OBD-II trouble code
P2279: Intake Air System Leak
Air is getting into the engine without being measured. Unlike the familiar lean codes, this one is set by an airflow plausibility check rather than by fuel trims, so it can appear before anything else does.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $20 – $900
- DIY difficulty
- Intermediate DIY
Browse every code in P2200–P229F, or start from the full code library.
What does P2279 mean?
The control module knows how much air it believes is entering the engine, from the mass airflow sensor or the manifold pressure sensor, and it also knows how much air ought to be entering given throttle angle, engine speed, camshaft position and idle control. P2279 sets when those two numbers stop agreeing in the direction that means unmetered air — more air arriving than the throttle should be admitting.
The usual way a driver meets an air leak is through P0171 or P0174: the mixture goes lean, the module adds fuel, and eventually the trim correction runs out of room. P2279 arrives by a different route. It is a rationality check on the air model itself, so it can set while fuel trims are still comfortably inside their limits — which is why a vehicle can carry this code with no lean code, no rough idle and no complaint from the driver beyond the light. Read it as an early warning rather than a late one.
The leak has to be downstream of whatever measures the air. A split in the ducting before the mass airflow sensor changes nothing, because that air still gets counted. A split after it does not. The boundary matters more than most people expect, and it extends further than the visible ducting: on most engines the crankcase is tied into the metered intake through the positive crankcase ventilation system, so a hardened PCV hose, a stuck valve, a loose oil filler cap or a dipstick that is not seated is an intake leak by another name.
Idle is where these leaks show themselves, because that is where the throttle plate is nearly shut and a fixed-size leak represents the largest fraction of total airflow. At higher speeds the same hole is a rounding error. Two exceptions are worth remembering: on turbocharged engines the charge pipework between the compressor and the throttle is part of the same system and leaks there appear under boost instead of at idle, and a leak that only opens when a rubber component flexes with engine torque will be invisible with the engine sitting still.
Common causes
- Cracked or split intake boot between the mass airflow sensor and the throttle body, usually in the accordion folds
- Hardened, split or disconnected PCV hose, or a PCV valve stuck open
- Leaking intake manifold gasket or throttle body gasket
- Perished brake booster hose or a failed booster check valve
- Loose or missing oil filler cap, or a dipstick not fully seated
- Cracked or disconnected vacuum lines to accessories and emissions components
- Leaking charge pipe, intercooler connection or clamp on a turbocharged engine
- Split or disconnected EVAP purge line feeding the intake
- Failed intake manifold runner control shaft seal on engines that use one
Symptoms
- Check engine light on, often with no other complaint
- Unstable, high or hunting idle
- Hesitation or stumble just off idle that clears as the throttle opens
- A hiss or whistle from the engine bay at idle that changes with engine speed
- Lean codes such as P0171 or P0174 appearing later as the leak grows
- Slightly reduced fuel economy
- On turbocharged engines, loss of boost or a rushing noise under load rather than an idle problem
Diagnostic steps
- 1.Read all codes and freeze frame. Whether the event was recorded at idle or under load tells you which half of the intake system to search — idle points at the throttle body and manifold, load points at charge pipework.
- 2.Inspect the intake boot between the mass airflow sensor and the throttle body with the engine running, flexing it by hand and opening the accordion folds. Splits close up at rest and open under vibration.
- 3.Remove the oil filler cap at idle. On most engines the idle should stumble noticeably; no reaction at all suggests the crankcase is already leaking air somewhere and the PCV path deserves attention.
- 4.Check the PCV valve and its hoses, the brake booster hose and check valve, and every vacuum line for hardening, splits and pulled-off ends.
- 5.Watch long-term fuel trims at idle and then at around 2,500 rpm. A leak that shows at idle and disappears as speed rises is the classic signature of a fixed-size leak.
- 6.Smoke-test the intake with the system sealed. This is the definitive test and it finds leaks that are invisible and inaudible, including manifold gasket and runner shaft leaks.
- 7.Inspect the intake manifold and throttle body gasket faces for the dark, clean streaking that marks a leak path, particularly after previous gasket work.
- 8.On a turbocharged engine, pressure-test the charge pipework separately from the pre-throttle intake, since a boost-side leak will not show at idle at all.
- 9.Avoid spraying flammable cleaner around a hot engine to hunt leaks. A smoke machine or propane at low pressure gives a better answer without the fire risk.
Repair cost
$20 – $900
A cracked vacuum hose or a loose oil cap can be fixed for $20 or less. A replacement intake boot or PCV hose runs about $60 to $250 fitted. Intake manifold gasket replacement is the expensive end at roughly $200 to $900 depending on engine layout, since a V-configuration engine needs the plenum and often the fuel rail removed. Expect $90 to $180 of diagnostic time for a smoke test if the leak is not visible, which is usually money well spent.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with vacuum leak repair preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.