2001 Toyota Corolla check engine light is on

Hai, I own a 2001 toyota corrola S model car, automatic transmission, 107,000miles on it. I had "check engine light" on and it was diagnosed to have a problem with catalytic converter six months back. I had it replaced. That time I had been driving up and down slopes for a while and then the check engine had come up, but the car mechanic said the problem was with catalytic converter and I replaced it. Everything went fine for next two months and then again I had my check engine lights coming up, this time the car mechanic said the catalytic converter has waranty and so we can replace once again with a newer one. He also said that probably the previous car owners hadnt maintained well (had not changed engine oils in time) so he said he thinks there are problems in the engine too and he is not sure how long the car will run. I was frustrated since it is not even a year since I had got my car. After changing the catalytic converter for the second time, everything seemed fine, untill today. Today I was driving up and down (very steep) slopes and the check engine light has come up. Also it is time for me to change engine oil, I dont what the real problem is. Is it driving my car in steep slopes a problem or is it indication for changing my engine oil or is it a serious problem with my car engine, I will be glad if someone can suggest me regarding my car problem. Thanks in advance.