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Code explained

P0171: your engine is running lean on bank 1

P0171
In plain words
The engine is getting too much air or too little fuel on bank 1.
Standard name
System Too Lean (Bank 1)
System
Fuel and air mixture

General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.

What the computer saw

Lean means more air than the fuel can match. The computer adds fuel to correct it, and when the correction runs past its limit, it sets P0171. Bank 1 is the side of the engine with cylinder 1; a four-cylinder engine has only bank 1.

Likely causes, most common first

  1. A vacuum leak. Air that sneaks in past the airflow sensor through a cracked hose, a worn gasket or the PCV system is the most common cause.
  2. A dirty or failing airflow sensor. If the mass airflow sensor under-reads the air coming in, the engine gets too little fuel for it.
  3. Low fuel pressure. A weak fuel pump, a clogged filter or a faulty pressure regulator starves the engine of fuel.
  4. Clogged or leaking fuel injectors. Injectors that spray too little fuel make that side run lean.
  5. An exhaust leak before the front oxygen sensor. Outside air at the sensor makes the exhaust look leaner than it is.

What you can note yourself

  • Note whether P0174 is also stored. Both banks lean points to something they share, such as a vacuum leak, the airflow sensor or fuel pressure.
  • Note when the car runs worst: at idle, when cold, or under load on the highway. Vacuum leaks tend to show most at idle.
  • Note any change in fuel economy, rough idle or hesitation when you press the pedal.

What to tell the shop

  • All the codes, and whether P0174 or a misfire code came with it.
  • When the problem shows: idle, cold start or highway.
  • Ask whether they will smoke-test for vacuum leaks and check fuel pressure before replacing sensors.

This page gives no verdict on whether a car is safe to drive; a mechanic can.

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