This could be a number of things but the most likely cause is the rear blend door is not operating.
Here is a guide on how to replace a front actuator so that it will help with some info but clearly this is going to be in the rear for you:
Below you will find the process from the manual on how to replace this and perform a calibration.
This is assuming that the front is operating properly at proper temps. There is a chance that the system being weak can cause the front to work for a short time after the rear stops. If you want to, you can check the pressures just to be sure the system is not low.
Here is a guide that will help with this:
Please run through this info and let us know if you have other questions. Thanks
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Saturday, July 10th, 2021 AT 3:27 PM