P0174

A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.

Safe to drive

Usually driveable briefly. Stalling, hesitation, a hard start or audible pinging under load means get it looked at now, not next month. If the brake pedal has gone hard or takes noticeably more effort, stop driving and have it towed - that points at a leaking brake booster, which is a braking problem before it is an engine one.

Urgency

Diagnose soon

Will it pass emissions

No

If ignored

Rough idle and stalling typically worsen as the leak grows. Sustained lean operation raises combustion temperatures, which invites detonation, damages exhaust valves and pistons over time and can overheat the catalytic converter. The vehicle will fail an emissions test, and a lean condition frequently develops into misfire codes on the affected bank.

Check engine light

Solid

Clearing the light

A completed drive cycle is normally required before the monitor re-runs and the light can clear.

• Rough idle, possible stalling
• Hesitation or hard cold start
• Power loss under load

Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.

These are the causes this code can have. For the order to work through them in, see what to check first.

Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.

In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.

1. Whether P0171 is also stored

   This is the highest-value single question on the code. Lean on both banks points to a shared cause such as the MAF, fuel supply, PCV or a central vacuum leak. Lean on bank 2 alone points to hardware unique to that side.

2. Which physical side of the engine is bank 2 on this vehicle

   Bank 1 is always the bank containing cylinder 1 - that part is standardized. What varies by manufacturer is cylinder numbering, so which physical side of the engine is bank 2 differs between makes. Confirm cylinder 1's location in service data before working on a side.

3. Fuel trim behavior at idle versus at 2500 rpm

   Trims that are badly positive at idle but settle near zero at higher rpm are the signature of a vacuum leak. Trims positive across the whole range point to fuel delivery or the MAF.

4. Every hose, boot and gasket downstream of the MAF

   A cracked intake boot, a brittle PCV hose, a disconnected dipstick tube or a leaking brake booster hose is free to find and accounts for a large share of these codes.

5. Brake pedal feel and effort

   A booster diaphragm leaking badly enough to skew fuel trim has also lost part of its vacuum reserve. A hard or high-effort pedal turns this from an emissions complaint into a brake repair.

6. Whether the intake was recently off for other work

   A disturbed gasket, an unplugged vacuum port or a hose left off after a repair is far more likely than a random component failure.

1. Log both banks' trims at idle, 2500 rpm and under load

   Tools Scan tool with two-bank live data

   Graph short-term and long-term trim on bank 1 and bank 2 together. The comparison between banks and the change with rpm narrows the field faster than any part swap.

   Expected

   Combined trims within roughly plus or minus 10% are normal. Bank 2 high with bank 1 normal isolates the fault to that bank.

2. Smoke test the intake and crankcase for unmetered air

   Tools Smoke machine, intake plug

   Introduce low-pressure smoke into the intake with the throttle body blocked and look for smoke escaping at manifold gaskets, injector seals, the intake boot, PCV hoses, the brake booster line and the throttle body base. This is the definitive test for a vacuum leak and the highest-yield step on this code.

   Expected

   No smoke escaping anywhere. Any visible escape is a confirmed leak.

3. Evaluate the MAF sensor against expected airflow

   Tools Scan tool live data

   Compare reported grams per second to a rough expectation, about one gram per second per liter of displacement at warm idle and roughly 0.8 grams per second per horsepower near wide-open throttle. A MAF under-reporting airflow starves both banks; if only bank 2 is lean, the MAF is unlikely to be the cause.

   Expected

   Airflow in the expected neighborhood and rising smoothly. Treat these figures as sanity checks, not specifications.

4. Look for an exhaust leak upstream of the bank 2 oxygen sensor

   Tools Inspection light, mechanic's stethoscope, smoke into the tailpipe as an option

   A crack or leaking gasket ahead of the sensor lets outside air get pulled in between exhaust pulses, so the sensor reports lean when the mixture is correct and the ECM adds fuel it does not need. Check the manifold, the flex joint and the sensor bung, listening with the engine cold when leaks are widest.

   Expected

   No ticking, sooting, or smoke escaping from joints ahead of the sensor.

