I took possession of a new Whirlpool side-by-side 7/1/23. 30 days later I noticed the temp rising in both the rif & freezer compartments. I called for service, & when he came, he showed me the back wall of the freezer & it was covered with ice. He removed the back wall panel, & underneath was also a wall of ice. He speculated it was a defective thermister, which senses when to start the defrost cycle. The defrost cycle didn't start, so the ice continued to build. Eventually, it locked up a fan, I believe it was. Essentially, the ice buildup eventually disabled the ability of the rif to cool down properly. He checked the diagnostic codes, & it did call out the thermister as being defective, confirming his suspicion. He spent most of the visit chipping out ice! Once the ice was removed, he put the panel back in, & started it back up. It once again worked properly, with the ice removed, altho over the long term, would repeat the failure if the part wasn't replaced. He was gone for ~ 2 weeks, & the rif cooled fine during that time. When he came back, there was some ice buildup on the coils, but not enough to interfere with the function. He used a heat gun to remove what ice had accumulated, replaced the thermister, & I've had no problems. since.