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Ron Z.
Ron Z., - Toyota Tech -
Category: Toyota
Satisfied Customers: 19027
Experience:  18+yrs experience. State Inspector and Toyota Diagnostics
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I have a 2007 prius. engine light came on 3 weeks ago

Customer Question

Hi, I have a 2007 prius. engine light came on 3 weeks ago for about 5 hrs then went out. Then 2 wks ago. Then yesterday, again light is out. auto zone test showed P1121 code. I just noticed my radiator overflow tank is below the low mark, need to fill. also there have been rat droppings on the engine. neighborhood issue. 60,000 miles on the car. clean, runs great. taking a trip. ? to go without dealing with this at this time.
JA: Sometimes things that you think will be really complicated end up being easy to fix with Toyota. The Toyota Mechanic I'm going to connect you with knows all the tricks and shortcuts. Are you hoping to fix this yourself?
Customer: yes
JA: Great! What have you tried so far?
Customer: nothing
JA: Is there anything else important you think the Toyota Mechanic should know about your Toyota?
Customer: no
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Submitted: 1 year ago.
Category: Toyota
Expert:  Ron Z. replied 1 year ago.
The P1121 pointing to a problem with the Coolant Flow Control Valve aka "Water Valve"). This system uses an electric pump to supply hot coolant stored in the heat storage tank into the cylinder head of the engine, in order to optimize engine starting combustion and reduce the amount of unburned gas that is discharged while the engine is started. Before the engine starts, the ECM operates the electric water pump to direct the hot coolant in the heat storage tank into the engine, in order to heat the cylinder head (this process is called "preheat mode"). The duration of the operation of the electric water pump is variable, depending on the temperature of the cylinder head. During the normal operation of the engine, the water valve opens the passage between the cylinder head and the heater and closes the passage between the cylinder head and the tank. During preheat mode in which the cylinder head is heated, the water valve opens the passage between the tank and the cylinder head, in order to allow the coolant to flow from the tank to the cylinder head. At this time, in order to warm up the intake port quickly before the engine is started, the coolant flows in the reverse direction. The water valve consists of a water valve, valve position sensor, and valve control motor. So the P1121 can be set when the water valve fails (or one if it's internal components) or when there is a clog in the cooling system. So, first thing to do here is to drain and flush the cooling system and make sure there are no clogs. If the trouble persists after the cooling system flush, then more commonly, the water valve will need to be replaced. For an image location view of the water valve, click here: my answer has helped you, please help me and rate using the stars at the top of the page or 'accept' my answer.Rating or accepting my answer does not cost you extra. It allows me to be credited for this Q&A. If you cannot rate or do not see where to rate, please let me know! You can ask follow-up questions after you rate or accept. Thank-you!
Expert:  Ron Z. replied 1 year ago.
HI! I noticed this thread was still open and wanted to check in and see if there was anything else I could do for you on this issue? Any other questions you had?