The P0304 diagnostic trouble code on your 2020 Toyota Prius indicates the engine control module has detected consistent misfires in cylinder 4. The hybrid uses Toyota's efficient 1.8L Atkinson-cycle engine.
1.8L Hybrid Engine Layout
The 2020 Prius uses the 2ZR-FXE 1.8L four-cylinder with hybrid system producing 121 hp combined. This Atkinson-cycle engine operates differently from conventional engines. Cylinder 4 is the rearmost.
Common Causes
- Ignition Coil Failure: Coil-on-plug units can fail
- Spark Plug Issues: Hybrid operation affects plug wear
- Fuel Injector Problems: Port injection deposits
- Engine Cycling: Start-stop operation affects diagnosis
- EGR System: Exhaust gas recirculation issues
Diagnostic Approach
Use Toyota Techstream for hybrid-specific diagnostics. The engine cycles on and off during driving. Ensure engine is running during misfire testing. Port injection is reliable.
Repair Solutions
Replace failed ignition coils with Denso OEM components. Install Toyota-specified spark plugs. Clean fuel injectors if needed. EGR system cleaning may help at higher mileages.