Unmetered air entering the intake after the mass air flow sensor on the bank 2 side, most commonly through a cracked or split intake boot, a deteriorated vacuum hose, or a leaking intake manifold gasket.; A faulty or contaminated mass air flow sensor that under-reports airflow, causing the module to add fuel on both banks but often setting the bank 2 adaptation first.; A leaking crankcase ventilation valve or breather hose that admits air into the intake tract on the bank 2 side.; A sticking or partially clogged fuel injector on bank 2 that reduces fuel delivery and forces the module to increase adaptation.; A weak or failing oxygen sensor on bank 2 that reports a leaner mixture than actually exists, driving the adaptation upward.; Low fuel pressure from a tired fuel pump or a restricted fuel filter, which affects both banks but can trip the bank 2 adaptation limit first.; An intake manifold or throttle body gasket leak that is temperature-sensitive and only shows up at idle.

DTC codes / BMW codes / P1161

P1161 · BMWFuel Trim Adaptation Additive Bank 2 High

Q: What does code P1161 mean?

Fuel Trim Adaptation Additive Bank 2 High. P1161 on BMW vehicles means the engine control module has detected that the additive fuel trim adaptation for bank 2 has exceeded its positive limit, indicating the engine is having to add extra fuel at idle and low-load conditions to keep the air-fuel mixture correct. The additive portion of fuel trim corrects for small, consistent unmetered air leaks that affect the engine mainly at idle, and a high positive value means the module is adding more fuel than expected on the bank 2 cylinder group.

P1161 on BMW vehicles means the engine control module has detected that the additive fuel trim adaptation for bank 2 has exceeded its positive limit, indicating the engine is having to add extra fuel at idle and low-load conditions to keep the air-fuel mixture correct. The additive portion of fuel trim corrects for small, consistent unmetered air leaks that affect the engine mainly at idle, and a high positive value means the module is adding more fuel than expected on the bank 2 cylinder group.

SystemPowertrain

Code typeBMW specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Often no symptoms besides the warning light, especially if the trim has only just crossed the threshold.
A rough or unstable idle that may smooth out as soon as the engine speed rises above idle.
Slightly higher fuel consumption around town and during prolonged idling.
Occasional lean misfire codes or a flashing check engine light if the mixture becomes severely lean.
A faint hissing or whistling noise from the engine bay on some cars with a split intake boot or failed gasket.
Cold-start idle quality that feels worse than when the engine is fully warmed.

Common causes, most likely first

Unmetered air entering the intake after the mass air flow sensor on the bank 2 side, most commonly through a cracked or split intake boot, a deteriorated vacuum hose, or a leaking intake manifold gasket.
A faulty or contaminated mass air flow sensor that under-reports airflow, causing the module to add fuel on both banks but often setting the bank 2 adaptation first.
A leaking crankcase ventilation valve or breather hose that admits air into the intake tract on the bank 2 side.
A sticking or partially clogged fuel injector on bank 2 that reduces fuel delivery and forces the module to increase adaptation.
A weak or failing oxygen sensor on bank 2 that reports a leaner mixture than actually exists, driving the adaptation upward.
Low fuel pressure from a tired fuel pump or a restricted fuel filter, which affects both banks but can trip the bank 2 adaptation limit first.
An intake manifold or throttle body gasket leak that is temperature-sensitive and only shows up at idle.

How to diagnose and fix it

Connect a scan tool and read live data for additive and multiplicative fuel trims on both banks to confirm the bank 2 additive trim is high and see whether the fault is isolated to idle or present across the range.
Perform a visual inspection of the intake tract on the bank 2 side, checking the intake boot, vacuum lines, crankcase ventilation hoses and intake manifold gaskets for cracks, splits or loose clamps.
Carry out a smoke test of the intake system with the engine off to pinpoint any unmetered air leak that is not visible to the eye.
Clean or test the mass air flow sensor and compare its reported airflow against a known-good value or a substitute sensor before condemning it.
Check fuel pressure and delivery volume at the rail, and inspect the bank 2 injectors for clogging or incorrect spray pattern if the leak tests are clean.
Swap the bank 2 oxygen sensor with the bank 1 sensor, or scope both sensors, to rule out a biased sensor that is falsely reporting a lean condition.
After repairing the root cause, clear the adaptations and take the vehicle on a mixed idle and cruise drive cycle so the module can relearn the fuel trim and confirm the code does not return.

What additive fuel trim really tells you

Additive fuel trim is the idle and low-load correction, so a high additive value on bank 2 points to an air leak that is proportionally worse at idle, not a general fuel delivery problem across the whole operating range.
A common misdiagnosis is replacing the bank 2 oxygen sensor first, when the sensor is usually reporting a real lean condition caused by unmetered air entering after the mass air flow sensor.
The bank 2 designation on BMW inline-six and V8 engines does not always match the side a technician expects, so confirm the bank layout in the service data before chasing the wrong cylinder group.
A failing crankcase ventilation valve can set this code without any visible split hose, because the internal diaphragm tears and allows air to pass into the intake in a way that a quick visual check will miss.
If both bank 1 and bank 2 additive trims are high, the fault is usually upstream of the intake split, such as the mass air flow sensor or a shared vacuum line, rather than a bank-specific leak.

Can I keep driving with P1161?

You can usually drive the car to a workshop, but avoid long periods of idling and heavy load until the fault is repaired, because a persistently lean mixture on bank 2 can overheat the catalytic converter and cause misfires. If the check engine light starts flashing or the engine runs rough, stop driving and have the vehicle towed.

Tools that help

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Guidance is general and written for BMW vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.