Is this related to sprintf, or just a coincidence?
somebody is going to say publish the program. It's 9 k. it uses a particular library and a particular I2C backpack. if I post it, one of our perennial fault finders will jump in. so, no, thank you.
is sprintf incompatible with I2C LCD?
The red wire can't be the problem. I'm using circular polarized DC
In my experience when you use circular polarised DC you have to be careful to get the polarity of the neutron flow the right way, try reversing it.