WL Statesman 5.7 error message and reduced power

[Porci]

WL Statesman 5.7L 2004 has had a message come up on the dash - Oil change required and then goes into reduced power mode. Carried out the reset after changing oil and filter, but still came up. I was told it was an oil pressure sensor problem and so replaced the sensor and reset. Message came up again, so took it to auto electrician to clear all codes. Error message still came up. Took it back to the auto electrician to check what message is coming up on his diagnostic machine. Only message appearing on his machine is - "App sensor 1 circuit".
Any advice or help would be very much appreciated - thank you.