2017 Prius Two Eco – Severe loss of hybrid power, no inverter coolant circulation, no warning lights

Hello everyone,

I’m looking for some advice diagnosing a problem with my 2017 Toyota Prius Two Eco (ZVW51). The car was imported from a US auction and currently has around 64,000 km (about 40,000 miles).

For the last 3–4 weeks, the car has had a significant loss of hybrid/electric power.

The gasoline engine itself seems to run perfectly normally. The problem is specifically with the hybrid/electric power. Before this started, the car had very strong electric acceleration. Even with light throttle, it could accelerate very quickly using electric power, and on suitable roads it could reach around 100 km/h (62 mph) without the gasoline engine running.

Now the car still drives electrically, but the electric power is extremely weak. On a flat road it struggles to gain speed using electric power alone. When I press the accelerator harder, the gasoline engine starts immediately and the car continues accelerating, but because the hybrid system is no longer providing the same electric assistance, acceleration is much weaker than it used to be. The gasoline engine itself sounds and feels normal.

The hybrid battery does not seem to discharge unusually fast. The state of charge stays reasonably stable and the battery generally holds charge well. When the car is weak, the battery level may drop somewhat faster, but I think that may simply be because the electric system is not working properly and the gasoline engine is doing more of the work.

There are no warning lights, no Check Engine light, and no obvious hybrid system warnings on the dashboard.

One very strange thing is that the problem is sometimes temporary.

For example, recently I stopped to refuel. Before stopping, the car was suffering from the weak acceleration problem. I turned the car completely off, refueled, and started it again. Suddenly the hybrid system worked almost perfectly again. For about 5–7 minutes, the car had its old electric power back. Even very light throttle was enough to accelerate strongly and the gasoline engine did not need to start. After several minutes, the weak hybrid power returned again.

I also noticed something concerning with the inverter coolant system.

The inverter coolant reservoir has the correct coolant level now, around FULL, but when the car is in READY mode there is absolutely no visible coolant circulation in the reservoir. I mean the coolant surface looks completely still — no small waves or movement at all.

I can hear a small electronic whining sound from the inverter coolant pump, so it sounds like the pump is running, but there is no visible circulation. I have only noticed a few very small bubbles deep in the reservoir once or twice, but not actual coolant movement.

Both the engine coolant reservoir and inverter coolant reservoir were around the LOW mark when I bought the car. They were not empty, but they had clearly dropped below FULL over time. I topped both of them up, but the hybrid power problem did not improve. The coolant levels have remained stable since then and I have not noticed any coolant loss or leakage.

Another thing I noticed recently is a slight whistling / air-blowing type noise when I press the accelerator. There has always been a small high-frequency electronic whine from the hybrid system, but now under acceleration I sometimes hear a slightly different whistle that sounds almost like air being blown. I am not sure where exactly the sound is coming from.

The 12V battery is new, replaced about 2–3 months ago. I am not referring to the hybrid battery here. The car has the original lithium-ion traction battery, and given the low mileage and the fact that it holds charge well, I have not replaced it.

At the moment I am wondering whether the problem could be related to the inverter cooling system:

• Could there be air trapped in the inverter coolant loop?
• Could the inverter coolant pump make noise but still fail to circulate coolant properly?
• Could poor inverter cooling cause the hybrid system to limit electric power without immediately producing a warning light or DTC?
• Could the system temporarily work normally after a restart and then reduce power again as the inverter heats up?

I am planning to have the car properly diagnosed with Toyota Techstream / hybrid live data, but before I do that I wanted to ask the Prius community.

What would you check first based on these symptoms? Could this be an inverter coolant pump / trapped air / cooling circulation problem, or should I be looking somewhere else such as the inverter, transaxle, MG2, or hybrid battery?

Any advice from experienced Prius owners would be greatly appreciated.

Thank You!