P0171

What it means

Your engine's computer has detected that the air-fuel mixture in the first bank of cylinders is too lean, meaning there's too much air or not enough fuel for proper combustion.

Is it safe to drive?

While you might not be stranded immediately, driving with a lean condition can lead to engine damage (overheating, misfires), catalytic converter damage, and reduced performance. It's best to diagnose and repair it promptly.

Most likely causes

• Vacuum Leaks

  high

  Leaks in vacuum hoses, intake manifold gaskets, PCV valve, or brake booster can allow unmetered air into the engine, causing a lean condition.

• Faulty Mass Air Flow (MAF) Sensor

  medium

  A dirty or failing MAF sensor can incorrectly report less air entering the engine than there actually is, leading the PCM to inject too little fuel.

• Low Fuel Pressure or Clogged Fuel Injectors

  medium

  A weak fuel pump, clogged fuel filter, faulty fuel pressure regulator, or dirty/clogged fuel injectors can prevent adequate fuel delivery to Bank 1.

• Exhaust Leak (before O2 sensor)

  low

  An exhaust leak before the upstream oxygen sensor on Bank 1 can draw in outside air, causing the O2 sensor to read lean and the PCM to try to add more fuel, but the actual mixture is still lean.

Related help

• Full P0171 diagnosis, causes & costs
• How to read check engine light codes
• Is it safe to drive with the light on?