How do I diagnose and fix a starting problem in my Vauxhall Vivaro?

My Vauxhall Vivaro won't start. I've tried replacing the battery and starter, but it still won't start. What other steps can I take to diagnose and fix this issue?

First, I would recommend checking all of the fuses to make sure none are blown. If any are blown, replace them and try starting the vehicle again. If that doesn't work, you may want to check the spark plugs and ignition coils. These can often be the cause of starting issues.

Another thing to check is the fuel pump. If it's not functioning properly, it can prevent the engine from starting. You may also want to check the fuel filter and make sure it's not clogged.

It's also possible that there could be an issue with the engine control unit (ECU). If you have access to an OBD-II scanner, you can plug it into your vehicle and check for any error codes. These can often point you in the right direction to diagnose the issue.