Hybrid battery problem

Hi, yesterday my car suddenly while driving gave me warning (check hybrid system) and all light on dash turned on, but did not stop. I shut off my car and then restart it but still showed same message. By chance I was near to mechanic garage so he immediately connect it to computer and showed faulty pack battery and he reset the computer car and every thing return to normal. He told me some cells in the battery are falling and weak and advised me to replace it.
Next day I check it again with computer and did not show any codes.
My question is, what is your opinion and advice and how to check the battery function with out taking it out of car? If cells are weak do you suggest I replace battery or replace the cells only?
Thanks.