Chevrolet Repair: Need some suggestion about the codes

Question
I have a Chevy Cobalt 2007 and my engine light is still coming on when i drive the car. I went to show this to a mechanic.He found a code P 128 and P 449 and he also gave me a statement for to fix that problem in $375 labor
What is your suggestion???
Please tell me or should i do this myself and
Is the quote the mechanic gave me is OK
Please reply me with some advice
Thank you

Answer
po 449: Check the the label under the hood for the routing of the evap system, trace all the lines and verify that none are cracked, split or broken. any leaks will set off this code. So do this.If nothing is found ,then you would be wise to have that mechanic perform the repair considering the part's cost. It's up to him to be sure that that is the problem ,so if he is mistaken and that part was not what you needed, you aren't responsible for covering it's cost.you're only responsible for what the estimate you signed totals. So actually that's irrelevant, as long as it's fixed at the end of the day. But if you find a leak and repair it, take it to a orielly autoparts or similar, where they will clear codes and check to see if any codes still exist, for free. If po 449 code does not come back ,you have solved the problem, and should then tackle the project described in the following paragraph. If that code shows back up, then stop and have the mechanic who gave that estimate perform all the repairs which included code po128. That is not an unreasonable estimate actually.          po128This means that the engine’s PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. You would do well to replace the thermostat and change out the system's antifreeze, Basically a cooling system seasonal service, something you should have done on a yearly basis anyway, killing hopefully two birds with one stone.So to get rid of po128 try changing out thermostat with a quality new one.Remember, spring side goes in block.Get specific coolant required for your car, and be sure to purge air from system when filling.Running till thermostat opens and allows trapped air to escape.Burping;some engines have a bleed screw, 7 or8mm, in the cooling jacket at high point of circulation, that you crack open to purge trapped air too.Be sure you have the system filled all the way up without trapped air.  Good luck