5. Test fuel pressure and, more importantly, fuel volume

   Tools Fuel pressure gauge with correct adapter, or fuel rail pressure PID on direct injection

   Relieve fuel system pressure before connecting a gauge, keep a fire extinguisher within reach, work on a cool engine and wear eye protection - fuel sprayed onto a hot exhaust is a fire, and on direct injection the high-pressure rail can exceed 2000 psi and will inject fuel through skin. Never open a direct-injection high-pressure line or rail with the engine running or hot; read that side from the scan tool's fuel rail pressure PID. On the low-pressure or port-injection side, a pump that holds pressure at idle can still fall short of the volume the engine needs under load, so watch pressure during a hard acceleration or a load test, not just at rest. Pressure specifications vary by vehicle, so use the correct figure from service data.

   Expected

   Pressure meeting the vehicle's published specification at idle and holding under load without sagging. There is no OBD-II-wide figure, so a value borrowed from another vehicle proves nothing. Pressure that looks correct at idle and falls off under load is a confirmed volume shortfall even when the static reading passed.

6. Check the PCV system and any bank-specific breather

   Tools Hand tools, smoke machine

   A PCV valve stuck open, a torn diaphragm in the valve cover, or a split breather hose admits a large amount of unmetered air. Some V engines route crankcase ventilation asymmetrically, so this can be a genuinely bank-specific cause.

   Expected

   PCV valve rattles when shaken, holds vacuum in the correct direction, hoses supple and intact.

7. Assess bank 2 injectors for restriction

   Tools Injector tester or bench service, scan tool

   If air metering, leaks and fuel supply are all good, the remaining path is fuel delivery on that bank only. Compare injector flow or have them bench tested, and consider a fuel injector service before replacement.

   Expected

   Bank 2 injectors flowing comparably to bank 1.

8. Verify the bank 2 upstream oxygen sensor is honest

   Tools Scan tool, propane or a controlled vacuum leak

   Force rich and lean conditions and confirm the sensor swings the full range and crosses quickly. A sensor biased lean makes the ECM add fuel until the trim clamps even though the mixture is correct.

   Expected

   A conventional sensor should sweep roughly 0.1 to 0.9 volts and respond within a fraction of a second. On a wideband or air-fuel-ratio sensor there is no 0.1 to 0.9 volt switching. Read the equivalence ratio (lambda) or sensor current PID instead: it should move smoothly rich under propane, lean under an induced vacuum leak, and return promptly to about 1.00. Judging a wideband by the narrowband voltage pattern condemns good sensors.

US independent-shop ranges, parts plus labor, assuming the diagnosis is complete. The most common repairs on this code sit at the cheap end, a hose or a gasket, but finding them properly takes a smoke machine and about an hour of diagnostic time, typically 100 to 200 dollars. Fuel pump cost swings hugely on whether the tank must come out. On V engines with the intake buried under a supercharger or a plenum, manifold gasket labor runs well above the range shown.

• Replacing the bank 2 oxygen sensor first. The sensor is usually reporting the truth about a genuine lean condition.
• Judging a wideband air-fuel-ratio sensor by the 0.1 to 0.9 volt narrowband pattern. Most V engines built since the mid-2000s use widebands, which never switch that way, and a good sensor gets condemned.
• Working on the wrong bank because bank 2 was assumed to be the near side. Confirm which physical side holds cylinder 1 for the specific engine.
• Using carburetor or brake cleaner sprayed around the intake to find leaks. It works, but it is a fire risk on a hot engine and it misses leaks in places the spray cannot reach. A smoke test is better.
• Treating a confirmed brake booster leak as an emissions repair to be deferred. It is a brake repair.
• Chasing the intake when P0171 is also present. Two-bank lean means a shared cause, and time spent on one bank's hardware is wasted.
• Testing fuel pressure only at idle. Many marginal pumps look perfect until they are asked for volume.
• Clearing the code and declaring the repair successful without watching long-term trim relearn back toward zero.

• Mass Air Flow (MAF) Sensor
• Fuel Pump
• Intake Manifold
• Fuel Pressure Regulator

• P0171System Too Lean (Bank 1)Severity level 3 of 5: Diagnose soon.moderate
• P0173Fuel Trim Malfunction Bank 2Severity level 3 of 5: Diagnose soon.moderate
• P0175System Too Rich (Bank 2)Severity level 3 of 5: Diagnose soon.moderate

Definition

Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.

Diagnostic guidance

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Review status

Reviewed

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