2003 Chevrolet Impala gas

A flashing Check Engine light indicates the most serious of conditions and means you're supposed to stop the engine right away. Too much unburned fuel is going into the exhaust system and is going to overheat and damage the catalytic converter. The place to start is by having the diagnostic fault codes read and recorded. The people at many auto parts stores will do that for you for free, but be aware those codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition.

If the catalytic converter has already been overheated, the catalyst may have melted. That will plug the exhaust system and make acceleration nearly impossible. One clue you can look for is the exhaust will sound like a steady hiss instead of the normal "putt putt" at the tail pipe. The converter has to be replaced to fix that, but you also have to diagnose the cause of the too much fuel and fix that. Misfiring spark plugs is a common cause of misfires, especially at the mileage you listed.