This just started happening in the last few days. I did replace the water pump last year on my 2016 Prius it had failed. But right now with the car off and Lock the water pump will run constantly in till the 12 volt battery is drained. It will trun off when I trun on the car when I trun it off right after the water pump will stay off for about 30 sec the will trun right back on. Yes the water pump is an oem part from Toyota when I replaced it. Need help on what going on
You likely have an engine temp sensor going bad or a relay sticking or a loose connection somewhere in that system. My guess is that you need a shop visit. Hopefully it will be a fairly simple fix for someone with the right information to know where to look.
If the water pump in Gen 4 is like the one in Gen 3, this post (and following) will explain how it is controlled. There is one wire that supplies the pump with +12 volt power, and another that tells the pump how fast to run. The "how fast" signal is generated by the ECM as a train of pulses, where the more time spent high, the faster the pump runs. The pulse train is generated by the ECM pulling down while the pump itself pulls up. So if that circuit is open somewhere or the connection is bad, the pump will see that as a "run as fast as possible" signal. That still leaves the question of how the pump has power to run at all. In Gen 3, the circuit that brings it +12 V power is controlled by a relay and would be off when the car is off. A look in the Gen 4 wiring diagram (more info) would clear up whether that is different in Gen 4 / indicate what else to check.
