My V40 wasn't starting unless the gas pedal was depressed, and then it would die if you let up on the gas. My son and I took the throttle body off and cleaned it and cleaned out the vacuum line connections. After reassembly I noticed it would idle if I left one of the vacuum hoses disconnected, but die when I hooked it back up. I took off the idle air valve and cleaned it, but it still didn't work. The next morning I took another look at it and gently pried the idle air valve open. It popped open and I cleaned the gum off the valve seat. I worked it back and forth and got it moving freely and when reassembled it worked!! Apparently, the only thing wrong was the idle air valve was gummed up and stuck closed!!

Roughly, where is the throttle body located? I want to clean the tb and idle control motor. My 02 S40 is new to me with 40K and it's not obvious to me where the throttle body is located.
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If you have a hot wire air mass sensor (used on turbo engines), be extremely careful not to damage it or to spray anything on the hot wire. They are around $1000 to replace!

From memory you have to take the plastic cover off the engine to get to any valves/controls.

good observation. Happened to me on a non-Volvo the other way. Gummed valve was stuck open and engine would not idle below 1600 rpm or so.