OBD-II Code · Fuel & Air
P0171
System Too Lean (Bank 1)
Severity
medium
Worth booking. It costs fuel, emissions or comfort until it is fixed.
Can you drive it
Only if running normally
A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety.
Indicative cost (USD)
$50–$600
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
A smoke test locates the leak before any part is bought.
The engine is running lean on bank 1 — too much air, not enough fuel. The oxygen sensor detected more oxygen than expected in the exhaust.
How a shop works this
Lean means the ECU is already adding fuel and the sensor still reports lean. So the question is not whether it is lean — it is whether air is getting in unmetered or fuel is not getting there. One comparison separates them.
Read short-term and long-term fuel trim on the affected bank, warm, at idle.
Positive means the ECU is adding fuel, negative means it is taking fuel away. Read short-term and long-term together rather than either on its own. As a rule of thumb rather than a specification, correction beyond roughly plus or minus 10% is worth chasing. This code is specific to bank 1. If the engine is a V and bank 2 is not also lean, the fault is local to bank 1 rather than a global fuelling problem.
Compare those trims at idle against roughly 2,000 to 2,500 rpm.
A leak is a fixed amount of air. At idle it is a large fraction of what the engine is breathing; at 2,500 rpm it is a small one. That is why the number moves.
- Strongly positive at idle, much closer to zero at 2,000 to 2,500 rpmUnmetered air — a vacuum or intake leak. At idle the leak is a large share of everything the engine is breathing; at higher rpm it is swamped by it, which is why the number improves. Go find the leak.
- Positive at idle and still positive as the rpm comes upFuelling or measurement, not a leak. Fuel pressure at the rail, a MAF reading low for the engine, or an exhaust leak ahead of the upstream oxygen or air-fuel sensor letting fresh air past it.
- NegativeThe ECU is pulling fuel out, so the engine is running rich rather than lean. Look at a leaking injector, a purge valve stuck open, or fuel pressure that is too high.
Find the leak.
Start with what you can see: the intake ducting and its boot, the vacuum hoses, the PCV system, and anything obviously cracked or disconnected. A smoke test is the reliable way to locate one, and most shops will do it quickly. Do not spray a flammable solvent around a running engine to hunt a leak — it is an old trick and it does start fires.
Still lean with no leak found: fuel pressure at the rail, then the MAF.
Most port-injection systems sit around 35 to 55 psi at idle; below spec points at the pump, the filter or the regulator. A rough sanity check at warm idle is around 1 gram per second per litre of displacement — near 2 on a 2.0-litre four, near 5 on a 5.0-litre V8. Displacement and conditions move it a lot, so treat it as a smell test rather than a spec.
Before condemning a sensor, check what is ahead of it in the exhaust.
An exhaust leak upstream of the oxygen or air-fuel sensor pulls in outside air, the sensor reads lean, and the ECU adds fuel the engine never needed. In the data it looks exactly like a vacuum leak, and it costs a lot less to find than a new sensor costs to fit.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
- Rough idle
- Hesitation on acceleration
- Poor fuel economy
- Hard starting
Likely causes
- Vacuum or intake leakMost common
- Exhaust leak ahead of the upstream O2 sensor
- MAF sensor reading low
- Weak fuel pump or clogged filter
- Dirty fuel injectors
- Failing O2 sensor
Read the full diagnostic procedureP0171 is the canonical "lean" code on bank 1 — too much air, not enough fuel.
P0171 is the canonical "lean" code on bank 1 — too much air, not enough fuel. The ECU has corrected fuel trim past its allowable +25% range trying to compensate.
Diagnosis order on a shop floor
- Smoke-test the intake to find vacuum leaks — by far the most common cause, especially on engines past 80k miles where the rubber boots to the throttle body or PCV valve crack with age.
- Check the MAF sensor reading at idle (should be 2-4 g/sec on a typical 2.0-2.4L 4-cylinder, 5-8 g/sec on a V6) — a dirty or failing MAF reports artificially low airflow, which makes the ECU under-inject fuel, which makes the O2 sensor cry foul.
- Fuel pressure test at the rail with a mechanical gauge — most port-injection systems run 35-55 psi at idle; below spec means weak pump, clogged filter, or failing regulator.
Vehicle-specific patternsVehicle-specific patterns common in shop work: 2004-2014 Ford F-150 / Explorer / Mustang with the 4.6L or 5.4L Modular V8 often throws P0171 from int…
- 2004-2014Ford F-150 / Explorer / Mustang with the 4.6L or 5.4L Modular V8 often throws P0171 from intake-manifold cracks at the rear-driver-side corner — Ford issued multiple TSBs and many engines need the upgraded composite manifold.
- 2007-2014GM 3.6L V6 (Camaro, Equinox, Acadia, Traverse) often throws P0171 from a failing PCV valve in the cam cover causing a manufactured vacuum leak — $40 part, 30-min repair.
- 2002-2010Volkswagen / Audi 2.0T FSI/TSI throws P0171 from intake-manifold gasket failure ~80k miles AND PCV diaphragm failure on the valve cover at ~100k miles — both are common, sometimes simultaneously.
- 2008-2015BMW N51/N52/N54/N55 inline-6 throws P0171 from valve-cover gasket leak (oil drips onto the exhaust manifold AND introduces a vacuum leak as the gasket degrades) — replace with VCG kit including bolts.
Look up another code
P0171 — frequently asked
What does a P0171 code mean?
The engine is running lean on bank 1 — too much air, not enough fuel. The oxygen sensor detected more oxygen than expected in the exhaust.
What causes a P0171 code?
Common causes: Vacuum or intake leak; Exhaust leak ahead of the upstream O2 sensor; MAF sensor reading low; Weak fuel pump or clogged filter; Dirty fuel injectors; Failing O2 sensor.
What are the symptoms of a P0171 code?
Typical symptoms: Check engine light; Rough idle; Hesitation on acceleration; Poor fuel economy; Hard starting.
Is it safe to drive with a P0171 code?
Only if running normally. A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety. Estimated repair: $50-$600. A smoke test locates the leak before any part is bought..
How do I diagnose a P0171 code?
Lean means the ECU is already adding fuel and the sensor still reports lean. So the question is not whether it is lean — it is whether air is getting in unmetered or fuel is not getting there. One comparison separates them. 1. Read short-term and long-term fuel trim on the affected bank, warm, at idle. 2. Compare those trims at idle against roughly 2,000 to 2,500 rpm. 3. Find the leak. 4. Still lean with no leak found: fuel pressure at the rail, then the MAF. 5. Before condemning a sensor, check what is ahead of it in the exhaust.
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