2007 Toyota Prius with Multiple OBD-II Codes: P1121, C1242, U0123, U0124, U0126, B1421 - Diagnosis and Repair Guidance

The P1121 code indicates a stuck water control valve or a shorted/corroded sensor circuit in the water valve system. This is typically caused by wiring issues or internal valve failure. For all these codes—P1121, C1242, U0123, U0124, U0126, and B1421—it's important to consult the official 2007 Prius wiring diagram and perform targeted electrical testing. While no single connector handles all these sensors, they are all part of the vehicle’s electronic stability control (ESC) and climate system networks. A common issue may be a faulty harness or grounding problem affecting multiple systems. The P1121 code has appeared before on this model; in my case, it was resolved by replacing the water valve—though I suspect it may need replacement again if symptoms persist.

This issue was ultimately traced back to the same water control valve. The part was still under warranty, so we replaced it using hose clamps instead of factory connectors—this saved significant labor and parts costs. This confirms that while multiple codes may appear simultaneously, they can often stem from a single underlying component failure in older 2007 Prius models.