P2187
System Too Lean at Idle (Bank 1)
Fuel trim on bank 1 hits the lean limit only at idle, while trim returns to normal at higher load. This pattern strongly indicates a vacuum leak — leaks become a larger fraction of total airflow at idle and so dominate the trim, but disappear into background noise above idle.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Symptoms
- Check engine light on
- Rough idle, possible stalling
- Hesitation or hard cold start
- Power loss under load
Common causes
Ordered by how often each turns out to be the cause.
- Vacuum leak (intake gasket, brake booster, hoses)Common
- PCV valve stuck openPossible
Parts involved
- Intake Manifold
- PCV Valve
- Brake Booster
- Vacuum Hose
P2187 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.