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It is commonly known that the CD4E transmission, first found in 1994 4-cylinder Probes and used until 2008 in Mercury Mariners, is not a bullet-proof design and is "prone" to failure. There are a couple of things you can do to attempt prolonging the transmission's life. You can install a transmission cooler which cools the fluid lowering the overall temperature of the trans. You can also switch to synthetic fluids of the proper specifications. The filter is not serviceable but is intended to last the life of the transmission. It can be replaced if the unit is torn down. To the best of my knowledge TransGo is the only manufacturer of shift kits for the CD4E. A shift kit will not lengthen the life of this unit, or almost any transmission, and will likely cause premature failure.