P2187: System Too Lean at Idle (Bank 1)
TL;DR
P2187 = system too lean at idle (Bank 1). Severity: medium. Drivable but may idle rough/stall. Top causes: vacuum/intake leak (50%), dirty MAF (25%), fuel delivery (15%), PCV (10%). A smoke test finds the leak fast.
Can I keep driving with P2187?
IF it idles acceptably → drive short-term while diagnosing. IF it idles very rough, stalls or you hear knocking → minimize driving; a sustained lean condition runs hot and stalling is a safety concern.
Symptoms
- Check engine light on
- Rough or unstable idle
- Stalling at idle/stops
- Hesitation just off idle
- Often a companion P0171 code
Top causes (ranked by probability)
| Likely cause | Probability | Notes |
|---|---|---|
| Vacuum / intake leak (unmetered air) | 50% | Worst at idle — smoke-test it |
| Dirty MAF sensor | 25% | |
| Weak fuel delivery (pressure/injectors) | 15% | |
| Stuck-open PCV valve | 10% |
What does P2187 mean?
Technical explanation
P2187 sets when Bank 1 fuel trims show a lean condition at idle (low engine load). Unmetered air has the biggest proportional effect at idle, so vacuum/intake leaks dominate. A contaminated MAF under-reporting airflow, a stuck-open PCV, or insufficient fuel pressure can also drive idle-lean trims. It often appears with P0171.
In simple terms
P2187 means your engine is getting too much air (or too little fuel) when idling. Small air leaks matter most at idle, so a cracked vacuum hose or intake leak is the usual culprit. A smoke test quickly shows where air is sneaking in.
How to diagnose P2187 (step by step)
- Read idle fuel trims. Confirm Bank 1 lean trims are worst at idle and improve off-idle (points to a vacuum leak).
- Smoke-test the intake. Find cracked hoses, intake/PCV leaks and gasket leaks.
- Clean/test the MAF. Rule out a contaminated MAF under-reporting air.
- Check the PCV valve. A stuck-open PCV is a common idle-lean source.
- Check fuel pressure. Low pressure can cause idle-lean trims.
Repair options & cost
- Repair vacuum/intake leaks
- Replace the PCV valve
- Clean or replace the MAF sensor
- Restore fuel pressure
| DIY cost | $0–$250 |
|---|---|
| Workshop cost | $100–$500 |
| Repair time | 20 min (hose/PCV) to 1.5 hours (fuel) |
Costs are local ballpark ranges and vary by region and vehicle.
Tools you’ll need
- OBD-II scanner (BlueDriver / ANCEL)
- Smoke machine
- MAF cleaner
Vehicle-specific notes
- Common on aging European engines (VW/Audi/BMW) with brittle plastic intake/PCV parts.
- Diagnose with any P0171 — they share causes.
- A small leak can be hard to see; a smoke test is worth it.
Common mistakes to avoid
- Replacing the O2 sensor first
- Skipping the smoke test on a clearly idle-lean code
- Ignoring the PCV system
- Using non-MAF cleaner on the sensor
Frequently asked questions
Why is P2187 only at idle?
Unmetered air from a vacuum leak has the biggest effect at idle, where airflow is low. At higher loads the extra air is proportionally smaller, so the lean condition shows up mainly at idle.
What is the most common cause of P2187?
A vacuum or intake leak. A dirty MAF sensor, stuck PCV valve, or weak fuel delivery are next.
Does P2187 come with P0171?
Often, since both indicate a lean Bank 1. Fixing the shared cause (usually a leak or MAF) typically clears both.
P2187 summary
| Meaning | System too lean at idle (Bank 1) |
|---|---|
| Severity | Medium |
| Safe to drive? | Yes, short-term (watch for stalling) |
| Top cause | Vacuum/intake leak (50%) |
| DIY cost | $0–$250 |
| Shop cost | $100–$500 |