One problem related to manifold/header/muffler/tail pipe has been reported for the 2012 Chevrolet Impala. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2012 Chevrolet Impala based on all problems reported for the 2012 Impala.
About 2 months ago the engine starting dieseling (the engine will still have combustion after the ignition was turned off). During a trip on city streets the engine suddenly stopped when driving. I was able to put the car in neutral and restart the engine. About 15 minutes later the engine stopped again this time while stopped on a traffic light. I could not restart the car. I had it towed to the autolab in Plymouth mi where they diagnosed it with engine flooding. You could smell the unburned gasoline on the tail pipe. However no error codes were reported. After a few days in the facility and without the even duplicated I had the car back. Couple weeks later it started dieseling again so I went back to the autolab. This time they were able to diagnose the injection pump as being faulty and the O2 sensor not being able to control the fueling. They replaced the injection pump and since then the vehicle is running without problem. The lack of error reporting by the obd and the sudden engine stop while the car was in motion makes me feel I was lucky. I usually drive 30 mile each way to my work place. If the engine stops on the highway it will cause an accident as I am not able to use the brakes (power brakes!) or the steering wheel (power steering) properly to try to get away from the traffic behind by going to the shoulder.