2000 Camry runs rough but no CEL
 

Hello, new here with a question about my mom's 2000 Camry 4 cylinder.
A while back while I was driving it the car started bucking at idle and would smooth out when giving it gas. It did not throw a Check engine light.
She called me today and said it did the same thing except while driving down the road. She said it cut off at the stop light and ran very rough all the way home. About 5 miles.
I had her drive it over and I put the OBDII code reader on it and it has no codes stored and no check engine light.
I thought just about anything that went wrong on one of this computerized engines would turn on the CEL light and throw a code to be stored until retrieved? ANy ideas why it didn't
Thanks
clint