Defrost/ heater

Hello all, I have a 2004 Camry XLE that I’m having trouble figuring out. I have good both under dash and defrost for a while. After a bit of driving the defrost goes cold and the heat under dash is good with the flow split between dash and defrost. If I put the temp on max ( doesn’t matter if it’s in auto or not) I have good defrost and heat once it’s turned back down the defrost slowly goes cold again but the under dash has good heat I did a diagnostic on the control panel and I’m getting a code 21 for the sunlight sensor but it’s gone when the the sun is out. Any ideas? Thanks