Figure 6: 2008 Toyota Prius ICE Coolant Flow. So, what is " Preheat " on a Prius? " Preheat " warms the cylinder head before start up to reduce HC emissions. When the engine has been at operating temperature, hot water is stored in the CHS Tank (think " thermos " ). " Preheat " is a system Toyota used to reduce emissions after cold start, sometime after a hot engine shut down. Operation of the preheat mode is determined by a timer in the ECM. If " soak " time is within parameters, the system will operate. When the " Preheat " system is commanded (the cylinder head is cold and the water in the CHS Tank is hot), the water valve moves to direct water from the CHS Tank into the cylinder head, through what is usually the water outlet to the heater core (reverse flow). The amount of water pumped during the " Preheat " cycle is determined by cylinder head temperature. March 2022 5 I have reviewed this system because of the difficulties encountered during service. A high percentage of Gen II Prius' with the CHS Tank are old enough to develop cooling system issues. From head gaskets to rotten spring-type hose clamps in the " Rust Belt, " the actual service is not much different from regular ICE vehicle service. Problems arise after the repair during the refill procedure. Note: when draining a Prius cooling system that is equipped with a CHS Tank, remove the inner fender/ splash shield fasteners at the LF corner of the vehicle. Pulling the shield back will expose the tank and the coolant drain valve in the lower hose. You will also see the tank temperature sensor and pump assembly. See Figures 7, 8. MACS Service Reports Mitchell Pro-Demand