I have a 2017 Toyota Prius Prime with approximately 168,000 miles.
Over a period of a couple of weeks, I noticed the engine coolant level dropping even though there was no visible coolant leaking under the car. I also experienced weak cabin heat one evening and received the high coolant-temperature warning several times while driving. Each time the warning appeared, I immediately slowed down, and it disappeared shortly afterward. The car otherwise continued to drive normally, with no obvious rough running or major loss of power.
I took the car to a mechanic. They pressure-tested the cooling system but found no external leaks. They also removed and inspected the spark plugs, which were dry, making a significant coolant leak directly into the cylinders less likely.
The dealership then inspected the exhaust system and confirmed that coolant was leaking internally through the exhaust heat exchanger. This is a known issue addressed by Toyota service bulletin T-SB-0135-19 for certain 2017–2019 Prius Prime vehicles.
The Prius Prime uses an exhaust heat-recovery system. A heat exchanger is integrated into the front exhaust-pipe assembly and uses heat from the exhaust gases to warm the engine coolant more quickly. The coolant and exhaust are supposed to remain separated by internal metal passages. In my vehicle, the heat exchanger failed internally, allowing coolant to enter the exhaust stream and leave the vehicle as vapor through the tailpipe. This explains why there was coolant loss without any visible external leak.
As the coolant level dropped, air entered the cooling system. This likely caused the weak cabin heat and intermittent overheating warnings.
The heat exchanger is not serviced separately. The required repair is replacement of the complete front exhaust-pipe assembly, which includes the catalytic converter and integrated heat exchanger. The updated Toyota OEM part is 17410-37D40, or any newer Toyota-approved superseding part matched to the VIN.
A D V E R T I S E M E N T S
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I have a 2017 Toyota Prius Prime with approximately 168,000 miles.
Over a period of a couple of weeks, I noticed the engine coolant level dropping even though there was no visible coolant leaking under the car. I also experienced weak cabin heat one evening and received the high coolant-temperature warning several times while driving. Each time the warning appeared, I immediately slowed down, and it disappeared shortly afterward. The car otherwise continued to drive normally, with no obvious rough running or major loss of power.
I took the car to a mechanic. They pressure-tested the cooling system but found no external leaks. They also removed and inspected the spark plugs, which were dry, making a significant coolant leak directly into the cylinders less likely.
The dealership then inspected the exhaust system and confirmed that coolant was leaking internally through the exhaust heat exchanger. This is a known issue addressed by Toyota service bulletin T-SB-0135-19 for certain 2017–2019 Prius Prime vehicles.
The Prius Prime uses an exhaust heat-recovery system. A heat exchanger is integrated into the front exhaust-pipe assembly and uses heat from the exhaust gases to warm the engine coolant more quickly. The coolant and exhaust are supposed to remain separated by internal metal passages. In my vehicle, the heat exchanger failed internally, allowing coolant to enter the exhaust stream and leave the vehicle as vapor through the tailpipe. This explains why there was coolant loss without any visible external leak.
As the coolant level dropped, air entered the cooling system. This likely caused the weak cabin heat and intermittent overheating warnings.
The heat exchanger is not serviced separately. The required repair is replacement of the complete front exhaust-pipe assembly, which includes the catalytic converter and integrated heat exchanger. The updated Toyota OEM part is 17410-37D40, or any newer Toyota-approved superseding part matched to the VIN.
The repair cost was $2658.16.
- Aidan C., Tacoma, WA, US