I have no personal experience what so ever on the code but from google searches seems like they say it could be a faulty thermostat. Here's a copy & paste from what i've found.

C/P

P0599

P0599 - Thermostat Heater Control Circuit High

Possible causes
- Open or short thermostat heather control circuit
- Poor thermostat heather control circuit connection
- Failed Engine Cooling Thermostat

Tech notes
If there are not coolant system leaks, replaced the thermostat to fix the problem.

When is the code detected?
The P0598 is triggered when there is an open or short thermostat heather control circuit.

End of C/P


I'm sure someone else can give you more insight on the matter. GL!