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A high positive LTFT means the ECM is consistently adding fuel over time to correct a lean air-fuel mixture. At warm idle, one of the most common reasons for a lean condition is unmetered air entering the engine, such as from a vacuum leak in the intake manifold. A failed oxygen sensor heater circuit can delay oxygen sensor readiness (especially after cold start) but does not typically create a stable, long-term lean condition by itself. A stuck closed EGR valve more commonly increases combustion temperatures and can contribute to spark knock rather than causing a persistent lean condition at idle. A malfunctioning throttle position sensor primarily affects load/acceleration calculations and driveability symptoms and is less directly associated with a stable +25% LTFT at warm idle.

A P0420 indicates the PCM has determined that catalyst oxygen-storage/efficiency is not meeting the expected threshold. The most appropriate first step is to confirm the conditions under which the code set and evaluate related data with a scan tool. Reviewing freeze-frame and readiness/monitor status, checking fuel trims, and comparing the upstream and downstream O2 sensor patterns can help determine whether the catalyst is truly inefficient or whether another issue (such as abnormal fuel control, an exhaust leak, or sensor response problems) is influencing the monitor. The choice about replacing the catalytic converter is premature without confirming the fault. The compression test is not a direct first step for confirming a catalyst-efficiency monitor failure. Coolant level and thermostat checks are not the most direct initial confirmation step for a P0420.

Poor acceleration under load is most directly confirmed by insufficient fuel pressure/volume reaching the injectors. The scan data shows a large gap between the desired rail pressure and the measured rail pressure during hard acceleration, so the value to address first is the parameter reporting the measured rail pressure. The fuel pump command being very high indicates the system is already requesting maximum pump output, which supports (but does not by itself prove) a fuel delivery problem. The desired rail pressure is only the target, not the achieved result. Ignition status being ON is expected and does not help isolate the cause of the acceleration loss.