I have a 2004 Volvo s40. The car wasn't starting unless I pushed on the gas. I took it to the dealership a month ago and they said that the spark plugs were the cause. They replaced those and the car seemed to start OK after that until recently. Now it won't start unless I push on the gas again. There are no engine codes. I am going to replace the fuel pressure regulator. Where is this located and how do I get to it?

 

Is this the 2004.5 or the early 2004?

Try in here:
https://volvoforums.com/forum/volvo-s40-11/frequently-asked-questions-34712/

 

Its the Idle Control Valve which when gets dirty overtime it will be sticking or stuck.Take it off and clean it with throttle cleaner.Like lyonman said which car do you have as in 2004 there were 2 different s40.In case you have the earlier here is a link how to do it.
http://s40concepts.net/forum/index.p...83fca6fcb96091

 

The links in the FAQ thread as well...