I bought a used car and I need help

What did the seller tell you that you didn't already know? Is your check engine light on? How are you planning to fix P0420?

 

Google "prius Pxxxx" for each code
and check this
3 Trouble Codes To Not Worry About On Your Toyota Prius | Torque News

 

Hello and welcome to Priuschat.

Those three codes are all related to the Coolant Heat Storage tank system. The CHS is a thermos bottle in left front fender that keeps several quarts of coolant hot for 2-3 days (only on US spec cars). On a cold start a small pump pushes that hot coolant into the engine so it warms up faster (and emissions are reduced). Whether this system functions or not has no detectable affect on driveability or reliability. If you don't have emissions inspection in your area then you don't NEED to repair this unless you choose to.

P1116 is for the temperature sensor that is mounted on the bottom of the tank. P1121 (and P1123) are both for the coolant "3 way valve" that controls engine coolant flow in and out of the CHS tank (super common failure on Gen2 Prius- best to get a new one from Toyota).

For diagnosing this you might want to find a copy of the factory service manual. Also it can be very useful to get a mini VCI cable (along with a bootleg copy of Techstream - Toyota's diagnostic software from amazon or ebay) and install it onto an old windows laptop. Use the search feature on this forum.

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