check for water ingress on the ABS module the expansion tank is above it sometimes leaks onto the module disconnect plug clean connector as the wires to power steering also go to ABS
power sounder under wing battery dead will bring it on has its own battery link to catalog and part number
https://catcar.info/opel/?lang=en&l=bW9kZWx0eXBlPT1YMDR8fG1jb2RlPT1PUE18fGxhbmc9PUdCfHxzdD09NTB8fHN0cz09eyIxMCI6Ik1vZGVsIiwiMjAiOiJPcGVsICBUSUdSQS1CICggMjAwNCAtICkiLCIzMCI6IlAgRUxFQ1RSSUNBTCIsIjQwIjoiIDE2IEJPRFkiLCI1MCI6IjUuQU5USS1USEVGVCBXQVJOSU5HIFNZU1RFTSJ9fHxzY29kZT09UHx8c3Njb2RlPT0gMTZ8fG1uY29kZT09MjJ8fHNtbmNvZGU9PS03fHxpa2V5PT0zOQ%3D%3D
opcom seems to work ok if you can sort the software out and run on a laptop for basic code reading on apps look for torque light and EOBD factfile if you just want codes
still available from dealers they have the technolgy to change it as it has toi be programmed to the vehicle. these people are good might suggest what you can do to fix or change it http://revivecaraudio.co.uk/vauxhall/
worth checking diesel pump are you sure no airlock in fuel system take off the filler cap make sure no sign of vac lock hissing when removing after that with all diesels it is down to pipes as no fault codes could be blocked filter or bad diesel