P2279 Code: Intake Air System Leak
Air is getting into the engine after the mass airflow sensor, so the computer isn't measuring all of it.
At a glance
Severity: Low
- Meaning
- Intake Air System Leak
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $40–$750
- DIY difficulty
- Easy
The ECM compares the air the MAF sensor measures with how much air should be entering based on throttle angle, rpm and manifold pressure. When the engine is clearly breathing more air than the MAF saw, and the fuel trims have to add fuel mostly at idle to compensate, it sets P2279. Any opening between the MAF and the cylinders can do it: a split intake boot or loose clamp, a cracked PCV hose or torn PCV valve diaphragm, a leaking intake manifold or throttle body gasket, the brake booster hose, a purge valve stuck open, or even an oil filler cap or dipstick that doesn't seal, since the crankcase is connected to the intake through the PCV system. Leaks matter most at idle, when manifold vacuum is high and total airflow is small; the same leak barely registers at highway speed, which is the classic fingerprint that separates P2279 from a dirty MAF sensor.
Can I keep driving? Generally fine to drive, but a large leak can cause stalling at stops or a racing idle. Fix it before it lets the engine run lean for months, which can lead to misfires and a damaged catalytic converter.
How to read P2279
- P Powertrain
- 2 Generic (SAE)
- 2 Fuel & air (injector circuit)
- 79 Specific fault
Symptoms
- Check engine light, often together with P0171 and/or P0174 (system too lean)
- Rough, high or hunting idle, sometimes stalling when coming to a stop
- A hissing or whistling sound from the engine bay at idle
- Fuel trims well positive at idle that come back toward normal at higher rpm
- Hard brake pedal feel if the brake booster hose is the leak
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Cracked or disconnected PCV hose, or failed PCV valve or diaphragm
- Split intake boot or loose clamp between MAF and throttle body
- Leaking intake manifold or throttle body gasket
- Brake booster hose, check valve or booster diaphragm leaking
- Oil filler cap or dipstick not sealing
- EVAP purge valve stuck open
- MAF sensor under-reading, which mimics a leak
Diagnosis step by step
Read fuel trims at idle and 2,500 rpm
With the engine fully warm, add short-term and long-term fuel trims together. More than about +10 to +15 % at idle that drops to within about 5 % at 2,500 rpm is a vacuum leak signature; lean at every speed points more toward the MAF sensor or fuel delivery.
Tools: OBD-II scanner
Do the quick checks
Make sure the oil filler cap and dipstick are fully seated with intact seals, and squeeze the intake boot between the MAF and throttle body to look for cracks in the folds. Tighten any loose clamps and listen around the intake for a hiss with the engine idling.
Tools: Flashlight, Screwdriver
Smoke-test the intake
With the engine off, feed smoke into the intake after the MAF at low pressure (no more than about 1 psi) and plug the air inlet. Watch for smoke at the manifold gasket, PCV hoses, throttle body, injector seals and the brake booster line; a smoke test finds in minutes what spraying cleaner can take hours to find.
Tools: Smoke machine, Flashlight
Check the PCV system
Remove the oil filler cap at idle: a light suction is normal, but strong suction that changes the idle or makes a whistle means a torn PCV diaphragm or stuck valve. Inspect PCV hoses underneath the intake, where they harden and crack out of sight.
Tools: Flashlight, Gloves
Isolate the booster and purge valve
Briefly pinch the brake booster hose, then the purge line, while watching idle and trims. If the idle smooths or the trims drop, that branch is leaking; a purge valve that holds vacuum with the engine off should be sealed.
Tools: Pliers, Vacuum gauge
Repair and relearn
After the repair, clear the codes so the trims start fresh, then check that they settle within about ±5 % at idle after a drive. On some engines an idle relearn procedure is needed if the throttle body was cleaned or replaced.
Tools: OBD-II scanner
Where the parts are
- 1MAF sensor
- 2Turbocharger
- 3Throttle body
- 4Intake manifold
- 5Fuel injector
- 6EGR valve
- 7Fuel pump
- 8Thermostat
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace intake boot or clamp | Yes | $40–$200 |
| Replace PCV valve or hoses | Yes | $50–$350 |
| Professional smoke test | No | $100–$200 |
| Replace intake manifold gasket | No | $250–$750 |
| Replace brake booster hose or check valve | Yes | $40–$150 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Toyota
Toyota and Lexus engines, including the 2AZ-FE four-cylinder, are known for PCV hoses that harden and crack underneath the intake manifold where they're hard to see. A smoke test shows them quickly.
Mercedes-Benz
On Mercedes M272 V6 and M273 V8 gasoline engines, the crankcase breather hoses on top of the engine harden and crack with age, a common source of P2279 together with lean codes on both banks.
Emissions / smog test impact
Will fail
P2279 with the check engine light on fails OBD-based state emissions tests. A leak that keeps the engine lean can also stop the catalyst and O2 sensor monitors from completing until it's repaired.
Frequently asked questions
What's the difference between P2279 and P0171?
P0171 simply says bank 1 is running lean. P2279 goes a step further: the ECM has compared measured and expected airflow and concluded that unmetered air is getting in, which points directly at a leak.
Can I find a vacuum leak with carburetor cleaner?
Spraying flammable cleaner around a running engine is a fire risk and imprecise. A smoke test is safer and shows the exact spot, and many shops charge little for it.
Can a loose gas cap cause P2279?
No. The gas cap is part of the EVAP system and sets its own codes. A purge valve stuck open, however, can pull in vapor and air and contribute to P2279.
Will a vacuum leak damage the engine?
A small one mainly causes a rough idle and a warning light. A large or long-ignored leak can cause lean misfires that overheat the catalytic converter, so it's worth fixing promptly.
Sources
- SAE J2012 – Diagnostic Trouble Code Definitions
- ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
- Manufacturer public service information (cross-checked)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy