Q: Check Engine light comes on and goes off 2005 Volvo S40

Hi there - without knowing whether the fuel pressure problem is "pressure high" or "pressure low", it would be speculating as to what component may be malfunctioning. Get the problem codes turning the Check Engine light on. For the fuel pump to be the problem, the symptom would pretty-much have to be low fuel pressure. Low fuel pressure could be caused by a worn fuel pump, clogged fuel filter, or inadequate electrical power to the pump - possibly from a fuel pump relay with burned contacts. Get the actual codes which are causing the Check Engine light - then you aren’t guessing as to the problem. I recommend a fuel system inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, and give you an accurate assessment of damage and cost estimate for repairs.