An evaporative-emissions code can indicate a leak, incorrect purge flow, a valve problem, a pressure-sensor fault, or a blocked path. It does not reveal the location, and visible smoke does not prove every component works. Reliable diagnosis combines a regulated smoke test with scan-tool commands and pressure data.<\/p>\n
This workflow applies to gasoline vehicle EVAP systems, but component layouts and test strategies vary. Always use the wiring diagram, vapor-routing diagram, and manufacturer procedure for the exact vehicle. The system contains fuel vapor, so ventilation, approved equipment, and controlled pressure are essential.<\/p>\n
The EVAP system stores fuel vapor in a charcoal canister instead of releasing it to the atmosphere. Under suitable operating conditions, the powertrain module opens the purge valve so engine vacuum draws stored vapor into the intake. A vent valve allows filtered air into the canister during normal operation and closes when the module performs a system-integrity test.<\/p>\n
Many systems use a fuel-tank pressure sensor. The module commands purge and vent valves, observes pressure change, and evaluates whether the system holds vacuum. Other designs use pumps, switching valves, natural-vacuum logic, or separate leak-detection modules.<\/p>\n
Perform a full scan before clearing codes. Record current, pending, permanent, and history faults, plus freeze-frame data. Common code families include gross or small leaks, incorrect purge flow, pressure-sensor range faults, and purge or vent electrical faults. Our OBD2 trouble-code guide<\/a> explains how code status changes the diagnostic plan.<\/p>\n Check service information for code criteria, enabling conditions, valve state, service-port location, fuel-level requirements, and approved pressure. Note whether the monitor failed during a cold soak, cruise, idle, or engine-off test.<\/p>\n Bosch’s smoke-machine guidance recommends inert gas for fuel-vapor testing because EVAP systems can contain potentially explosive fumes. Pressure limits differ by vehicle and equipment, so follow the specified procedure rather than applying a universal number.<\/p>\n Check the filler seal, filler neck, hoses, canister, and connectors. Look for disconnected lines, cracked quick connectors, impact damage, corrosion, and liquid fuel in the canister.<\/p>\nSafety Before Smoke Testing<\/h2>\n
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1. Inspect the Simple Causes First<\/h2>\n