Hello I have a 09 ss Im having an issue with. When I get under 1/2 tank I get kind of a power surging issue which kicks on engine reduced power, service esc and service traction. I was thinking it might be the fuel pump but there is a P0121 in stored codes but engine light is not on. Any body here had this issue before, Thanks!

 

The P0121 code directs you to the most likely cause , read in

https://www.obd-codes.com/p0121

 

Stored as in "history" or "pending".
Could it be more like hard shifting?
most likely is you need a new accelerator pedal. Possibly a buggered wire on the accel pedal or .MAF.