BMW 98 528i OBD2 Fault Codes P1423, P1421, and P0420 Explained - What They Mean and How to Fix

I recently encountered OBD2 fault codes P1423, P1421, and P0420 on my 1998 BMW 528i with a manual transmission. The vehicle has 153,000 miles and all maintenance was performed by the previous owner at an authorized dealer. I can temporarily clear these codes using a diagnostic tool, but I'm unsure of their exact meanings or what actions are required to resolve them. I'd like to understand what each code indicates—especially P1423, which doesn't have a clear name in my research—and whether the issues are serious enough to require professional repair or if they can be addressed with DIY fixes. I'm looking for accurate technical explanations and guidance before deciding to take it to a mechanic or attempt repairs myself. Any insights from owners of similar BMW 528i models or experts on these specific OBD codes would be greatly appreciated.