Vacuum leak from a cracked or disconnected intake boot, vacuum hose, or intake manifold gasket; Failed or leaking crankcase ventilation valve (CCV) or its associated hoses; Dirty or contaminated mass air flow (MAF) sensor reporting less air than actually enters; Leaking intake manifold gasket or deteriorated throttle body gasket; Weak or failing fuel pump delivering insufficient volume at idle; Clogged or restricted fuel filter or fuel injector screens; Faulty pre-catalyst oxygen sensor giving a false lean reading

DTC codes / BMW codes / P1159

P1159 · BMWFuel Trim Adaptation Additive Bank 1 High

Q: What does code P1159 mean?

Fuel Trim Adaptation Additive Bank 1 High. P1159 on BMW vehicles means the engine computer has detected that the additive fuel trim adaptation for Bank 1 has exceeded its positive limit, indicating a persistent lean condition that the system can no longer compensate for. Additive trim corrects mixture deviations at idle and low load, so a high value points to unmetered air entering the engine or a fuel delivery shortfall at idle.

P1159 on BMW vehicles means the engine computer has detected that the additive fuel trim adaptation for Bank 1 has exceeded its positive limit, indicating a persistent lean condition that the system can no longer compensate for. Additive trim corrects mixture deviations at idle and low load, so a high value points to unmetered air entering the engine or a fuel delivery shortfall at idle.

SystemPowertrain

Code typeBMW specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Rough or unstable idle, sometimes with a slight stumble when coming to a stop
Occasional lean misfire at idle that may feel like a light shake
Slightly higher fuel consumption as the computer adds extra fuel to compensate
In some cases, a delayed or harsh downshift feel at low speed due to idle instability

Common causes, most likely first

Vacuum leak from a cracked or disconnected intake boot, vacuum hose, or intake manifold gasket
Failed or leaking crankcase ventilation valve (CCV) or its associated hoses
Dirty or contaminated mass air flow (MAF) sensor reporting less air than actually enters
Leaking intake manifold gasket or deteriorated throttle body gasket
Weak or failing fuel pump delivering insufficient volume at idle
Clogged or restricted fuel filter or fuel injector screens
Faulty pre-catalyst oxygen sensor giving a false lean reading

How to diagnose and fix it

Connect a scan tool and read live fuel trim data to confirm additive trim is high on Bank 1 and check for stored freeze-frame data
Perform a visual inspection of all intake and vacuum hoses for cracks, loose clamps, or disconnected lines
Use a smoke machine to pressurize the intake system and locate any vacuum leaks precisely
Clean the mass air flow sensor with a dedicated MAF cleaner and verify its readings against expected values
Test fuel pressure and volume at idle, comparing against BMW specifications for the engine
Check the crankcase ventilation valve and hoses for leaks or blockage, replacing if faulty
If no leaks are found, inspect the pre-catalyst oxygen sensor signal for erratic or biased readings and replace if necessary

Additive trim points to idle air leaks

Additive fuel trim is learned only at idle, so a high additive value almost always means unmetered air is entering the engine at idle rather than a general fuel delivery problem.
A leaking crankcase ventilation valve on BMW engines is a very common cause of P1159 that many technicians overlook because the valve is hidden under the intake manifold.
A dirty mass air flow sensor can under-report airflow and cause the computer to add fuel, setting this code even when no actual vacuum leak exists.
Do not replace the oxygen sensor first; a false lean reading from a worn sensor is possible but far less likely than an intake leak on BMW vehicles.
After repairing the leak, the additive trim must be cleared and relearned through a drive cycle before the code will stay off.

Can I keep driving with P1159?

You can drive the car short distances to a repair shop, but avoid prolonged idling and heavy load until the lean condition is fixed. Continuing to drive with a persistent lean mixture can overheat the catalytic converter and cause costly damage.

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.