P2187: System Too Lean at Idle (Bank 1)
Updated November 14, 2026
The engine computer is adding more fuel than normal at idle to keep the air-fuel mixture right on bank 1, and it has reached its limit. Because so little air enters the engine at idle, even a small amount of extra air that the computer doesn't know about makes the mixture lean, so vacuum leaks are the classic cause.
COMPZA may earn a commission when you buy through links on this page, at no extra cost to you. How we make money.
🔧 Want to check this yourself? An OBD2 scanner reads the code; models with live data also show the sensor readings used in the checks below. See code readers and scanners from $24.99 ↓ · Which scanner do I need?
Common causes
- A vacuum leak, such as a cracked hose, a leaking intake manifold gasket or a brake booster hose
- A failed PCV valve or cracked PCV hose
- A cracked intake air boot between the mass airflow sensor and the throttle body
- An exhaust leak ahead of the upstream O2 sensor, which pulls in outside air
- Low fuel pressure from a weak fuel pump or clogged fuel filter (less common at idle only)
Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.
Symptoms
- Check engine light
- Rough or uneven idle
- Hissing noise from the engine bay on some vehicles
- Sometimes P0171 or misfire codes along with it
Is it safe to drive?
Usually drivable short term with a steady light, but get it fixed soon; a lean engine can start misfiring. If the light flashes or the engine stalls at idle, stop driving and get it checked.
How to check it with a scanner
- Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
- Look at long-term fuel trims at idle, then hold the engine at about 2,500 RPM. If the trims are high at idle but come down at higher RPM, a vacuum leak is likely; if they stay high or get worse, look at fuel delivery or the MAF sensor.
- With the engine off, inspect the intake boot, PCV hose and vacuum hoses for cracks, especially on the underside.
- A smoke test is the safest and most reliable way to find vacuum leaks; many shops can do one. Don't spray flammable cleaners around a running engine to hunt for leaks.
- Keep hands clear of the belts and fan whenever the engine is running.
- After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.
A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.
Code readers and scanners in stock
From partner stores' live product feeds. Check that the tool covers your vehicle's make, model and year on the retailer's page before you buy.
LAUNCH CR319 OBD2 Code Reader & Emissions Readiness Tester$24.99at LAUNCH X431 Online 
LAUNCH CR529 OBD2 Code Reader with Live Data Stream$45at LAUNCH X431 Online 
Autel MaxiLink ML529$52at AUTELOnline 
AutoLink AL319$59at AUTELOnline 
MaxiAP AP2500E$60at AUTELOnline 
LAUNCH Creader VII+ Code Reader for Engine, ABS, SRS & Transmission$119at LAUNCH X431 Online Affiliate links. AUTELOnline prices updated Nov 14, 5:26 PM EST · LAUNCH X431 Online prices updated Nov 14, 4:18 PM EST.
Search eBay for your vehicle
Affiliate links: COMPZA may earn a commission from qualifying eBay purchases at no extra cost to you. Add your vehicle in eBay's parts finder to see parts that fit.
Questions
What does P2187 mean?
P2187 is the generic OBD2 code for "System Too Lean at Idle (Bank 1)." The engine computer is adding more fuel than normal at idle to keep the air-fuel mixture right on bank 1, and it has reached its limit. Because so little air enters the engine at idle, even a small amount of extra air that the computer doesn't know about makes the mixture lean, so vacuum leaks are the classic cause.
Is it safe to drive with P2187?
Usually drivable short term with a steady light, but get it fixed soon; a lean engine can start misfiring. If the light flashes or the engine stalls at idle, stop driving and get it checked.
How do I check P2187 myself?
Read the code and freeze-frame data with an OBD2 scanner, then: Look at long-term fuel trims at idle, then hold the engine at about 2,500 RPM. If the trims are high at idle but come down at higher RPM, a vacuum leak is likely; if they stay high or get worse, look at fuel delivery or the MAF sensor. With the engine off, inspect the intake boot, PCV hose and vacuum hoses for cracks, especially on the underside. A smoke test is the safest and most reliable way to find vacuum leaks; many shops can do one. Don't spray flammable cleaners around a running engine to hunt for leaks. Keep hands clear of the belts and fan whenever the engine is running.
Related codes
📬 Get the weekly deals email
One email a week with the best current deals from our partner stores: home, kitchen, tech, car and gifts. No spam, unsubscribe anytime.
Emails include affiliate links; COMPZA may earn a commission at no extra cost to you. See our privacy policy.