Coolant Monitoring System
 

The RX400h have 2 coolant area that needs to be topped off and monitored, one for inverter and one for internal combustion engine.
Now, the dash only shows one gauge for the temperature. So my question is, what does the gauge show exactly? (The temperature of the ICE, inverter, or a combination of both?)

The dash gauge is the ICE coolant. A hybrid system coolant warning would be displayed in the center of the gauges via a message. Hybrid coolant stays much cooler than ICE. I doubt it gets over 140 degrees f. And the hybrid system is not pressurized like the ICE one.

Do you happen to know which system is more prone to being failed? The seller replaced a water pump on the car, and it doesn't say whether it is the one for ICE or Inverter?

Most likely it was the ICE water pump. Neither system is prone to failure however many mechanics and individuals change the ICE water pump when changing the timing belt as everything is already apart. Lexus dealers generally do not change water pumps during first timing belt change as they say ICE pumps may last the life of the car. ICE pump failures start off as seepage of pink coolant dripping down the side of the engine so it probably won't leave you stranded on the side of the road.

Hybrid pump failures are very rare. I have only seen 2 or so posted in this forum. Most seem to get a "check hybrid" warning. Once they scan for codes and find a hybrid temp warning they check coolant levels and/or replace the pump. The pump is behind the radiator on the inverter side. Look for a black unit on a bracket with smaller hoses around 3/4". You can see it by looking down from above. Removing the cover from the inverter may help your view.

Thank you Smassey321!