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300h hybrid system overheat after dealer changed fluid

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I took my 2015 300h with 60,000 miles in for routine service and they promptly told me i need 3 coolant system fluids changed in addition to the oil change I came in for.  A few days later i had to do a fill in day at a clinic 3 hours from my home where the staff veterinarian had covid and her office was the only clinic in the small town.  About 1 - 1 1/2 hour into my commute, the hybrid system over heat light came on.  I was on a 2 lane road with no should in the middle of nowhere and could not even pull over for another 20 miles.  I googled the error message then checked all the coolant fluid reservoirs and they were full, shut the engine off and since my mpg had significantly dropped, I refueled and drove the rest of the commute with the climate control off.  I was in the mid 30's outside.  I made it to the clinic, saw patients then started the return trip with no heat on and driving very gently.  This time the overheat alert did not come on until 2.5 hours into the trip and going up a steep hill.  Again shut car off checked levels and made it home.  

The next day I returned to lexus and they said air bubbles in system after fluid change caused over heat and all is fixed now.  I specifically asked about Battery damage and they said no there would be no issues.

Finally, I get to my question for the brain trust.  My prior long term mpg according to the car computer is 42.4mpg.  After the last 2 refuelings, each tank has been in the 31 mpg average.  All with similar style of driving and same roads and terrain.  When the overheat errors happened my mpg on that tankful dropped to 28 mpg.  So, what do I do now?  Is there a way to test Battery function?  My wife's hybrid camry  had a huge drop in mpg when the Battery needed replacing and now I have a 25% drop in fuel efficiency after the dealer messed up his recommended service.

So what say ye?



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Looks like this was your first post so welcome to the forum!

You raise some interesting questions and it would be good if you could clarify some points.

What were the coolants that were changed? Does your bill specify what was done?

You mention an overheat light, as well as an error message. Do you remember which light it was and what the wording of the error message was?

And just for background was it mid 30s outside in fahrenheit (you needed heating) or centigrade (you needed cooling). If it's fahrenheit I'm guessing you're in the US and maybe your mpg figures are based on US gallons.

I have read that bubbles in the inverter water cooling system on the Prius can cause overheating but I'm not sure if it's the same on the IS300h. Some more expert contributors will no doubt provide some more info.

As for the coolant changes, in the UK as far as I remember the only extra task at 60,000 miles is spark plug replacement.

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Yes, in the US. Mid 30s F outside temps and I had heater running.  Fuel efficiency in miles per gallon. 
Dealership changed radiator, transmission and hybrid coolant fluids. 
Error message was service required hybrid system overheating. 
The engine temperature gauge stayed in middle range the whole time. 

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Reset the mpg function before a trip and the computer will relearn the correct air to fuel ratio resulting to the previous mpg reading.


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The is300h Battery is air cooled not fluid cooled. I believe the fluid is only for the inverter up front. There is a fan that cools the Battery which sucks air out of the cabin. Check if this inlet is not clogged or blocked by objects such as a jacket. It will be in the door sill of the right rear door (behind passenger on a LHD car). I believe if this is clogged the car may decide to protect the Battery and therefore rely less on its assistance during driving, hence increasing your fuel consumption.

There is also a how to on here on how to clean the filter which is not usually cleaned by a dealer. I recently did my 2013 with 140k KM and it was pretty clogged. My gas mileage improved after cleaning it.

Also how did you get an is300h in the US. Private import? I think it was never officially sold there. 

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On 1/25/2021 at 11:20 PM, GeorgePeterman said:

My prior long term mpg according to the car computer is 42.4mpg.  After the last 2 refuelings, each tank has been in the 31 mpg average.  All with similar style of driving and same roads and terrain.  When the overheat errors happened my mpg on that tankful dropped to 28 mpg. 

One possibility for the fuel consumption is that it's winter. Consumption always rises when it's cold because the engine runs to keep the cabin warm whereas in summer it would stop. There are also a lot more demands on power like heated seats, wipers, lights etc.

The effect is worse if you do a lot of short trips of 10 minutes or less. If the engine never reaches normal operating temperature fuel consumption will suffer.

Your long term mpg is 42 mpg (US gallons). The car computer usually overstates the number by 8-10%, so that's a real 38 US mpg. (For non-US readers that's around 47 UK mpg or 6 litres per 100 km.) This is pretty good. The average for the IS300h on the fuel consumption website is around 35 US mpg or 44 UK mpg or 6.47 l/100km.

If your inverter overheated, fuel consumption would suffer, so the drop in fuel consumption for that tank is not surprising. But my guess is that the computer would cut back use of the inverter if it was overheating to avoid damage. There's also a  limit on charge and discharge of the Battery which is constantly adjusted to avoid damage so I could be wrong but it seems unlikely that the Battery would have been affected. Instead, the use of the Battery would have been reduced if the inverter wasn't operating to full capacity. If the inverter had stopped working altogether, I don't think the car would be able to move.

After the problem was fixed consumption has been 31 mpg (let's say a real 28 US mpg, 35 UK mpg, 8.2 l/100km). That's disappointing but not extraordinary for winter driving. And you can't compare a one-off 31 mpg in winter with a long-term average of 42 which has been built up over many months including the summer when consumption is best.

If you have figures for previous winters you could compare and see if this is worse.


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My 2013 F-sport has been reading (car) over 9l/100km for the last 2 months - reset or not - all short journeys - never seen it climb this high before (3 years of ownership - Ireland). It could be a few things but with 125,000 on clock I would assume a new Battery would help improve the situation ...

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