So my 2002 s40 was having problems cranking so I replaced the fuel pressure regulator and that fixed the cranking problem. It starts right up now. The problem that still exists is that when I give the car full throttle the check engine light flashes then goes off. Before I got the car the check engine light was on and the previous owner took it to two mechanics who checked the codes and said that it was due to a weak catalytic converter so they turned the light off and said not to worry about it. I was just wondering why my light flashes when it's full throttle. I was thinking new spark plugs??? Not sure though. Thanks in advance guys.

 

It seems you have severe misfire.Read the codes but do not trust it when it says which cylinder misfired.I'd start with the sparkplugs but you probably should replace both coils as well unless the problem is at the electrical connector which tends to get loose and lose contact.First thing is to read the codes and go from there.

 

The 2000-2004 1.9L models always seem to set the code for cat convertor below efficiency. The first fix is to replace the lower O2 sensor. If both O2 sensors are over 100k miles then replace both lower and upper sensor. If that does not help then this little device will:
Oxygen Sensor Spacer (CEL Boss)

And you can get your codes checked for free at places like Autozone, Advance, and others.