when i installed my high flow cat i had to take out the o2 sensors from my stock cat. the o2 was in there tight, i just unplugged the wires for the o2 sensors (the wire for the secondary o2 sensor i just cut and left out..dont need..no emissions where i live..but u can cut the wires then solder them back together afterwards). then i grabbed my army surplus oxy-acetalene (sp?) torch and warmed up the cat where the o2 went in..yeah wd40 didnt work for me..maybe it will for u tho..it ended up being real easy to take out the o2 sensor when the metal was heated up.