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P2187 and P2189: system too lean at idle only
A lean condition that exists at idle and vanishes under load points almost exclusively at unmetered air. The load-dependence is the diagnosis, and it costs nothing to observe.
P2187 “System Too Lean at Idle Bank 1” and P2189 “System Too Lean at Idle Bank 2” are the idle-specific relatives of P0171 and P0174. Neither has a public reference page here, but the “at idle” qualifier makes them far more informative than the general lean codes.
Why “at idle only” is the whole answer
Think about proportions.
At idle a small engine moves perhaps 2 grams of air per second. At 3000 rpm under load it moves ten to twenty times more. A vacuum leak admits roughly the same amount of extra air in both cases.
So the same leak is a large percentage of idle airflow and a trivial percentage of loaded airflow. The ECU compensates easily under load and runs out of correction range at idle.
A lean condition that exists only at idle is a vacuum leak until proven otherwise. If it were a fuel supply problem, it would be worse under load, when demand is highest — the opposite pattern.
The fuel trim test that confirms it
Ten minutes, no tools beyond the adapter.
Log short-term and long-term fuel trims at warm idle, then at a steady 2500 rpm.
| Idle trim | 2500 rpm trim | Conclusion |
|---|---|---|
| +20% or more | Under +5% | Vacuum leak — confirmed |
| +20% | +20% | Fuel delivery or air metering, not a leak |
| Under +5% | +20% | Fuel supply limit at higher demand |
| One bank high, other normal | Leak on that bank specifically |
That last row matters on V engines. P2187 without P2189 means the leak is on bank 1 only, which halves the area you need to search — and rules out anything shared, like the fuel pump or the MAF sensor.
Background on interpreting these numbers: fuel trim explained.
Where the air gets in
Vacuum hoses. Small-bore rubber to the brake booster, PCV system, purge valve and various actuators. Cracks form at bends and at the ends where the hose stretches over a barb. On a car over eight years old, this is the first place to look.
Intake manifold gasket. Especially common on engines with plastic manifolds. A leak that only opens cold, closing as the engine expands, gives a lean condition on the first few minutes of every drive.
PCV valve or breather hose. A stuck-open PCV valve is a calibrated leak. A split breather hose is a large uncalibrated one.
Throttle body gasket and the throttle shaft itself, which wears and admits air past its bearings on high-mileage engines.
Brake booster. A failed diaphragm is a large leak that also makes the pedal hard. Related: spongy brake pedal.
Injector seals. Hardened O-rings at the base of the injectors let air into individual runners.
Purge valve stuck open, feeding both air and vapour into the manifold — P0440 and P0446.
Dipstick tube or oil filler cap not sealing. Genuinely common and genuinely overlooked.
Symptoms beyond the code
- Hunting or unstable idle — fluctuating idle RPM.
- Idle speed higher than commanded, which is P0507 territory.
- Hesitation from a standstill.
- Occasionally a stall when coming to a stop.
If the engine also misfires, remember that a lean mixture misfires. Chasing P0301-P0304 while ignoring the lean code means replacing coils for a cracked hose.
Finding the leak
Listen. Engine idling, in a quiet space. A hiss localises it faster than anything else.
Smoke test. The reliable method, and cheap at a workshop.
Propane or spray test. Introduce a small amount near each suspect joint and watch short-term fuel trim on screen. When trim swings negative, you are standing over the leak. Be sensible about fuel sources near hot exhaust.
Watch trim while pinching hoses. Clamping the PCV hose and seeing trim change identifies the crankcase side immediately.
What it is not
Do not start by cleaning the MAF sensor. A dirty MAF that under-reports causes the ECU to under-fuel, which does produce lean codes — but it does so across the whole rev range, not just at idle. The load-dependence rules it out. Detail: P0101.
Likewise a weak fuel pump fails under load, not at idle.
Cost expectations
- Vacuum hose: $5–40.
- PCV valve: $15–60.
- Intake manifold gasket: $40–120 in parts.
- Injector seal kit: $20–80.
- Smoke test at a workshop: $40–100.
After the repair
Confirm idle trims return to within a few percent, idle speed settles at target, and the trims at idle and 2500 rpm now agree with each other. Drive a full cycle and check nothing pending returns.
Try it on the free tier of the app — pair an ELM327 adapter and compare fuel trims at idle against 2500 rpm before touching a single part. That one comparison is the difference between finding a $5 hose and replacing a $200 sensor for nothing.
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