All controls are working properly. Lower oven works perfectly. Upper oven
fan for convection blows. Control panel shows oven is preheating however
it is not heating at all. No error codes. What part needs replacing? Thank you!
I am assuming that the unit stays in preheat and never comes ready.
Here are your parts
Replacement parts for WHIRLPOOL GBD307PDB09 30" BUILT-IN ELECTRIC DOUBLE OVEN CONVECTION-UPPER SELF-CLEAN (GOLD LINE) | AppliancePartsPros.com
It could be that the control board is toast but more likely is that the thermal fuse (Item 37 in Section 2) for the upper oven has blown.
If you click on the parts picture a new page will open with a Q&A section.
Remove power from the unit and one wire from the fuse and measure it with a meter, should be 0 ohms.
See the attachment for the tech sheet.
Also here is a manual which should help.
KR-30 Whirlpool Electric Built-In Double Oven Service Manual - ApplianceDigest.com
You have to join the site to download the manual but it is free and this is a very good site for manuals and other info for the appliance repair DIYer.
RBD275.pdf (517.1 KB)