If your Ford Focus has an automatic transmission, you need to change the transmission fluid filter every two and a half years or 30,000 miles--the same time that you need to change the fluid itself. The filter is bolted in place on the transmission within the fluid pan on 2003 models. This model does not include a drain plug for the transmission fluid, so you need to drain the fluid as you remove the pan.
Raise the car with the floor jack and support it on jack stands. Place a six-quart drain pan directly under the transmission.
Remove all of the bolts for the transmission fluid pan except the three in the front of the pan near the driver's side. Loosen those bolts with the wrench by two full turns without removing them.
Separate the transmission pan from the transmission using a plastic scraper to break the sealant. Let the old transmission fluid drain into the drain pan, then remove the remaining three bolts and disconnect the pan.
Disconnect the electrical connector for the fluid temperature sensor on the transmission.
Remove the mounting bolts for the transmission filter with your wrench and remove the filter. Discard it with its seal.
Scrape the gasket material off the transmission pan with the scraper and then clean all traces of old fluid from the pan using solvent and compressed air to dry it; don't wipe the pan with a rag.
Connect a new seal to the replacement filter and bolt the filter in place on the transmission with the wrench.
Apply RTV sealant to the transmission pan's flange.
Press the pan in place on the transmission and apply the bolts. Tighten the bolts with your wrench in a criss-cross pattern, starting with the middle bolts and then working your way one bolt at a time at different ends of the pan, installing the four corner bolts last.
Lower the car off the jack stands and let the RTV sealant cure; the amount of time needed will be on the sealant's container.
Fill the transmission with fresh Mercon V transmission fluid (specifically made by Ford) through the filler tube near the battery--add a little at a time, using a funnel and allowing it to drain into the transmission, and check the level with the dipstick.
