I have a 2004 Mitsubishi Galant engine type:2.4L SOHC and my service engine light is on and when the codes are read it says that the warm up cat is below efficiency level what does that mean and how can it be fixed I will try to get the code number asap

If the Powertrain Control Module (PCM) detects that the efficiency of the converter is below a certain predetermined point, that light and the code, either P0421 for cylinder's 1 & 4, or P0431 for # 2 & 3, to alert you to potentially excessive emissions.

What is your mileage? You should check with your local dealership regarding possible coverage for a faulty converter. Most major emission items are covered far longer than the original factory warranty.

Should I replace my o2 sensors to see if that works?

should i replace my o2 sensors to see if that works?

You could... I wouldn't. Not till I know more.
That would be far less expensive, and quicker, than replacing a converter... if that fixes the problem.
The sensors may be old and needing replacement due to slow contamination. But, I just don't think it will fix what's broke.

Why? Because the oxygen sensors are just that: sensors. If they "sense" something isn't what is should be, they set a code.
But, they are reporting the condition of the exhaust gasses, NOT the condition of the sensors. If they indicate lean conditions in the exhaust, chances are that there may actually be lean conditions.

But--- I still say you should check with your local dealer or repai shop. Being as both converter codes are setting simultaneously, there may be only a simple software update that adjusts the threshold criteria. Either way, they will have the diagnostic scan tool that, in the hands of a skilled automotive technician, can pin-point the problem and fix only what needs fixed.

How much would it be to replace a warm up cat?

how much would it be to replace a warm up cat?

That can vary significantly from one place to another.
Have you contacted a local dealer or service facility to see if they have any info for you? It's usually just a phone call away.