P0171 Code Causes and Fixes

Overview of how to inspect, diagnose, and repair the causes of a P0171 fault code.

DIY Cost: $120-$320
Est. Time: 30 min - 2 hr
Difficulty: Intermediate

What does P0171 code mean?

A P0171 code is when a lean condition occurs on the first bank of the engine. A "lean" condition means there is too much air or not enough fuel in the fuel-air mixture required for optimal engine operation.

Fault code P0171 usually comes on when there is a vacuum leak present or the fuel system is running weak.

What is running lean?

Running lean is when the engine is getting too much air or not enough fuel.

If there is too much air, the Engine Control Module (ECM) may try to compensate by injecting even more fuel. 

Possible Causes

The most common problems that trigger P0171 fault code are:

  • Bad MAF (mass airflow sensor)
  • Bad injectors
  • Bad O2 sensors
  • Vacuum leak
  • A malfunctioning or bad PCM
  • Clogged fuel filter
  • Weak fuel pump
  • Bad fuel pressure regulator


Common symptoms that you notice when the check engine light comes on due to the P0171 code. 

  • Engine stall
  • Engine derate
  • Check engine light
  • Engine hesitation
  • Poor starting
  • Possible catalytic converter damage

Possible Repair

  • Repair or replace intake/vacuum hoses
  • Replacement of bad fuel system components (filters, injectors, pumps)
  • Replacement of bad intake or exhaust gaskets
  • Repair or replacement of O2 sensors
  • Air filter replacement
  • PCM repair or replacement
  • Mass airflow sensor replacement


troubleshoot p0171 code common casuses

Disconnected or leaking vacuum/air hoses:

  • The supply tube from the airbox to the throttle body (or the snorkel) can become loose. Check all fasteners and check the tube for any punctures or looseness
  • Listen for whistling or rushing air sounds while your engine is running. Follow the noise and inspect for holes, dry-rotted hoses, open ports, or loose hoses.

Faulty/dirty/plugged mass airflow sensor or air filter

  • Debris blocking the mass airflow sensor will prevent the sensor from sending the correct readings to the PCM.
  • The air filter is probably the most overlooked component. Inspect your air filter for debris and damage. If the filter looks damaged, make sure you look to make sure no debris got past the filter into the airbox snorkel.

Damaged or disconnected engine intake components

  • Check your plenum gasket for dry rot or other indication that the gasket wouldn’t be sealing.

(to do this, removal of the air filter box and plumbing is often required as well as the engine cover. Once the plenum is exposed, bolts are fastening the plenum to the cylinder head. Remove these and remove the plenum. Inspect the gasket for damage.)

  • While the plenum is off, inspect for any possible damage to the cylinder head.

If there are no obvious signs of damage or possible leak points, put everything back together and tighten all bolts and fasteners to OEM specifications.

  • Once everything is put back together, you can use a carburetor cleaner to check for possible leaks.

(with the engine running, spray carb cleaner in the suspected leak area. If a leak is present, the engine will either run smoother or rougher than before you sprayed the carb cleaner)

  • Check the area where you got the most response from again for any of the possible causes mentioned earlier. (if you still can’t find the source of your problem, taking your car to a certified shop will help you. Technicians have computers that can graph engine performance and help them find the system that isn’t performing correctly)

Fuel system checks

  • If your fuel pressure is too low, your engine isn’t getting the correct amount of fuel. Begin by checking fuel filters, fuel injectors, and fuel pumps for blockages.
  • Borrow a fuel pressure gauge from your local auto parts store and perform a fuel pressure test. 

Exhaust system checks

  • Check for leaking exhaust flex pipes and manifold gaskets.
  • Check for properly functioning O2 sensors. (sensors sending incorrect signals to the PCM can trigger a P0171 code)