P2279: Intake Air System Leak
The engine control unit has detected air entering the engine without being measured.
Normal risk
Usually safe to drive. Have it checked soon.
Short answer · Can I keep driving with P2279?
Usually yes. The engine may idle unevenly or hesitate. Have it checked soon, because a lean mixture can cause misfires.
What does P2279 mean?
P2279 means the engine control unit compares the measured air with the air it calculates from manifold pressure, throttle angle or fuel trims and finds more air than it measured. A split intake hose after the air-flow sensor, a leaking intake gasket, a loose oil filler cap or dipstick, a faulty PCV valve or a leaking brake servo hose are typical causes.
What the car does when the code is set
The check engine light comes on. The engine keeps running but its fuel correction is at its limit, so it may idle unevenly, hesitate when you accelerate and, in some cases, misfire. Fuel economy may change, and the engine may also store misfire codes if the mixture becomes too lean.
How the system works
The engine control unit calculates the fuel quantity from the air measured by the air-flow or manifold pressure sensor. The front oxygen sensor then reports whether the burnt mixture was rich or lean, and the control unit corrects the fuel quantity. This correction is called fuel trim: short-term trim reacts instantly, long-term trim learns a lasting offset. Both are limited. When the correction reaches its upper limit and the mixture is still lean, the engine sets a lean code, because something outside the control unit's measurement is adding air or removing fuel.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Vacuum leakA split hose, intake gasket or PCV valve lets in air the air-flow sensor does not measure.
- Dirty or faulty air-flow sensorThe sensor under-reports the air, so the engine adds too little fuel.
- Low fuel pressureA weak pump, clogged filter or faulty pressure regulator supplies too little fuel.
- Clogged injectorsDeposits reduce the amount of fuel injected.
- Exhaust leak before the front oxygen sensorOutside air makes the sensor read lean.
How it is diagnosed
- Look at the fuel trimsA scan tool shows short- and long-term fuel trims. Trims that are high at idle but fall at higher revs suggest a vacuum leak; high everywhere suggest fuel delivery or the air-flow sensor.
- Smoke-test the intakeA smoke machine shows leaks in hoses, gaskets and the PCV system quickly.
- Check the air-flow sensor readingCompare the measured air flow at idle and at higher revs with the value the service data gives for your engine.
- Measure fuel pressureA pressure gauge or the live data on direct-injection engines shows whether the fuel supply keeps up.
Questions about P2279
How is an intake leak found?
A smoke test is the most reliable method: smoke is pushed into the intake and shows where it escapes. Listening for a hiss at idle also helps.
Can I just clean the air-flow sensor?
If the sensor is dirty, cleaning it with a dedicated sensor cleaner can help. Check for vacuum leaks first, as they are the more common cause.
Can a lean code damage the engine?
Running lean for a long time raises combustion temperatures and can cause misfires and, in extreme cases, burnt valves or pistons.