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.