Two problems related to check engine light on have been reported for the 2009 Toyota Prius. The most recently reported issues are listed below.
My car recently has a check engine light and the obdii scanner show the fault code p1121 which is a failed three-way coolant control valve. Toyota has issued a technical service bulletin (TSB) on this problem: eg001-08 m. I. L. It should be a recall because it's a common problem for 2nd generation Toyota Prius.
When my 2009 Toyota Prius reached approximately 65,000 miles, the check engine light came on, and the error code was p1121, indicating a problem with the coolant control valve. I took the car to my local Toyota dealership, and their charge for replacing the part was $400. The mechanic who inspected the valve said it was not closing securely. He said making the repair would not improve the car's performance or affect its value, and therefore he did not recommend making the repair. Unfortunately, I have realized that opting out of this repair does affect the car's safety. The check engine light is constantly on. Even if I manually turn it off, it comes right back on, so it is unlikely that I will be aware if other mechanical issues ever arise. My mother drives a 2008 Toyota Prius which just hit 65,000 miles, and her car produced the exact same error code. It is statistically impossible that two cars of the same make and model, only a year apart in age, in the same household, would produce identical error codes after reaching roughly the same mileage for any reason other than defective manufacturing. I believe this error code is the result of a manufacturing error that Toyota should address through a safety recall. Toyota's own mechanic admitted that the repair is financially impractical for customers, yet if the repair is not made, drivers are left without a functional check engine light which is a safety hazard.