ok so me and my friend were putting my cat back on and after taking the test pipe off looked inside and it was white and my friend said that meant i was running lean and well my car isnt pulling as hard as it use to and i maintain it very good working at a dealership but it seems it lost power but going up hill its struggles more than it use to, so what should i do? i am chaninging the fuel filter

 

Change O2 sensor.
TPS could be taking a crap but I doubt it...