Kitchen Aid KSRB25FNSS00 top shelf warmer?

Brand: kitchen aid
Model Number: KSRB25FNSS00 Fridge
Main Symptom: top shelf warmer
What happens & when: top shelves is 50 degrees in the fridge, next lower shelf is 45, next lower 35 and bottom two drawers are 30 degrees. The freezer is a stable 0 degrees.

what part needs to be replaced to ensure the temp. in fridge is even through-out

Based on your description the issue is likely a damper or damper thermistor. If these are not the issue check air flow from the evaporator fan.
https://www.appliancepartspros.com/whirlpool-thermistor-wp2188819-ap6006067.html
https://www.appliancepartspros.com/whirlpool-air-diffuser-assy-wp2216112-ap6006658.html

Are there any test I can run to better pinpoint whether the issue is the damper, damper thermistor or evaporator fan?

Other than the fact that the temperature at the top of the fridge is warmer, there is ice build up on the cabinet louver and ice build up at the bottom of the freezer.

Are there any diagnostics that you could refer me to, like a video or any guidance?

Or which part would you recommend purchasing first?

Thank you for further guidance.

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