An initial route for throttle, airflow, boost, EGR, and timing issues when the vehicle enters limp mode.
A throttle-limited engine, weak boost, or airflow plausibility problem belongs in the reduced-power bucket. A vehicle stuck in one gear with harsh shifts belongs in the transmission limp-mode bucket even if the driver also says reduced power.
Reduced power can involve powertrain plus electronic throttle or transmission modules, so do not stop at one generic PCM code if the scan tool can reach more.
MAF, throttle position, cam timing, and boost readings should agree with load and RPM. If they do not, the ECU may be limiting torque on purpose rather than reacting to a true mechanical power loss.
Connector fitment, low oil, carbon buildup, air leaks, and low system voltage can all mimic failed sensors or force protective derate behavior.