How the system works
Follow energy, material, information, and mechanical force through the system before focusing on a single component.
A high-level system map. Component roles and exact circuit behavior vary by vehicle.
- 01InputCombustion creates exhaust gases
- 02Flow 2Sensors report mixture and system behavior
- 03Flow 3Aftertreatment converts targeted pollutants
- 04Flow 4Vapor systems contain and meter fuel vapor
- 05Protect / verifyOn-board monitors compare expected and actual response
Follow the sequence, stop when evidence changes the path, and verify the exact vehicle information before acting.
- 01Combustion creates exhaust gases
- 02Sensors report mixture and system behavior
- 03Aftertreatment converts targeted pollutants
- 04Vapor systems contain and meter fuel vapor
- 05On-board monitors compare expected and actual response
Major components in context
Reports exhaust oxygen for control or monitor comparison
Open component →Supports 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 path →SYMPTOMP0455 gross EVAP leak
Inspect sealing and use controlled leak testing rather than smoke or pressure guesswork.
Follow the diagnostic path →SYMPTOMExhaust odor in the cabin
Stop exposure, ventilate, and arrange immediate inspection for leakage.
Follow the diagnostic path →A system-level diagnostic method
Identify the exact code, monitor, and enabling conditions
Use the result to support or reject a cause group before moving to a more invasive test.
Correct engine performance faults first
Use the result to support or reject a cause group before moving to a more invasive test.
Inspect exhaust and vapor-system sealing
Use the result to support or reject a cause group before moving to a more invasive test.
Use supported live data and controlled tests
Use the result to support or reject a cause group before moving to a more invasive test.
Complete the exact verification drive only when safe and legal
Use the result to support or reject a cause group before moving to a more invasive test.
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.