hi i have a dumb question , i need to know when is it time to change the transmission fliud in my 02 corolla ? the last time i got rip off , when i brought my car to a gas station to do this. they told me a transmission flush will be 89.00. so i said ok and did it there. but later found out that the gas station only poured the old transmission fluild out , and just poured the new fluid in right after. it took them only about 8-10mins to finish. then about 3 weeks later i brought my car into another station for a check up on something else . what they told me was i need to change my transmission fluid because its dirty. so this time im trying to get more info before doing this again. thanks for any info.