What could be the source of this overheating?

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. I'm sorry to hear you're having trouble with your vehicle. It sounds like you've been very thorough with replacing the cooling system components in your XJ L. The P0126 code indicates a coolant temperature that is too low. One of the first things you could try is using a diagnostic tool to reset all of the computer's learned values and then verify proper temperature sensor operation. If the temperature sensors are good, the overheating might be due to a partially clogged radiator, even after flushing, which could prevent proper coolant flow. Another possibility could be an issue with the catalytic converter, especially with a P0420 code present. This could cause excessive heat buildup, especially if it's getting clogged. Additionally, you may have an intermittent issue with electrical components such as the thermostatic switch in the radiator or the radiator cooling fan module. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!