Help with trans code
 

i have a 00 galant & it just threw a cel for Output Speed Sensor (p0720). its had a code for cat warm up, p0421 i think, but this one just popped up. Only thing thats changed is the addition of some atf+4 to top it off since i noticed it was low. SP-III is impossible to get around here outside of the dealer & i looked online & a few sites said that +4 is fine for this tranny. Do you guys think maybe some spillage got on the sensor or that the sensor actually went bad? & how will this affect drivability? there was some issue on the highway when i slightly cut throttle when doing around 65-70 at 2.5k RPM but a buddy of mine said it could also be because the cat is bad & choking at high speed. any help would be great