Number P0174: System Too Lean Bank 2
With closed-loop control the engine module corrects the injected fuel using the oxygen sensor: short-term fuel trim reacts within fractions of a second, long-term trim stores a learned base correction per load and speed range. Positive values mean fuel is being added because the mixture was lean, ne
Can you drive with P0174?
Severity: Plan the repair. Yes — this is an emissions-system fault. The car will generally drive normally, but it will fail an emissions inspection with the light on.
If you ignore it: Fuel economy and emissions creep up, and some faults in this family (rich/lean running) stress the catalyst over time.
This is the #25 most common code in our top-150 library, in the fuel trim & air metering family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0174 means
With closed-loop control the engine module corrects the injected fuel using the oxygen sensor: short-term fuel trim reacts within fractions of a second, long-term trim stores a learned base correction per load and speed range. Positive values mean fuel is being added because the mixture was lean, negative values mean fuel is being removed; around ±10 % is normal. P0174 is the bank 2 counterpart of P0171 and only exists on V and boxer engines with separate mixture control per bank. The limits match: GM a long-term factor of about 1.30 (engine-specific), Ford above +25 % with short-term trim above +1 %; an older Saab V6 flagged more than 25 % deviation for 20 s. Two trips, no limp mode. P0174 alone points to a leak, injector or exhaust leak on bank 2; together with P0171 to a shared cause such as the air flow sensor, fuel pressure or crankcase ventilation – exactly what several GM bulletins describe. Diagnosis hint: trims very high only at idle and normal under load point to an air leak; trims rising with load point to fuel shortage or an air flow sensor reading low. The oxygen sensor only reports what is in the exhaust and is rarely the cause.
Systems involved: oxygen sensor bank2 sensor1, mass air flow sensor, fuel system, intake manifold.
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Air leak after the air flow sensor on bank 2 (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line) | most likely |
| 2 | Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick) | most likely |
| 3 | Air flow sensor dirty or drifting (measures too little air) | possible |
| 4 | Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty) | possible |
| 5 | Injectors clogged, or exhaust leak ahead of the oxygen sensor | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine light on (after two drive cycles)
- Rough or unsteady idle, possibly stalling
- Hesitation or lack of power when accelerating
- Hissing or whistling from the engine bay (air leak, PCV)
Misdiagnosis traps
- Same lean playbook as P0171, one bank only: an intake leak or exhaust leak on that bank, not a global fuel problem.
- On V engines a one-bank lean code with the other bank normal rules out the fuel pump and filter — they feed both banks.
How a good shop confirms it
- Compare trims bank to bank; a one-sided fault means a one-sided leak or sensor.
- Check that bank's intake runners, injectors and upstream sensor.
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| Intake / vacuum leak repair (gaskets, hoses) | $338 – $838 | $150 – $400 (parts only) | 3/5 |
| Mass air flow (MAF) sensor | $158 – $420 | $120 – $320 (parts only) | 1/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor). Published cross-check — Mass air flow (MAF) sensor: CarMD 2026 national average $346.
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.5–5 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
P0171 and P0174 together — what does that mean?
Both banks lean at once points to shared causes: low fuel pressure, a contaminated MAF, or a large vacuum leak. Fix the shared cause, not the sensors.
How much does it cost to fix P0174?
The fix most associated with this code — intake / vacuum leak repair (gaskets, hoses) — runs $338 – $838 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0174?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0174 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0174?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0174?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.