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Severity: Low Code type: Generic (all makes)

P2187 Code: System Too Lean at Idle (Bank 1)

Bank 1 runs lean at idle but recovers once rpm rises, the classic fingerprint of unmetered air.

At a glance

Severity: 2/5 – Low2/5

Severity: Low

Meaning
System Too Lean at Idle (Bank 1)
System
Fuel & Air Metering · Powertrain
Safe to drive?
With caution
Typical repair cost
$20–$700
DIY difficulty
Moderate

P2187 is limited to the idle range: the PCM needs so much added fuel on bank 1 at a warm idle that the correction passes its limit, while trims at higher rpm stay reasonable. At idle only a few grams per second of air pass the MAF sensor, so even a pencil-sized leak behind it can be a fifth of the total airflow; at 2,500 rpm the same leak barely registers. That makes P2187 a far more focused code than P0171. The usual sources are the crankcase ventilation system, a cracked vacuum line, a leaking throttle body or intake gasket, or a purge valve that stays open. An exhaust leak ahead of the upstream sensor behaves the same way, because the exhaust pulses pull in outside air mostly at low rpm.

Can I keep driving? Driving is usually fine, but the engine may stumble or stall when you come to a stop, which is a risk in traffic and at intersections. Get it checked soon, and have the hose or gasket fixed before the leak grows.

How to read P2187

How to read P2187P12213874
  1. P Powertrain
  2. 2 Generic (SAE)
  3. 1 Fuel & air metering
  4. 87 Specific fault

Symptoms

  • Check engine light, idle-related trims high
  • Idle speed hunting, surging, or sitting higher than normal
  • Stumble or stall when rolling to a stop
  • Hissing or whistling from the engine bay at idle
  • Idle speed code P0507 on some vehicles
  • Normal power and smooth running once moving

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Crankcase ventilation fault: torn PCV valve diaphragm, split PCV or breather hose 28%
  2. Cracked or disconnected vacuum line (brake booster, EVAP, HVAC or accessory lines) 20%
  3. Throttle body gasket, intake manifold gasket or intake runner shaft seal leaking 16%
  4. Exhaust leak ahead of the upstream sensor drawing air in at low rpm 10%
  5. EVAP purge valve stuck open, pulling air through an empty canister 10%
  6. Leaking upper injector O-rings 8%
  7. Crankcase openings leaking: oil filler cap seal, dipstick O-ring, valve cover gasket 8%

Diagnosis step by step

Diagnostic flow: P21871Prove it's an idle-only problem2Read manifold vacuum3Smoke the intake4Pinch-test the suspects5Check the rear sensor to rule out anexhaust leak

  1. Prove it's an idle-only problem

    On a warm engine, total trim (STFT + LTFT) of around +18 to +25% at idle that drops to within ±5% at a steady 2,500 rpm confirms unmetered air. If the numbers stay high at 2,500 rpm, go to the P2177 or P0171 approach instead.

    Tools: OBD-II scanner

  2. Read manifold vacuum

    Connect a vacuum gauge to a manifold port. A healthy warm engine at sea level typically shows a steady 17–21 inHg at idle; a low but steady reading supports a leak, while a needle that flutters points more to valves or misfires.

    Tools: Vacuum gauge

  3. Smoke the intake

    Feed smoke into the intake after the MAF with the engine off and watch the throttle body, runner shafts, injector bases, PCV valve and every vacuum hose. It's safer and more precise than spraying flammable cleaner around a hot engine.

    Tools: Smoke machine, Flashlight

  4. Pinch-test the suspects

    With the engine idling and trims on screen, briefly pinch the brake booster hose, then the purge line. If STFT drops sharply while one line is closed, the leak is downstream of that point (booster check valve, booster diaphragm or purge valve).

    Tools: Pliers, OBD-II scanner

  5. Check the rear sensor to rule out an exhaust leak

    With a true vacuum leak both O2 sensors read lean at idle. If the upstream sensor reads lean while the downstream sensor still swings rich, suspect an exhaust leak at the manifold or flange ahead of the front sensor; listen for ticking on a cold start.

    Tools: OBD-II scanner, Flashlight

Where the parts are

Air intake and fuel metering – P218712345678
Air intake and fuel metering
  1. 1MAF sensor
  2. 2Turbocharger
  3. 3Throttle body
  4. 4Intake manifold
  5. 5Fuel injector
  6. 6EGR valve
  7. 7Fuel pump
  8. 8Thermostat

Repair options & costs

RepairDIY?Estimated cost
Replace cracked vacuum or PCV hose Yes $20–$150
Replace PCV valve or oil separator Yes $40–$350
Replace throttle body gasket Yes $60–$200
Replace intake manifold gasket No $250–$700
Reseal injectors with new O-rings No $150–$400

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Volkswagen

VW scan tools label this code 'System too lean at idle (additive)'. On EA888 2.0T engines the PCV valve bolted to the cam cover is the first suspect; a whistle at idle and idle trims well above normal with good trims at cruise is the typical picture.

Audi

On Audi 2.0 TFSI engines in the A4 and Q5, the same PCV valve failure is common, and the intake runner flap shafts on FSI engines can also leak. Smoke testing is quicker than swapping parts on these crowded engine bays.

Emissions / smog test impact

Will fail

A stored P2187 with the check engine light on fails the OBD-II test. Fix the leak before clearing codes, since the readiness monitors need a few drive cycles to reset.

Frequently asked questions

Why only at idle?

Because the leak is a fixed-size hole. At idle the MAF measures only a few grams of air per second, so a small leak makes up a large share of it; at higher rpm the engine breathes so much more air that the leak hardly changes the mixture.

Can I find the leak with carburetor cleaner?

People do, but spraying flammable solvent around a hot exhaust manifold is a fire risk and the results are vague on modern engines. A smoke test finds the leak visually and is safer.

Will a new O2 sensor stop P2187?

Almost never. The sensor is correctly reporting extra oxygen. Replace it only if the trims look normal at idle in live data while the sensor itself is stuck lean.

Could the brake booster cause this?

Yes. A torn booster diaphragm or leaking check valve pulls air into the intake. A hint is that trims or idle speed change when you press the brake pedal.

Sources