2007 Vauxhall Astra Car Engine Light Permanently On Dashboard M Driving Will

my car has an engine light permanently on the dashboard, when I'm driving the car will start to shake and chug then the light will flash and the car will have hardly any power, my foot would be to the floor on the accelerator and still hardly pick up speed especially on hills. i went to a mechanic who ran it through a diagnostic machine and said there where a number of faults one being co2 trasnmitters theres one in the front of the engine and one in the back, i think thats what they were called, he said he thinks its the exhaust causing them to misfire, i would love another opinion to make sure im not getting ripped off.

Thanks