S80: Engine Problems won't go away. What else could there be?

[venomtail]

So my S80 was in a dire state. Loved the car and decided to bite the bullet and get it repaired.

• Replaced brakes
• Wheel bearing
• New tires
• Complete transmission change
• Complete engine oil change
• New alternator
• New maf sensor
• A full swirl cap kit replacement
• Actuator arm replacement
• Soot filter proffesionally cleaned
• And likely something else that I've forgotten

and after all of this, my car STILL will barely run in a sort of limp mode state with a more than average white smoke (or is it steam? Maybe from leftover chemicals used to clean the soot filter?), and has an "Engine system Service urgent" message on the dashboard. No check engine lights. My 2 OBD2 readers also says everything's fine as doesn't read any error codes.

My mechanic said that after I get the filter cleaned I should go back to him so he can do a quick 5 minute one last check just so nothing else pops up but sadly he's gonna have more work cut out for him.

Does anyone have the slightest suggestion as in what could be going wrong? What is there left to do? Could it literally just be a computer glitch that's causing all of this?

I've spent so much money on it I will never be able to sell it and even get a quater of the value. Kinda hopeless. But incase I have no other options but to sell it, what does a limp mode running S80 go for? £200? £300?