Engine sputterss on high RPM, anywhere over 2000 RPMs the engine starts stuttering and acceleration is choppy. The Fuel economy has also jumped to be about 50% more then it usually is. The battery voltage also seems to stay low even though I just replaced the battery about a week ago. Engine does have an error code of p1000 but it’s unclear what it is.

This is a ford fusion 2006 around 300000kms.

I think it could be the fuel injectors or the fuel pump. How should I diagnose this and what else could it be?