I have a 2008 impala LS with the 3.9L engine in it.when driving down the road it has a surge to it between 1400 and 2200 rpm.I've replaced every sensor on the engine and did a complete tune up,changed coil,checked all the wiring and still having the issue.I keep getting a P0420 engine code which is saying an emission code,possibly catalytic converter but its not clogged up or over heating the converter.I've even changed the O2 sensors.I've had it to 2 dealerships and they can't find the problem either.anybody out there got a clue as to what this might be causing it.

A P0420 just means the cat isn't working to its expected efficacy. There is a O2 sensor before and after the cat, the PCM compares the readings of the two sensors and makes sure the cat is working properly. As far as the surge problem it'* really hard to diagnose a drivability problem such as this without a scanner that reads live data. Right off hand most surge issues are caused by a bad mass air flow sensor but you say all the sensors have been replaced. It could be a vacuum leak around the intake. Just my inshallah thoughts.

they put those codes there for a reason. if it says the cat is bad its bad. or the o2 are bad or there is an exhaust leak messing up the sensors readings.