P0171 Code: System Too Lean (Bank 1)
The engine computer is adding the maximum extra fuel it's allowed on bank 1 and still sees a lean mixture.
At a glance
Severity: Moderate
- Meaning
- System Too Lean (Bank 1)
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $10–$1,200
- DIY difficulty
- Moderate
The PCM fine-tunes fuel delivery using the upstream oxygen or air-fuel ratio sensor, and stores the corrections as short-term and long-term fuel trims. P0171 means the combined correction on bank 1 has hit its positive limit: the PCM is adding a lot of extra fuel and still can't bring the mixture back to ideal. So the code doesn't point to a part; it tells you there's either extra air the engine didn't measure or not enough fuel arriving. On a four-cylinder or most inline engines, bank 1 is the whole engine. On a V6 or V8, P0171 without P0174 points to something only bank 1 sees, such as an intake gasket, injector or exhaust leak on that side.
Can I keep driving? You can usually drive it to a shop, but avoid towing, hard acceleration and long trips. A lean mixture burns hotter and can lead to misfires and a damaged catalytic converter; if the check engine light starts flashing, pull over and stop driving.
How to read P0171
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 71 Specific fault
Symptoms
- Check engine light on
- Rough or unstable idle, sometimes with a hissing sound from the engine bay
- Hesitation or stumble on acceleration
- Stalling at idle or when coming to a stop
- Misfire codes (P0300 range) on some vehicles
- Pinging or knocking under load
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Vacuum leak: cracked PCV hose or failed PCV valve, intake manifold gasket, brake booster hose
- Dirty or failing MAF sensor reading less air than is actually entering
- Low fuel pressure or volume (weak fuel pump, clogged filter or strainer, faulty regulator)
- Exhaust leak ahead of the upstream O2 sensor drawing in outside air
- Clogged or dirty fuel injectors
- EVAP purge valve stuck open, acting like a vacuum leak
- Faulty upstream O2 or air-fuel sensor reporting a false lean reading
Diagnosis step by step
Compare fuel trims at idle and 2,500 rpm
With the engine fully warm, note long-term fuel trim at idle and again at a steady 2,500 rpm. If trims are high at idle (for example +20%) but drop toward normal at higher rpm, a vacuum leak is the likely cause. If they stay high or get worse under load, suspect fuel delivery or the MAF sensor.
Tools: OBD-II scanner
Hunt for vacuum leaks
Listen for hissing at idle and check the PCV valve and hoses, the brake booster line and the oil filler cap and dipstick seals, which leak into the crankcase. A smoke test is the fastest reliable way to find leaks you can't hear.
Tools: Smoke machine, Flashlight
Check and clean the MAF sensor
Compare MAF grams per second at idle with what's typical for the engine size and check the intake duct after the sensor for cracks. Clean the element with MAF cleaner; if trims drop noticeably afterward, the sensor was under-reading.
Tools: OBD-II scanner, Screwdriver
Test fuel pressure
Connect a gauge to the fuel rail test port and compare with spec at idle and under load; many port-injected engines run somewhere around 40 to 60 psi. Pressure that falls off when you snap the throttle or on a road test points to a weak pump or clogged filter. Relieve pressure before disconnecting anything and work away from ignition sources.
Tools: Fuel pressure gauge
Check for exhaust leaks upstream of the O2 sensor
On a cold engine, listen for a ticking sound near the exhaust manifold and look for black soot trails around the manifold gasket and flange. Air drawn in through a leak makes the O2 sensor read lean even when the mixture is fine.
Tools: Flashlight
Isolate the purge valve
With the engine idling, pinch or disconnect and plug the purge line at the intake. If the fuel trims drop back toward zero, the purge valve is stuck open and needs replacing.
Tools: OBD-II scanner, Pliers
On V6/V8 engines, compare both banks
If bank 1 trims are high but bank 2 trims are near normal, focus on the intake gasket, injectors and exhaust manifold on the bank 1 side only. Anything shared by both banks, like the MAF or fuel pump, would push both sides lean.
Tools: OBD-II scanner
Where the parts are
- 1MAF sensor
- 2Turbocharger
- 3Throttle body
- 4Intake manifold
- 5Fuel injector
- 6EGR valve
- 7Fuel pump
- 8Thermostat
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean MAF sensor | Yes | $10–$20 |
| Replace PCV valve or cracked vacuum hose | Yes | $20–$180 |
| Replace intake manifold gasket | No | $250–$700 |
| Replace EVAP purge valve | Yes | $90–$250 |
| Replace fuel pump module | No | $500–$1,200 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Volkswagen
On many VW 1.8T and 2.0T turbo gasoline engines, a torn diaphragm in the PCV valve or oil separator is a well-known source of lean codes, often with a whistle at idle. Pulling the oil filler cap with the engine idling and feeling strong suction is a quick hint.
Audi
Audi's 1.8T and 2.0T four-cylinders share the VW crankcase ventilation design, so a failed PCV valve or cracked breather hose is one of the first things to check for P0171 on an A3, A4 or TT.
BMW
On BMW inline-six engines, a failed crankcase vent valve or cracked intake boots commonly cause lean codes. Because many of these engines use two upstream sensors, you'll often see P0171 and P0174 together even though there is only one cylinder bank.
Field data
| Chevrolet | 30 | ||
|---|---|---|---|
| Toyota | 14 | Corolla, RAV4, Camry | |
| Ford | 8 | Focus Bev, F-150, Flex | |
| Subaru | 5 | Wrx, Brz, Forester | |
| GMC | 4 | Terrain, Acadia | |
| Mazda | 4 | CX-5 | |
| Mercedes-Benz | 3 | C-Class, E-Class | |
| Volkswagen | 3 | Gti, Passat |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
A lit check engine light fails the OBD-II emissions test in states that require one. A lean mixture can also raise NOx emissions in states that still do a tailpipe test, so fix the cause rather than just clearing the code before your smog check.
Frequently asked questions
Will replacing the O2 sensor fix P0171?
Rarely. The O2 sensor is usually just the messenger reporting extra air or missing fuel. Replacing it first is one of the most common wasted repairs for this code; test for vacuum leaks, MAF and fuel pressure before you buy a sensor.
Why does P0171 only set at idle or when the engine is cold?
That pattern points strongly to a vacuum leak. At idle, a small leak is a big share of the total airflow, and rubber gaskets and hoses often leak more when cold before they swell with heat.
Can a loose gas cap cause P0171?
No, a loose gas cap sets EVAP leak codes, not a lean code. However, a loose or bad oil filler cap or dipstick seal can, because the crankcase is connected to the intake through the PCV system.
I have a four-cylinder. Why does it only say bank 1?
Inline engines have only one cylinder bank, so bank 1 simply means the whole engine. You'll never see P0174 on a typical four-cylinder, because there is no bank 2.
Can I use a fuel injector cleaner for P0171?
It's cheap and may help if dirty injectors are the cause, but it won't fix a vacuum leak, a bad MAF or a weak fuel pump. Try it only after checking fuel trims so you know whether fuel delivery is the likely issue.