Summary
P2187 means the engine's air-fuel mixture on Bank 1 is too lean specifically at idle. Unlike P0171 (lean at all speeds), P2187 only flags a lean condition at idle RPM. The most common causes are small vacuum leaks (35%), PCV valve or hose issues (20%), and dirty or clogged fuel injectors (15%). Because the lean condition is idle-specific, it points to causes that affect low-speed airflow or fuel delivery — not high-load systems like the fuel pump.
Severity: Moderate — rough idle, slightly elevated emissions
Safe to drive: Yes, short distances — rough idle but generally safe
Repair cost: $20–$500 depending on cause
DIY difficulty: Easy to Moderate
What does P2187 mean?
The PCM maintains a precise 14.7:1 air-fuel ratio using O2 sensor feedback and fuel trim adjustments. P2187 is a manufacturer-specific enhanced code (common on European manufacturers like Volkswagen, Audi, BMW, and Volvo) that specifically identifies a lean condition at idle.
At idle, the throttle plate is nearly closed and airflow is very low. Small sources of unmetered air — a cracked vacuum hose, a leaking intake gasket, or a stuck-open PCV valve — represent a much larger percentage of total airflow at idle than at cruising speed. This is why P2187 can set without P0171: the lean condition exists only when the engine is at low RPM with minimal airflow.
The PCM tracks the lean condition through fuel trims. When the long-term fuel trim (LTFT) at idle exceeds the manufacturer's threshold (typically +15% to +25%), P2187 is set. You will likely notice a rough or unstable idle, slight hesitation when pulling away from a stop, and possibly a slight fuel smell if the PCM is over-compensating.