my volvo s40 2005 will not start and has message key error any idea what may be causing this problem . I have fitted a new battery and have changed the battery in the key fob but no change
Almost always the problem with the FOB is that the conductive carbon coating on the keypad wears out. If you replaced the battery and it still doesn't work, try this.
Take the fob apart, and you'll see a circuit board under the rubber key pad. The circuit board has a mesh under the rubber contact for unlock, start, etc. Take a screw, or a rolled up section of foil, and touch the mesh contacts under the one that doesn't work - if the car starts (unlocks) then the rubber keypad is worn out and needs to be replaced.
Just go on Amazon and search for '2005 Volvo FOB repair kit' or similar, they are really cheap - $10 - $20, then replace the old keypad and you should be back in business. No need to re-key. I just did this on my 2009 Chevy HHR and it works great. Of course the dealer was trying to get us to re-key for $400!