The rear heater core should be above the radiator fill cap in its installed place which may cause less coolant in the engine and create air bubbles in the heater, resulting in engine overheating and ineffective rear heater performance. Before installing the heater hoses, pre-fill the rear heater core with the correct coolant, fit the heater core into the housing for the rear A/C unit and install the side housing into the vehicle to avoid these problems. Take care not to spill the coolant on the carpet and stick a funnel into the top hose connection on the heater core to ensure the coolant comes out of the bottom. After you clean up the spill, put the heater hoses back in with the engine cooling system fully refilled. If the rear heater core was emptied and not filled before, you should fill the engine cooling system, let the engine run until the
Thermostat is open and cool, check and top off the coolant level, then start another thermal cycle two more times before you check the rear heater. After the engine is started and idled to operating temperature with the front heater off and the rear heater and rear
Blower Motor at highest speed, you should check the amount of air coming from the rear. The temperature of air exiting the rear heater at the back of the right C-pillar should be measured and it is expected to fall between 57 °C to 63 °C (135 °F to 145 °F). An unsuccessful check could mean there is air in the rear heater core or that the plumbing system is blocked.