VW EPC Light

I often get asked why the EPC light can be illuminated without the MIL light.

EPC -electronic power control consists of the pedal position sensors, MAF sensor, ECU, and throttle valve.
A classic input - logic -output system.
The ECU (ME7) has a torque based architecture, that is to say it manages outputs based on torque demands of the driver and the vehicle systems.

One of these outputs is the throttle valve.

If a fault occurs in the throttle valve, ECU or wiring, the ECU defaults into a limp home mode.
The Electronic Power Control (EPC) light will illuminate.

Depending on the fault one of three modes can be selected.

Mode 1
If one throttle position sensor fails, torque output is reduced.

The customer may complain of a lack of power.
This mode requires one functional throttle position sensor with an intact signal, and
engine load readings from the Mass Air Flow (MAF) sensor.

Mode 2
If the throttle valve actuator fails or malfunctions, the voltage to it is shut off and the
throttle valve defaults to its emergency running position. Torque requests are executed via ignition and charge pressure regulation. As a result, the engine will show very little
response to the throttle. This mode requires signals from both throttle position sensors.

Mode 3
If the throttle valve position sensor fails (implausible signals), the voltage to it is shut off and the throttle valve goes to its emergency running position. The engine speed is limited to 1200
RPM by restricting the fuel injector pulse width and ignition timing.

Therefore a technician can predict the fault from observing which limp home mode has been selected. As the engine continues to run correctly, there is no excessive emissions and therefore no MIL.