P2187 means the engine control module (PCM) has detected a fuel mixture that is too lean (not enough fuel for the amount of air entering the cylinders) specifically while the engine is idling on Bank 1. The PCM watches the long-term fuel trim at idle; when it has to add a large amount of fuel to keep the mixture stoichiometric, this code sets. A lean idle condition is most often caused by unmetered air entering the engine or by an under-delivery of fuel.
Unmetered air entering downstream of the mass airflow (MAF) sensor leans the mixture, and the effect is largest at idle when the engine tolerates even small air errors poorly. This is the single most common cause of P2187.
A contaminated MAF under-reports incoming air mass, so the PCM injects less fuel than the engine actually needs. Symptoms are often most pronounced at idle and low RPM.
Excess air bypassing the closed throttle plate leans the idle mixture. Carbon buildup on the throttle plate edge or a sticking IAC can hold idle airflow higher than commanded.
Insufficient fuel delivery cannot meet the engine's demand at idle and part-throttle, producing a lean condition. Fuel pressure should be tested against factory specification.
A compromised PCV system admits extra metered-by-leak air into the intake, leaning the mixture most noticeably at idle and low load.
A pre-catalyst exhaust leak or a lazy/biased upstream O2 sensor can mislead the PCM into thinking the mixture is richer than it actually is, pulling fuel trims lean.
Almost never. Drivers are frequently told to replace the PCM for P2187, but that is usually incorrect. The vast majority of P2187 cases are caused by vacuum leaks, MAF sensor issues, throttle body / IAC problems, or fuel delivery faults. The PCM should be considered only after every other cause has been ruled out with real measurements.
P0171 means the system is running lean across the full operating range (idle and part-throttle and wide-open throttle). P2187 is set only when the lean condition is detected specifically at idle or low RPM, so it is a more focused fuel-trim code that points more strongly at idle-specific issues such as vacuum leaks or a sticking idle air passage.
Short trips are usually possible, but a persistent lean condition can cause engine overheating, misfires, and eventual damage to the catalytic converter. The code should be diagnosed and repaired as soon as practical.
P2187 is a lean code at idle on Bank 1; P2188 is the opposite, a rich code at idle on Bank 1. The two codes are often set in alternation when the underlying cause (commonly a vacuum leak or MAF issue) makes fuel trims swing.