P0171Moderate severityFuel/AirDIY: Easy
P0171: System Too Lean (Bank 1)
Too much air is reaching the engine for the amount of fuel being injected. Almost always an unmetered air leak or a dirty/failing mass airflow sensor.
Can I keep driving?
Generally yes for short trips. Left alone, a lean engine can start to misfire and run hotter, so don't put it off for months.
Symptoms you might notice
- Rough or surging idle
- Hesitation when you accelerate
- A hiss or whistle from the engine bay
- Check engine light
What to check first
- Inspect the intake tube between the air filter box and the engine for cracks or a loose clamp, and check the PCV hose.
- Clean the mass airflow (MAF) sensor with a cleaner made for MAF sensors. Other sprays can damage it.
- On a scan tool, compare fuel trim at idle and at about 2,500 rpm. A lean reading that gets better as rpm rises points to a vacuum leak.
- If the air side checks out, test fuel pressure and check the fuel filter.
Most common causes
- Vacuum leak (cracked intake boot, PCV valve)
- Dirty or failing MAF sensor
- Weak fuel pump or clogged fuel filter
- Leaking fuel injector seals
Check it against your exact vehicle
A code means the same thing on every car, but where the parts are and what to check first depends on your year, make, model and engine. Tell CaliperAI what you are seeing and it will help narrow down the likely causes for your vehicle.
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General information about this code, not a diagnosis of your specific vehicle. Safety-critical work such as brakes, fuel and airbags is best left to a qualified mechanic.