figure it out either they did a pressure check to make sure I didn’t have a antifreeze leak and a vacuum and a fuel pressure test but there’s not too many of these types of trucks on the roads anymore like that was a big issue with Chevy in that era


November 3, 2018

Chevy dealer knew exactly what it was, bad fuel pump on ZR2 blazer 2004, charged me 1000. scammers


November 6, 2018

My car was getting occasional P0171 errors. I’d clear the error and some time later would get an error hit. Never several errors, and not following any particular sequence to cause the error.
Now back in May 2018 my car club had arranged to drive on Talladega. I drove maybe 20 laps speeds were 110 to 140 MPH. Whatever happened, I guess cleared up the fuel flow, I haven’t had the problem since. Bottom line, running the engine hard fixed this problem in just one case. And my Gosh it was fun!!!

Kenneth M Williams

December 13, 2018

My check engine light came on and I was just about to purchase a diagnostic tester to see what was going on. I then saw your ad on the web and purchased one. Long story short it worked; it cleared my light. Thanks guys!

FIXD Marketing

December 13, 2018

No problem Kenneth! Glad we could help!