P0171 OBD-II Trouble Code: System Too Lean (Bank 1)

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Cost of diagnosing the P0171 code

P0171 is a diagnostic trouble code (DTC) for "System Too Lean (Bank 1)". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $114.99 . Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.0 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

Cars Estimate Credit towards follow-up repair Earliest Availability
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Toyota $114.99 $20.0
BMW $124.99 $20.0
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P0171 code definition

System Too Lean (Bank 1)

What the P0171 code means

The P0171 OBD-II code means that, on the first bank of the engine, the fuel system is running weak or a vacuum leak exists near this side of the engine. A lean condition occurs when the engine either receives too little fuel or too much air.

What are the causes of the P0171 code?

Causes of the P0171 code may include:

  • A faulty fuel pressure regulator
  • A weak fuel pump
  • A clogged fuel filter
  • A faulty powertrain control module
  • A vacuum leak
  • Faulty injector(s)
  • Faulty oxygen sensor(s)
  • A faulty mass air flow sensor

What are the symptoms of the P0171 code?

Symptoms of the P0171 code may include:

  • Loss of power
  • Check Engine Light
  • Hesitation or stumble from the engine
  • Engine may be difficult to start
  • Engine may die
  • Catalytic converter damage may result if this code is stored for a long period of time

How does a mechanic diagnose the P0171 code?

  1. Assuming that no other trouble codes exist, a mechanic may diagnose the P0171 code by checking the engine for vacuum leaks with a vacuum gauge. A vacuum leak will create a hissing sound usually only audible when the engine is turned off.

  2. The fuel pressure sensor and MAF sensor (mass air flow) will also be checked with a fuel pressure gauge.

  3. In order to maintain a proper air/fuel ratio, the vacuum and fuel pressure of the engine must be within the manufacturer’s specifications. The cause of this trouble code will likely be discovered during one of these two tests.

  4. If the cause of this issue is still not found after the mechanic has performed these two tests, the problem likely lies with a sensor.

  5. The mechanic would then run tests on the mass airflow sensor and the oxygen sensors using the manufacturer’s procedure.

  6. If all of these tests have been performed and no issues are found, the problem is likely the powertrain control module.

Common mistakes when diagnosing the P0171 code

A mistake that often occurs is not looking for any technical service bulletins for your specific car model that may be related to this issue.

Although not every trouble code will have a technical service bulletin associated with it, it is always wise to check. They can save you a world of time and effort.

How serious is the P0171 code?

The P0171 code is a serious issue. If a vehicle has this code stored in its powertrain control module, the engine of that vehicle simply will not run well. If the engine does not maintain proper air/fuel ratio, there will be a major waste of fuel and a lack of power while driving. It is a good idea to have this code diagnosed and repaired as soon as possible to have your vehicle running smoothly.

What repairs can fix the P0171 code?

Additional comments regarding the P0171 code

It is unwise to drive a vehicle with this trouble code stored for a long period of time. If an engine runs improperly for a long period of time, further damage may result that could end up being much more costly to the owner of the vehicle.

Need help with a P0171 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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Related questions

After 'check engine' light, diagnostic revealed P0171 and P0402
The first code, P0171 (https://www.yourmechanic.com/article/p0171-obd-ii-trouble-code-system-too-lean-bank-1-by-andrew-quinn), is an indication the engine is running lean. This code can indicate anything from clogged air and fuel filters to problems with O2 sensors and mass air flow sensors to intake gaskets. The other code,...
Too lean in bank 1 and bank 2
Hi and thanks for contacting YourMechanic. When the engine becomes too lean, then combustion chamber becomes very hot causing the head gasket to burn, valves begin to burn, and the rings being to seize up. Check the camshaft sensor (https://www.yourmechanic.com/services/camshaft-position-sensor-replacement)...
My car has recently been taking about 5 minutes of cranking to start - do I need to replace the engine?
Before resorting to removing the engine, have a certified technician properly diagnose your hard starting condition (https://www.yourmechanic.com/services/car-is-hard-to-start-inspection). Hard starting or a long cranking situation can be caused by a number of things. The "system too lean" code can contribute to...

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