How the system works
A high-level system map. Component roles and exact circuit behavior vary by vehicle.
- 01Combustion outputCombustion creates exhaust gases
- 02System feedbackSensors report mixture and system behavior
- 03AftertreatmentAftertreatment converts targeted pollutants
- 04Vapor managementVapor systems contain and meter fuel vapor
- 05Monitor checksOn-board monitors compare expected and actual response
Major components in context
Reports exhaust oxygen for control or monitor comparison
Open componentSupports chemical conversion after proper warm-up
System contextContains fuel vapor and meters it into the engine
System contextCarry gases without unwanted air entry or cabin exposure
System contextManage combustion conditions that affect emissions
System contextSymptom patterns and first direction
P0420 catalyst-efficiency code
Check engine condition, exhaust sealing, monitor data, and sensor behavior before replacing a converter.
Follow the diagnostic pathSYMPTOMP0455 gross EVAP leak
Inspect sealing and use controlled leak testing rather than smoke or pressure guesswork.
Follow the diagnostic pathSYMPTOMExhaust odor in the cabin
Leave for fresh air and arrange qualified inspection.
Follow the diagnostic pathA system-level diagnostic method
Identify the exact code, monitor, and enabling conditions
Correct engine performance faults first
Inspect exhaust and vapor-system sealing
Use supported live data and controlled tests
Complete the exact verification drive only when safe and legal
Evidence worth preserving
Freeze-frame and readiness state
Fuel-control and misfire context
Upstream and downstream sensor relationships
Controlled smoke or sealing-test evidence
Use the VIN, installed equipment, current manufacturer information, and applicable safety rules.
Safety and professional boundary
Toxic exhaust and flammable-vapor warning
Never run an engine in an enclosed space. Exhaust can contain deadly carbon monoxide, and EVAP work involves flammable fuel vapor.