Remove the transmission oil pan. CAUTION the pan has no oil drain meaning that if you remove the bolts and hit the pan five quarts of fluid will come out. There is no gasket on the pan it is sealed with silicone.
Most likely you still have the old silicone gasket stuck in the bore of the valve body. To remove it, use a 3/8" extension bar from your socket set. Insert it into the bore and tilt it. The gasket will come right out.
Repairing intake gasket not preferred, you can use red silicone to seal it, but to do so, you must remove the intake, in which case, you may as well replace the gasket.
Just scrape it off with a scraper or a knife but be careful not to gouge the metal. If someone has made a silicone gasket instead of a fibre gasket, you might have to use a sharp edged knife to cut it off, and then scrape it to remove the rest.
Drain off the oil first, then remove the sump bolts. Hey presto, The gasket it there, remove and refit new one, or bodge it with silicone sealant.
Remove the transmission oil pan. You'll need a new filter and gasket. After you remove the pan, clean the pan, replace the filter and put the pan back in place using the new gasket. Then fill the transmission
I have had the same problem. What i did to fix it is to remove the two bolts from the center and ensure to remove all old gasket material. on the left side(passenger side) there is a half circle rubber that tends to dry up. i replaced the gasket and applied a GENEROUS coating of heat resistant silicone to all gasket surfaces including the rubber and ensure that the rubber bolt seals get a good coat as well. WAIT AT LEAST an HOUR before starting to allow the silicone to set up some.
Drain oil. Remove pan and catch remaining old oil. Remove old gasket material off mating surfaces. Ensure surfaces are properly cleaned and coat each surface with black silicone (or equivilant). Install pan with gasket in proper position. Properly torque bolts*. *If you are not familiar with how to torque bolts, and/or you do not have torque wrench, you can tighten bolts as if you were making them as tight as you can with a screwdriver (without leverage). Do not overtighten because it can damage gasket.
Remove transmission oil pan for access to filter Change fluid, filter, and gasket
The oil pan gasket on a 1999 Honda Civic is found under the oil pan. Lower the down pipe and drain an oil within the engine. Lower the oil pan and remove the old gasket plus and silicone residue from the mating surface. Install the new gasket using silicone sealer to ensure leak proof replacement.
Remove transmission oil pan for acces to filter Replace fluid, filter, and gasket
Remove the transmission oil pan for access to filter Replace fluid, filter and gasket