P0171
Fix within a few daysThe engine's computer detects that the air-fuel mixture in Bank 1 is too lean, meaning there's too much air or not enough fuel.
Code P0171 means your engine is running too lean on Bank 1. Common causes: Vacuum leaks (40%), MAF sensor (35%), fuel system issues (15%). Drivability: Yes, for short trips. Repair cost: $50-$200 DIY, $150-$500 professional.
Can I Drive With P0171?
Yes, for short trips only
Essential trips under 10 miles, avoid highways and heavy acceleration. Fix immediately.
Stop driving immediately if: Flashing CEL, significant power loss, loud engine noises, or a strong burning smell.
Symptoms of P0171
- 1Check Engine Light is on
- 2Rough idle or engine hesitation
- 3Reduced engine power
- 4Poor fuel economy
- 5Engine stalling
Common Causes of P0171
Parts most likely causing this code, ranked by probability
Unmetered air enters the engine, causing the oxygen sensor to detect a lean condition.
40% of P0171 cases are fixed by repairing a vacuum leak
A dirty or faulty MAF sensor provides incorrect air intake readings, leading the engine to inject too little fuel.
35% of P0171 cases are fixed by cleaning or replacing the MAF sensor
Insufficient fuel delivery to the engine causes the air-fuel mixture to become lean.
15% of P0171 cases are fixed by addressing a fuel system issue
A faulty upstream oxygen sensor can send incorrect lean readings to the engine's computer.
5% of P0171 cases are fixed by replacing the O2 sensor
An exhaust leak before the upstream oxygen sensor can draw in outside air, tricking the sensor into reading a lean condition.
5% of P0171 cases are fixed by repairing an exhaust leak
Check These First (Before Buying Parts)
- Visually inspect all accessible vacuum hoses, the PCV valve, and the intake manifold area for any cracks, disconnections, or loose fittings.
- Clean the Mass Air Flow (MAF) sensor with a specialized MAF sensor cleaner. Do not use other cleaners.
How to Diagnose P0171
- 1Perform a thorough visual inspection for cracked or disconnected vacuum hoses, loose intake manifold bolts, or a damaged PCV valve.
- 2Clean the Mass Air Flow (MAF) sensor with a dedicated MAF sensor cleaner.
- 3Use an OBD-II scan tool to monitor live data, specifically fuel trims (Short Term Fuel Trim and Long Term Fuel Trim) and Bank 1 Sensor 1 oxygen sensor readings.
- 4Check fuel pressure at the fuel rail to ensure the fuel pump is delivering adequate pressure.
- 5Perform a smoke test on the intake system to pinpoint any hidden vacuum leaks.
P0171 on Popular Vehicles
Toyota Camry
2007-2024Commonly linked to a failing Mass Air Flow (MAF) sensor or a cracked intake manifold gasket. Also check the PCV valve and its hoses.
60% fixed by MAF sensor or intake gasket
Honda Accord
2003-2024Frequently caused by a dirty or faulty Mass Air Flow (MAF) sensor. Vacuum leaks from the intake manifold gasket or brake booster hose are also common.
55% fixed by MAF sensor or vacuum leak repair
Ford F-150
2004-2024P0171 on F-150s is often due to vacuum leaks, particularly from the PCV system (hoses, valve) or the intake manifold gasket. A dirty MAF sensor is also a frequent cause.
65% fixed by vacuum leak repair or MAF sensor
Frequently Asked Questions
What does P0171 mean?
P0171 indicates that the engine's computer has detected a lean condition in Bank 1, meaning there's too much air or not enough fuel in the exhaust gases.
Can I drive with P0171?
Yes, you can drive for short trips, but you need to fix this within a few days. Prolonged driving with a lean condition can cause serious engine damage and catalytic converter failure.
Will P0171 clear itself?
No, P0171 will not clear itself. The underlying issue causing the lean condition must be diagnosed and repaired before the code will clear, either automatically after several drive cycles or by manually clearing it with a scan tool.
How do I fix P0171 myself?
