Error 121 issue

I just have an issue with my raspberry pi i was wondering if anyone can help with. 

I bought the rpi 3+ a few

months back and its been alot of fun, learning lots, everything going super. Made a program with some sensor modules to measure ambient and infrared temp. All worked great prototyped at home. Then i made some proper fly leads for the sensors and went to run it and suddenly stopped working saying no sensor found error, to check cabling and config. Process of elimination i progressively checked and replaced all the wiring until i was back to my original prototype setup and even replaced the sensor modules, then checked the config and the code, which was all as it should be and hadnt been touched between working and not working. I did something that gave me a different error response and got an errno 121 line. Which I researched and as i understand is something to do with a break down in communication within the pi to the bus. At my wits end, where the config is as it should be, the code is unchanged from original and all the wiring is sure and new. What should be my next step or potential issue?