S40 engine power loss

Sounds like it could be a defective throttle valve. Has the fuel efficiency dropped too? When the check engine light is on and power's reduced, the car's probably in limp mode to protect the engine.

Limp mode, got it. Fuel efficiency has taken a hit, yeah. Could it be the throttle body itself, or something else entirely? How would I check it?

Given the age and symptoms, it's likely the accelerator pedal module or the electronics inside are defective. I'd recommend having a workshop check it out; a faulty throttle valve can cause further issues. Repair cost should be around 265€.

Thanks! Sounds like I was on the wrong track. Took it to the shop, and you nailed it – throttle valve issue. They're fixing it now.