both warm and cold how much is it for a new sensor you reckon? and to jonhvk7 yea still got the keypad system.

May also be a problem with the fuel pump/ fuel pressure. It may prime OK to start up but when the engine wants to idle the fuel pressure drops off and it stalls. I a similar problem recently and had to get the fuel pump replaced.

Air flow meter for an '04 wrx is approximately $900.00 new. So you want to be sure that's the issue. Although i've seen them on Ebay for a lot less. But once again, there's no point spending money on a part if its not the problem. My money is on alarm issue. You need to take it to a workshop for diagnosis.

Don't bother buying a second hand AFM if that is the issue, there is no guarantee the 2nd hand one isn't faulty and most place don't take electronic parts back.