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.

Vaux Pop P1120 P1550

This 2001 Zafira (engine code Z18XE) came from another garage, who had extracted the fault codes. The codes were P1120 (Accelerator Pedal Position Sensor 1 Low Input)
and P1550 (Electronic Throttle Control Reduced Power)
They looked at the live data and pedal position sensor 1 did not respond to the pedal movement.
So it looked like a simple sensor fault, but another garage had replaced the pedal box containing the sensors and the fault remained. They had also removed and cleaned the throttle, and could not perform the throttle re-learn using their diagnostic tool.
Due to the cost of the repairs so far, they asked for a second opinion.
As always we started with a visual inspection. Then confirmed the fault, poor performance, stalling and MIL on. Then we looked for fault codes in all modules as the TCS light was also flashing.
The same codes were presented in the engine control module.
This is where system knowledge is a key ingredient to your diagnostic routine.
P1550 is stored as a result of the P1120 code, it is a symptom not a cause. The ECU has gone into a safe mode of operation. (reduced power mode).
So what could be the cause of the pedal position sensor 1 low input?
The list includes, wiring faults, sensor faults, Module faults. How to work out which one is the cause requires some information. Using a wiring diagram we are able to determine which ECU pins were supplies, earths and signals. A multi-meter could then be used to test the wiring, we elected to test at the ECU. This allows you to test the entire circuit at once.
The supplies and earths were good, and both sensors appeared to be working. One sensor output is twice the other.
This means the fault is inside the ECU. Not uncommon with this engine.
Now the customer is left with a choice, new ECU and coding at the Vauxhall dealers, or a refurbished second unit. The customer sourced his own ECU and it came with the transponder and key chips. Once fitted it required no coding, other than a throttle reset which was carried out using the original repairers equipment.