P0303: Cylinder 3 Misfire Detected
The ECM detects crankshaft deceleration during Cylinder #3 combustion stroke. On Subaru boxer engines, Cylinder 3 is on the passenger side rear.
Driveability & Safety Advisory
Drive directly to repair garage. Avoid highway cruising under load.
Official Manufacturer TSB Advisories
Factory Condition: Oil consumption causes carbon buildup on Cylinder 3 exhaust valve, leading to partial valve seating failure.
Physical Component Location & Service Access
Third cylinder from front on inline engines; rear passenger side on Subaru boxer 4-cylinder engines.
Electrical Specifications & Live PID Benchmarks
Scan Tool Live Data Targets
Digital Multimeter (DMM) Specs
Documented Model-Specific Case Studies
Root Cause: Cylinder 3 plug boot carbon tracking caused by high humidity in intake air charge.
Verified Fix: Replace Cylinder 3 coil boot and spark plug.
Technician Inspection & Testing Workflow
Swap coil with Cylinder 1; check code migration.
Inspect spark plug condition.
Verify injector signal.
Recommended Diagnostic Tools & Verified Parts
BlueDriver Bluetooth Pro OBDII Scan Tool
Wireless sensor telemetry & enhanced diagnostics (Engine, ABS, Airbag). Delivers verified vehicle repair reports.
Autel AutoLink AL519 / AL319 OBD2 Scanner
Color screen standalone scanner. One-click I/M emissions readiness status, freeze-frame data, live DTC reset.
Ram-Pro Inline Engine Ignition Spark Tester & Gap Gauge
Quickly verify if spark plug coils are firing with high-intensity visual spark indicator. Includes electrode gap measure tool.
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Cost Breakdown
Tools & Parts Matrix
Diagnostic Questions & Answers
Where is Cylinder 3 located?
On standard inline 4-cylinder engines, it is the 3rd cylinder counting from the front timing belt back toward the transmission.