overheating warning but engine is cool

Just about to drop my Prius off at the dealer but wanted a bit of independent advice.

I've been getting an engine overheating warning on a GenII 2000 Prius when driving up hills in hot weather. When checking the engine it's not showing any signs of overheating though. Haven't lost any coolant, pipes not hot, no strong smells. After leaving the car to cool down for 15min it continues normally until it heats up on hills again. Driving around the city is fine and it only happens on hot 25c days. Just before the warning comes on the engine starts to lose a little power and you can hear the revs surging.

I've had one dealer take a look and they couldn't find anything wrong on the testing equipment.

Has anyone had any experience with a problem like this? My guess is that since the engine itself dosen't seem hot that the temperature sensor might be gone and there by causing the system to go into overheating protection mode.

Could this be air in the coolant or would lead to the engine actually heating up?

Thanks for any suggestions. Only had the car a month and I'm loving it! Getting 20k/l.

 

Thanks for the fast reply.

Just had a full 90,000 km service. Coolant wasn't replaced. Dealer said everything was in good condition. Coolant colour looks good. It's a Japanese import so I don't know when it was last replaced. Might be worth doing as well.

Haven't noticed any coolant leaking at all. Level is exactly the same as a month ago. Have driven 1,200 km.

Next time it happens I'll check the fan.

Thanks for the advice.

 

After driving around with the mechanic and a tester plugged in we tracked it down to the inverter over heating. It turned out the small water pump for the inverter cooling system had failed. As the inverter system can almost cool itself we didn't notice any problems day to day driving around town. On longer trips the inverter would heat up and then trigger the warnings and slow the car down.

Overall I'm impressed at how well the car looks after the safety of it's own systems. We were driving around without a water pump for three months!

Funnily enough the car now drives faster on the open road thanks to the inverter running properly. Like having a software upgrade!

 

Nope it's a NHW11. I was told not to get a NHW10 as they're not well supported by Toyota internationally as they were intended as a domestic test model only.

Just plugged in a standard tester I believe and we were off being able to watch temps in real-time.

Most enjoyable and nerve racking part was when the warnings came on and he got me to keep driving up the hill. As the temp in the inverter increased you could feel the power step down until we were standing still in the middle of the road! We then rolled backwards off to the side of the road and waited for the inverter to cool down after a few minutes.

Actually, they pulled out the pump and bench tested it. I don't think it was 100% clear from the error codes that the coolant pump had failed.