Idea: Low Coolant Sensor to Prevent Head Gasket Failure on Gen 4 Prius

hence the idea

Mendel -- totally agree that fixing the leak source is the right answer.
If there’s an EHRS/exhaust-side manufacturing defect (or any hard-part failure), that should be diagnosed and repaired. My point isn’t to avoid the fix; it’s to add a cheap early-warning layer so owners don’t learn about a leak only after the head gasket is cooked.

Why an early-warning sensor still helps (even with known leak modes):

• Leaks don’t announce themselves on a schedule. A pinhole that opens on a hot day, a failing cap, a hose clamp, a slow seep at the water pump--any of these can drain a reservoir long before a MIL or temp light fires.
  
  
• MIL is late for level-loss. By the time the dash warns, the system may already be ingesting air and spiking temps under load. An earlier alert at the tank buys time to pull over and save the engine.
  
  
• Complement, not substitute. Think of it like a smoke detector. It doesn’t fix wiring; it buys you time to keep a bad situation from becoming catastrophic.
  

If EHRS defects are common, great--then we can be specific:
If you’ve got a TSB/campaign/article that documents those weld failures on Gen 4, please link it. That actually strengthens the case for an early-warning kit: a known failure mode + a simple alert = fewer totaled engines while owners schedule the repair.

Design thoughts to avoid false alarms (and slosh):

• Dual-threshold + debounce: Trigger only when level is below a set point and remains low for, say, 10–20 seconds (ignores slosh).
  
  
• Temperature gating: Only monitor when coolant is warmed (prevents “cold contraction” noise).
  
  
• Self-test & fail-safe: Periodic test pulse; if sensor/wire fails, it warns “sensor fault” (not silent).
  
  
• Non-invasive install: Float switch or capacitive probe at the reservoir, add-on buzzer/light in cabin. No CARB/EPA entanglement since it’s not emissions-control--just a warning.
  

Who benefits:

• People out of warranty, high-mileage drivers, rideshare/delivery folks (lots of heat cycles), and anyone far from a dealer. A $100–$200 kit that prevents a $3–$5k repair is basic risk management.
  

Happy path (both/and, not either/or):

1. Diagnose/repair the leak source (EHRS, pump, clamps, cap, hoses, HG, etc.).
   
   
2. Add an early-warning level sensor so the next issue doesn’t escalate unseen.
   

If you happen to have links or TSB refs on the EHRS issue, that’d be super helpful for owners and for calibrating the alert threshold.