Unmetered air entering through a cracked or disconnected vacuum hose, intake boot, or intake manifold gasket.; A faulty or dirty mass air flow sensor that under-reports the air entering the engine.; A leaking intake manifold gasket or throttle body gasket that opens up only at idle vacuum.; A stuck-open or leaking purge solenoid in the evaporative emissions system, allowing fuel vapor or air to upset the mixture at idle.; Low fuel pressure from a weak fuel pump, clogged fuel filter, or restricted fuel line.; A dirty or failing fuel injector that delivers less fuel than the computer commands.; A lazy or contaminated oxygen sensor on bank 1 that reports a false lean reading.

DTC codes / P21xx · Fuel and air metering / P2187

P2187System Too Lean at Idle (Bank 1)

Q: What does code P2187 mean?

System Too Lean at Idle (Bank 1). P2187 means the engine computer has detected a lean condition at idle on bank 1, which is the side of the engine containing cylinder one. The computer calculates that the air-to-fuel mixture is too high in air, so it has added more fuel than expected and still cannot bring the oxygen sensor reading back to the target range.

P2187 means the engine computer has detected a lean condition at idle on bank 1, which is the side of the engine containing cylinder one. The computer calculates that the air-to-fuel mixture is too high in air, so it has added more fuel than expected and still cannot bring the oxygen sensor reading back to the target range.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Often no symptoms besides the warning light, especially if the leak is small.
Rough or unstable idle that may smooth out as soon as you press the accelerator.
A hissing or sucking sound from the engine bay at idle, which points to a vacuum leak.
Occasional misfire or shaking at stoplights, sometimes with a flashing check engine light if the lean condition is severe.
Slightly higher fuel consumption because the computer is adding extra fuel to compensate.
A lean exhaust smell or a faint whistle from a cracked intake component.

Common causes, most likely first

Unmetered air entering through a cracked or disconnected vacuum hose, intake boot, or intake manifold gasket.
A faulty or dirty mass air flow sensor that under-reports the air entering the engine.
A leaking intake manifold gasket or throttle body gasket that opens up only at idle vacuum.
A stuck-open or leaking purge solenoid in the evaporative emissions system, allowing fuel vapor or air to upset the mixture at idle.
Low fuel pressure from a weak fuel pump, clogged fuel filter, or restricted fuel line.
A dirty or failing fuel injector that delivers less fuel than the computer commands.
A lazy or contaminated oxygen sensor on bank 1 that reports a false lean reading.

How to diagnose and fix it

Connect a scan tool and record fuel trims at idle and at 2500 rpm; if the lean condition disappears off idle, focus on vacuum leaks.
Inspect all intake and vacuum hoses, the intake boot, and the PCV system for cracks, loose clamps, or disconnected lines.
Spray a small amount of carb cleaner or use a smoke machine around intake gaskets and hoses to pinpoint a vacuum leak.
Clean the mass air flow sensor with dedicated cleaner and verify its reading against expected values at idle.
Test fuel pressure with a gauge and compare it to the manufacturer's specification, then check fuel filter restriction.
Check the purge solenoid for sticking open and verify it holds vacuum when commanded closed.
If all external causes are ruled out, perform an injector balance test and inspect the bank 1 oxygen sensor signal for a lazy or biased response.

Why idle-only lean codes fool many technicians

A vacuum leak that only opens under high idle vacuum is the most common cause, so a leak that does not show up during a rev test can still be the culprit.
A dirty mass air flow sensor often under-reports air at idle while reading correctly at higher rpm, which sets P2187 without any other code.
A leaking purge solenoid can pull fuel vapor into the intake at idle and create a lean condition that looks like a vacuum leak but is actually an evaporative emissions fault.
Do not replace the oxygen sensor first; a lean reading from a healthy sensor is the computer's correct response to unmetered air or low fuel delivery.
A clogged fuel filter or weak fuel pump can cause a lean condition at idle that disappears under load, mimicking a vacuum leak.

Can I keep driving with P2187?

You can usually drive the car short distances at low speed to a repair shop, but avoid hard acceleration and heavy loads because a severe lean condition can overheat the catalytic converter and cause misfires. If the check engine light flashes or the engine runs very rough, stop driving and have it towed.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.