The P0304 diagnostic trouble code on your 2021 Toyota Sienna indicates the engine control module has detected consistent misfires in cylinder 4. The redesigned 2021 Sienna is exclusively a hybrid.
2021 Sienna Hybrid Powertrain
The 2021 Sienna uses a 2.5L Dynamic Force four-cylinder with hybrid motors producing 245 hp combined. This hybrid-only minivan uses D-4S dual injection. Cylinder 4 is the rearmost.
Common Causes
- Ignition Coil Failure: Coil-on-plug units can fail
- Spark Plug Issues: D-4S specific requirements
- Fuel Injector Problems: D-4S dual injection system
- Hybrid Operation: Engine cycling affects diagnosis
- Carbon Buildup: D-4S helps prevent but possible
Diagnostic Approach
Use Toyota Techstream for hybrid diagnostics. Ensure engine is running during testing. D-4S helps prevent carbon issues. Test ignition components systematically.
Repair Solutions
Replace failed ignition coils with Denso OEM components. Install Toyota-specified spark plugs. D-4S system is generally reliable. Dealer service for warranty coverage.