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Excess fuel pressure forces injectors to overfuel, overwhelming PCM corrections. Negative trims and rich O2 confirm enrichment. Vacuum leaks would cause lean conditions, dirty injectors cause restriction, and MAF sensors underreport airflow leading to lean, not rich. ASE emphasizes fuel pressure testing in diagnosing systemic richness.

If all injectors show low pressure drop, they are not receiving sufficient fuel volume despite proper pressure at idle. This points to restriction in supply, such as a partially clogged filter or line. MAP, ignition, and exhaust faults would not cause universal low injector balance. ASE stresses combining pressure and balance tests to confirm systemic versus localized injector faults.

A P0171 lean condition is set when oxygen sensors detect excessive oxygen in the exhaust. The fact that short-term fuel trim is high at idle but normalizes under load suggests unmetered air entering the intake system at idle, such as through a vacuum leak. At higher RPM, the additional airflow diminishes the relative effect of the leak, normalizing trims. A restricted fuel filter or weak fuel pump would cause lean conditions under load, not idle. A faulty oxygen sensor would not produce consistent trim patterns across operating conditions. ASE’s diagnostic objectives stress analyzing fuel trim data in real time to pinpoint root causes. Understanding how trim readings change under different loads helps separate fuel delivery issues from air leaks, a critical competency for the L1 exam.

If the PCM commands low IAC and idle airflow is insufficient, the valve may be sticking shut. Fuel, TPS, or injectors would affect trims or cause misfires, not airflow mismatch. ASE emphasizes comparing commanded IAC to actual airflow to confirm diagnosis.

Small pressure drops in injector balance testing indicate restricted injectors that fail to deliver proper fuel. This aligns with lean trims for that bank. MAP sensors and exhaust restrictions affect both banks. O2 sensors report mixture, but the root cause is injector restriction. ASE emphasizes injector balance testing as a definitive way to identify cylinder- or bank-specific fueling issues.