Trouble codes › P0171
P0171 System Too Lean (Bank 1)
Fuel and air
P0171 means bank 1 is running lean (too much air or too little fuel), most often from a vacuum leak or a dirty MAF sensor.
The engine computer is adding a lot more fuel than normal to bank 1 to keep the air-fuel mixture right, and it has reached its limit. Either extra air is getting in that the computer isn't measuring, or not enough fuel is getting delivered.
Usually drivable for now, but a lean engine can hesitate, misfire and run hotter. Get it fixed soon, and stop if it starts misfiring or the check-engine light flashes.
Common causes
- A vacuum leak: a cracked hose, intake boot or PCV hose, or a leaking intake gasket (most common)
- A dirty or failing mass airflow (MAF) sensor that under-reports the air coming in
- Weak fuel delivery: a failing fuel pump, clogged filter or faulty pressure regulator
- Clogged fuel injectors
- An exhaust leak before the front oxygen sensor
Symptoms
- Check-engine light
- Rough or hunting idle
- Hesitation when accelerating, sometimes misfires
- Sometimes no symptoms at all
Common on: Ford F-150, Chevrolet Silverado 1500, Toyota Camry, Honda Civic, Honda Accord, Honda CR-V, Toyota RAV4, Toyota Corolla, Toyota Tacoma, Ram 1500, Nissan Altima, Chevrolet Equinox, Ford Escape, Ford Explorer, GMC Sierra 1500, Nissan Rogue, Jeep Wrangler, Toyota Highlander, Chevrolet Malibu, Honda Odyssey, Hyundai Elantra, Ford Fusion, Chevrolet Tahoe, Chevrolet Traverse, GMC Acadia, Chevrolet Suburban, GMC Yukon, Ford Expedition, Honda Pilot, Nissan Sentra
What to check first
- Check for a whistle or hiss at idle, and inspect vacuum hoses, the PCV hose and the air intake boot after the MAF sensor for cracks.
- With a scan tool, look at long-term fuel trim: high (above about +10%) at idle that improves at 2,500 rpm suggests a vacuum leak; high at all speeds suggests the MAF or fuel delivery.
- Clean the MAF sensor with MAF cleaner (not brake or carb cleaner) and recheck.
- A smoke test finds vacuum leaks you can't see or hear.
- If you also have P0174 (bank 2), look for something that affects both sides: the MAF, a big vacuum leak or fuel pressure.
What does P0171 mean on your vehicle?
The likely cause depends on your engine. iRepair researches your exact year, make, model and engine (service bulletins, recalls and owner reports) and tells you what to check first. Your first 3 questions are free.
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Related codes
General guidance for P0171 on most vehicles. Some makers use the same code a little differently; confirm in the service information for your vehicle, put safety first, and see a professional when in doubt.