Start by visually inspecting all vacuum lines and the PCV hose for cracks or disconnections. Clean your Mass Air Flow (MAF) sensor with MAF cleaner. If those don't work, you'll need a scan tool to check fuel trims and potentially perform a smoke test for leaks.
How much does it cost to fix P0171?
DIY repairs for P0171 typically cost $50-$200, depending on the part needed (MAF cleaner, vacuum hose, intake gasket). Professional repairs at a shop can range from $150-$500, including diagnosis and labor.
P0171 keeps coming back - why?
P0171 keeps coming back because the root cause was not fully addressed. This often happens when only one potential issue (like a MAF sensor) is replaced without properly diagnosing other common causes like vacuum leaks or fuel system problems.
Will P0171 fail emissions/smog test?
Yes, P0171 will cause your vehicle to fail an emissions or smog test. The Check Engine Light being on is an automatic failure, and the lean condition itself means your vehicle is not meeting emissions standards.
What's the difference between P0171 and P0174?
P0171 indicates a lean condition on Bank 1 of the engine, which is the side containing cylinder #1. P0174 indicates a lean condition on Bank 2, which is the opposite side of the engine. If both codes are present, it often points to a common issue affecting both banks, like a dirty MAF sensor or a widespread vacuum leak.
P0171 on Toyota Camry?
On Toyota Camry models, P0171 is frequently caused by a faulty Mass Air Flow (MAF) sensor or a cracked intake manifold gasket. The PCV valve and its associated hoses are also common culprits for vacuum leaks on these vehicles.
Questions to Ask Your Mechanic
Did you check the fuel trims at idle and under load?
Good answer: Yes, the long-term fuel trims are high (e.g., +25%) at idle but drop significantly under load, which points to a vacuum leak. If they stay high, it's more likely a MAF or fuel delivery issue.
Did you perform a smoke test for vacuum leaks?
Good answer: Yes, we smoke tested the intake system and found a leak at the PCV hose/intake manifold gasket. We also checked the brake booster line.
Repair Cost Estimate
DIY Repair
Parts only
$50-$200
Shop Repair
Parts + labor
$150-$500
Save $100-$300 by doing it yourself
Difficulty
intermediate
Time
1-3 hours
Intermediate - requires some diagnostic skills and potentially a scan tool to accurately pinpoint the cause, but many common fixes are accessible to a DIYer.
Cost of Waiting
Ignoring P0171 can lead to severe engine damage due to excessive heat from the lean condition, potentially burning valves or pistons. It also sends unburned fuel to your catalytic converter, which can destroy it. A $50 MAF cleaning or $100 vacuum hose job now prevents a $1,500-$3,000 engine or converter replacement later.
While You're In There...
If you're cleaning or replacing the MAF sensor, it's a good time to check and replace the air filter, as a dirty filter can contribute to MAF issues and poor engine performance.
If you're addressing vacuum leaks, especially around the intake manifold or PCV system, replacing the PCV valve is a low-cost item that often fails and is easy to access at the same time.
Tools You'll Need
- OBD-II Scan Tool
- MAF Sensor Cleaner
- Basic Hand Tools (screwdrivers, wrenches)
- Vacuum Gauge (optional)
- Smoke Machine (for advanced leak detection)
P0171 is one of the most common engine codes. While it indicates a serious condition if ignored, the repair is often straightforward and involves replacing a relatively inexpensive part like a vacuum hose or cleaning a sensor.
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Your Next Steps
Step 1: Conduct a thorough visual inspection of all vacuum lines, the PCV valve, and the intake manifold for any obvious cracks, disconnections, or loose components.
Step 2: Clean the Mass Air Flow (MAF) sensor using a dedicated MAF sensor cleaner. Clear the code and drive to see if it returns.
Step 3: If the code persists, use an OBD-II scan tool to monitor live data, focusing on fuel trims (STFT and LTFT) and Bank 1 Sensor 1 oxygen sensor readings. High positive fuel trims indicate a lean condition. Consider a smoke test to pinpoint elusive vacuum leaks.