Intake vacuum leak from a cracked or disconnected vacuum hose; Leaking intake manifold gasket or throttle body gasket; Faulty crankcase ventilation (PCV) valve or breather hose allowing unmetered air; Dirty or failing mass air flow (MAF) sensor under-reporting airflow; Weak or failing fuel pump or clogged fuel filter reducing fuel delivery at idle; Leaking fuel injector seals or a partially clogged injector on Bank 1; Intake air leak after the MAF sensor, such as a cracked intake boot

DTC codes / BMW codes / P1158

P1158 · BMWFuel Trim Adaptation Additive Bank 1 Low

Q: What does code P1158 mean?

Fuel Trim Adaptation Additive Bank 1 Low. P1158 on BMW vehicles means the engine computer has detected that the additive fuel trim adaptation for Bank 1 has hit its low limit, indicating the engine is running too lean at idle and the computer cannot add enough fuel to compensate. The additive portion of fuel trim corrects mixture errors at idle and low load, so this code points to a vacuum leak or unmetered air entering the engine rather than a problem with the main fuel supply.

P1158 on BMW vehicles means the engine computer has detected that the additive fuel trim adaptation for Bank 1 has hit its low limit, indicating the engine is running too lean at idle and the computer cannot add enough fuel to compensate. The additive portion of fuel trim corrects mixture errors at idle and low load, so this code points to a vacuum leak or unmetered air entering the engine rather than a problem with the main fuel supply.

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 that may smooth out as engine speed increases
Occasional hesitation or stumble when accelerating from a stop
Slightly higher fuel consumption than normal
In severe cases, a lean misfire code or reduced engine power

Common causes, most likely first

Intake vacuum leak from a cracked or disconnected vacuum hose
Leaking intake manifold gasket or throttle body gasket
Faulty crankcase ventilation (PCV) valve or breather hose allowing unmetered air
Dirty or failing mass air flow (MAF) sensor under-reporting airflow
Weak or failing fuel pump or clogged fuel filter reducing fuel delivery at idle
Leaking fuel injector seals or a partially clogged injector on Bank 1
Intake air leak after the MAF sensor, such as a cracked intake boot

How to diagnose and fix it

Connect a scan tool and record live fuel trim data at idle and at 2500 rpm to confirm the additive trim is maxed out lean on Bank 1
Perform a visual inspection of all intake hoses, vacuum lines, and the intake boot for cracks, splits, or loose connections
Use a smoke machine to pressurize the intake system and locate any vacuum leaks that are not visible
Clean the mass air flow sensor with dedicated MAF cleaner and recheck fuel trim readings
Check fuel pressure and volume at idle against BMW specifications to rule out a weak pump or restricted filter
Inspect and test the crankcase ventilation valve and hoses for leaks or blockage
If no leak is found, perform an injector balance test and inspect injector seals on Bank 1

Why additive trim points to unmetered air

Additive fuel trim only affects idle and low-load operation, so a P1158 almost always indicates a vacuum leak rather than a failing fuel pump or clogged filter, which would show up in multiplicative trim instead.
A smoke test is far more reliable than spraying carb cleaner around hoses, and BMW intake boots and PCV hoses are common hidden leak points that are easy to overlook during a visual inspection.
A dirty MAF sensor on BMW engines can under-report airflow and cause a lean additive trim without setting a MAF-specific code, so cleaning it is a cheap first step before replacing expensive parts.
This code is bank-specific, so if only Bank 1 is affected, focus on intake runners, injectors, and gaskets on that side rather than shared components like the air filter housing.
After repairing the leak, clear the adaptation values with a BMW-capable scan tool and drive the car through a complete idle and cruise cycle so the additive trim can relearn properly.

Can I keep driving with P1158?

You can drive the car short distances to a repair shop, but avoid prolonged idling and heavy acceleration because a lean condition can cause misfires and overheating. If the check engine light starts flashing or the engine runs very rough, stop driving and have it 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.