Originally Posted by donbrew

As I said; A bad ref for the TPS and APP causes a fault, so the computer reduces power so you can't blow the motor up.

The computer can only control the fuel supply, that's why fuel trims are so hard to understand.

For instance; if you have a cracked flex pipe the #1 O2 sensor can read lean so the computer will increase the injector duty cycle causing a rich condition. Or, it can read rich so it reduces the cycle causing a lean condition. Either of those would cause the computer to go into reduced power mode if the conditions are right. Sometimes a crack will admit air (too much Oxygen) into the system making the sensor read rich sometimes exhaust gasses escape before they get to the sensor (not enough Oxygen) causing a lean reading.

Any sensor can short internally causing the entire ref circuit to go bad; engine power reduced mode. In a computer driven car everything that you used to call a switch is now a sensor. There are 2 ref circuits, the APP and TPS use both of them, other sensors use one or the other.

Many faults can cause a reduced power mode, the message tells you that it is activated, not that it is sensing a reduction in power.

Much more complicated than that, but for us amateurs enough.