If your vehicle is showing dashboard warning message says 'engine power reduced' or 'drivetrain malfunction', here are the most likely causes and the parts you'll need to fix each one.
The throttle body controls the air entering the engine. If its internal sensors fail or the butterfly valve gets jammed with carbon buildup, the computer restricts power to prevent a runaway engine.
The gas pedal contains redundant sensors that tell the engine how much you are pressing it. If these sensors disagree with each other, the car enters limp mode to prevent unintended acceleration.
The MAF sensor measures air entering the engine. It is incredibly common for this sensor to be accidentally left unplugged or damaged after someone checks or replaces the engine air filter.
A failing ignition coil or worn spark plug can cause a severe misfire. The computer reduces power and usually flashes the check engine light to protect the catalytic converter from unburned fuel.
Common on modern direct-injected vehicles (especially European models), a failing HPFP starves the engine of fuel under heavy load, instantly triggering a drivetrain malfunction warning.
If the engine detects too much or too little turbo boost, it cuts power to prevent catastrophic engine damage. This is often caused by a blown-off intercooler pipe (especially after recent engine work) or a failing boost sensor.
VVT solenoids rely on clean engine oil pressure to adjust engine timing. Neglected oil changes can clog the solenoid screens, throwing off engine timing and forcing the car into reduced power mode.