The P0304 diagnostic trouble code on your 2021 Toyota Highlander indicates the engine control module has detected consistent misfires in cylinder 4. The 4th generation Highlander offers V6 and hybrid options.
2021 Highlander Engine Options
The 2021 Highlander offers the 3.5L 2GR-FKS V6 (295 hp) or 2.5L hybrid (243 hp combined). On the V6, cylinder 4 is on the rear bank. On the hybrid inline-4, cylinder 4 is rearmost.
Common Causes
- Ignition Coil Failure: Individual coil-on-plug failures
- Spark Plug Issues: D-4S specific requirements
- Fuel Injector Problems: D-4S dual injection
- Carbon Buildup: D-4S helps reduce accumulation
- Hybrid Operation: Engine cycling affects diagnosis
Diagnostic Approach
Use Toyota Techstream for detailed diagnostics. Identify your engine type first. Monitor misfire counts. D-4S on V6 helps prevent carbon issues. Test ignition components.
Repair Solutions
Replace failed ignition coils with Denso OEM components. Install Toyota-specified spark plugs. D-4S system is reliable. Dealer service for warranty coverage.