System Too Lean at Idle Bank 1
The Powertrain Control Module (PCM) has detected that the air-fuel mixture is too lean (too much air or not enough fuel) specifically when the engine is idling on Bank 1. This code is set when the short-term and long-term fuel trims exceed their maximum positive thresholds during idle conditions to compensate for the lean condition.
While the vehicle can usually still be driven, running lean can eventually cause increased combustion temperatures, potentially leading to catalytic converter or internal engine damage over time. It should be diagnosed and repaired soon.
Common symptoms
- Illuminated Check Engine Light
- Rough or fluctuating idle
- Engine stalling at idle or when coming to a stop
- Hesitation or stumbling upon initial acceleration
- Hissing sound from the engine bay (if a vacuum leak is present)
Likely causes
- Vacuum leak (intake manifold gasket, vacuum hoses, PCV system)
- Faulty PCV (Positive Crankcase Ventilation) valve
- Defective or dirty Mass Air Flow (MAF) sensor
- Low fuel pressure (failing fuel pump, clogged fuel filter)
- Faulty or clogged fuel injectors on Bank 1
- Exhaust leak upstream of the Bank 1 oxygen sensor
What to check before replacing parts
Perform a visual and auditory inspection of all vacuum hoses and the PCV system.
Look for: Listen for hissing sounds and look for cracked, disconnected, or degraded rubber hoses that could allow unmetered air into the intake.
Monitor live fuel trim data using an OBD-II scanner at idle and at 2500 RPM.
Look for: If the positive fuel trims drop back to normal levels at 2500 RPM but are high at idle, it strongly indicates a vacuum leak. If they remain high at all RPMs, suspect a fuel delivery or MAF sensor issue.
Perform a smoke test on the intake manifold and vacuum system.
Look for: Watch for smoke escaping from intake gaskets, vacuum lines, or the PCV valve, which pinpoints the exact location of unmetered air entry.
Check the Mass Air Flow (MAF) sensor readings on the scan tool.
Look for: Compare the MAF g/s reading at idle to the manufacturer specification (typically around 2-4 g/s for a 4-cylinder). A lower than normal reading can cause a lean condition.
Test the fuel pressure using a mechanical gauge.
Look for: Verify that the fuel pressure meets the manufacturer's specifications at idle. Low pressure indicates a failing fuel pump, restricted fuel filter, or faulty fuel pressure regulator.
This page explains the generic P2187 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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