P2179: System Too Lean Off Idle Bank 2
What to do
Ask whether the equivalent bank 1 code is also stored. If both banks are lean, the cause is something they share — fuel supply, the airflow sensor, the intake or the crankcase ventilation — and treating each bank separately doubles the bill for one repair. If only this bank is lean, the problem is physically on that side of the engine.
What this code means
The same off-idle lean condition described at P2177, measured on the second cylinder bank. Only engines in a V or flat configuration have one.
Whether the matching bank 1 code is also stored is the most useful single fact about this code, and it is free to establish.
- One bank only. Whatever is wrong is on that side of the engine and nowhere else. A bank has its own injectors, its own intake runners, its own exhaust manifold and its own upstream oxygen sensor — so the candidates narrow to that hardware: a leaking gasket on one head, a cracked vacuum branch feeding one side, an exhaust leak ahead of one sensor, or a sensor of its own that reads inaccurately.
- Both banks together. Then the fault is almost certainly in something both banks share, and looking for two separate problems wastes the diagnosis. Fuel supply, fuel pressure, the airflow sensor, the throttle body, the crankcase ventilation system and the evaporative purge all feed both sides from one place.
This is the argument the corpus returns to more than any other — one shared thing failing looks exactly like many separate things failing. Here it is unusually easy to apply, because the codes themselves do the sorting.
One caution before concluding a bank is faulty: oxygen sensors are frequently swapped between banks during a repair, and a sensor giving a slightly optimistic reading takes its error to whichever bank it is fitted to. If the fault follows a sensor across banks rather than staying with the hardware, the sensor is the fault.
Likely causes
Start here
- Vacuum leak in intake manifold or hoses · moderate
- Leaking intake manifold gasket · moderate
- Exhaust leak before the sensor · moderate
Less common
- Faulty oxygen sensor · moderate
- Contaminated or failing mass air flow sensor · easy
- Clogged fuel injectors · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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