Because this recent article is under pay wall, here's entire Q and A - "Q. I have a 2018 Toyota Prius Prime (only 89,000 miles) and twice now I had to add coolant to the engine. When this happens, I don’t get much heat out of the heater and sometimes the check engine light comes on. I add coolant and all is fine again until it isn’t. I took it to a hybrid repair shop, and they told me it could be a problem with the exhaust heat exchanger, and it will be expensive to repair. I purchased this car to save money of gas, not pay for expensive repairs. How common is this? A. If in fact the exhaust heat exchanger is leaking the repair is covered under the California (most Northeast states) emissions warranty. This warranty covers certain components for 180 months or 150,000 miles whichever comes first. Return to a Toyota dealer and have them check out the problem and reference TSB 0135-19" https://www.saratogian.com/2022/01/22/questions-and-answers-from-the-car-doctor-53/?fbclid=IwAR1RkaJKJZuctvkl3vaYB2A-Hl3_ZJJjj1Ba45YdW6xfeYUK5imnZTGhmno
Here's Toyota Technical Service Bulletin. Note: for 2017-2019 PPs only. This repair is covered under the Toyota Federal Emission Warranty. This warranty is in effect for 96 months or 80,000 miles, whichever occurs first, from the vehicle’s in-service date. Prius Prime vehicles, sold, registered, and normally operated in California, Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Rhode Island, and Vermont. The California Emissions Defect and Performance Warranty coverage is 180 months from the date of first use or 150,000 miles (whichever occurs first). https://static.nhtsa.gov/odi/tsbs/2019/MC-10166037-9999.pdf
I have a 2020 Prime and noticed that my ICE coolant was suddenly at the LOW mark on the reservoir - at 18k miles. The dealership topped it off and advised me to watch it, while suggesting this is common with Primes. Roughly 1 month afterwards the ICE coolant level had again dropped to halfway between the FULL and LOW marks on the reservoir. This has been all summertime driving with no use of the heating system. I've seen no evidence of any leaking or coolant loss on the garage floor. I'll continue to watch this, but I'm suspecting a leak in my exhaust heat exchange system.
In my experience with both 2016 and 2017 Gen 4 Hybrids, not Primes though, the coolant level seems to like the Low mark. Whenever I top it up, it inevitably returns to the low mark. As long as it hangs there, I would not suspect the heat exchanger leak. Those situations result in complete loss of coolant, i.e., empty reservoir.
