If you have a check engine light and showing error code P1121, your coolant flow valve needs to be replaced. Luckily, it’s not necessarily an urgent issue (unless you need to pass state inspection asap)!
The 2004-2009 Prius utilizes a 3-way valve to direct coolant flow between the engine, the hot coolant storage tank and the heater core. It’s involved in heating the passenger cabin when necessary as well as reducing cold start emissions.
The vehicles engine control monitor (ECM) monitors the position of the valve and checks for malfunctions by monitoring the various temperature sensors. If the valve changes position and no changes or slow changes in temperature occur, it will set the P1121 code. The only way to resolve this problem is to replace the coolant flow valve and flush the system. You may have additional codes (P1151, P1150) if the coolant storage tank is malfunctioning.
Luckily, you can restrict or stop coolant flow to either of these components and it won’t have any effect on the coolant flowing through the engine block and cylinder head. The coolant flow valve is mounted along the driver’s side frame of the engine compartment, near the radiator, and can be removed by elevating the inverter and pulling the valve out from the top.