Why Your “Head Gasket Problem” Might Be Something Else
What is this problem?
If your 2016-2019 Toyota Prius or 2017-2019 Prius Prime is losing coolant, running warm, or giving you poor heater performance, you might be worried about a blown head gasket. That’s an expensive repair—often $2,500 or more. But before you panic, there’s a known issue that mimics head gasket failure and may be covered under Toyota’s extended warranty.
Toyota’s fourth-generation Prius uses a heat exchanger built into the front exhaust pipe. This system captures waste heat from the exhaust to warm the engine coolant faster, improving fuel economy and getting cabin heat to you sooner on cold mornings. The problem? These heat exchangers can develop internal leaks, allowing coolant to seep into the exhaust system. When this happens, you lose coolant—but you won’t see a puddle on the ground because it’s going out the tailpipe as steam.
How does this mimic head gasket failure?
A failing exhaust heat exchanger produces symptoms that look almost identical to a blown head gasket:
• Coolant disappearing with no visible external leak
• White steam from the exhaust (coolant burning off)
• Sweet smell from the tailpipe
• Engine running warm or overheating
• Check engine light with code P148F00
These are the same warning signs technicians look for when diagnosing head gasket failure. Without proper inspection, it’s easy to assume the worst.
Why is “no heat” often the first symptom?
Here’s what typically happens: the heat exchanger develops a small leak, and coolant slowly escapes into the exhaust system. Your coolant level drops gradually over weeks or months.
The heater core sits at the highest point in the cooling system. As coolant level drops, the heater core is the first component to lose circulation. Long before your engine overheats, you’ll notice the cabin heater blowing lukewarm or cold air—especially when idling or at low speeds.If your Prius heater has suddenly stopped working well, don’t just top off the coolant and hope for the best. That missing coolant is going somewhere, and the cause needs to be identified. Often owners continue driving and cause overheating and more substantial engine damage.
What warranty coverage is available?
Toyota acknowledged this issue with Technical Service Bulletin T-SB-0135-19 and covers the repair under their Federal Emission Warranty.
Location Coverage Period
All 50 states 8 years / 80,000 miles from original in-service date
CARB states* 15 years / 150,000 miles from original in-service date
*California Emissions states include: California, Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Rhode Island, and Vermont.
Vehicles covered: 2016-2019 Toyota Prius and 2017-2019 Toyota Prius Prime
You can read the full service bulletin from Toyota here: Toyota Technical Service Bulliten
What should you do?
Don’t assume you need a head gasket. We’ve seen shops quote clients for head gasket replacement on vehicles that actually needed a heat exchanger that may be covered by Toyota!
Bring your vehicle in for proper diagnosis. We can pressure-test the cooling system and inspect the exhaust heat exchanger for evidence of internal coolant leakage. This is the only way to determine whether you’re dealing with a heat exchanger leak, a head gasket failure, or something else entirely.
Check your warranty eligibility. If your Prius is within the coverage window, you may be able to have this repair performed at a Toyota dealership at no charge. Even if you’re outside the warranty period, knowing the actual problem saves you from paying for a repair you don’t need.
Don’t keep driving and adding coolant. Continued coolant loss can lead to overheating, which can cause actual head gasket damage. A problem that would have been a simple repair can become a complete engine replacement if ignored!
—Atomic Auto
Portland’s Hybrid & EV Specialists
Document reflects information from Toyota TSB T-SB-0135-19, issued September 2019.
Document last updated: January 2026