Circuit Description

By-pass valve is used with the Hydrocarbon Adsorber Catalyst (HCAC) system. Before engine is started, by-pass valve is open. When engine starts, Engine Control Module (ECM) delivers an output signal to HCAC Vacuum Switching Valve (VSV). HCAC VSV opens, allowing vacuum to be applied to actuator, which causes the by-pass valve to close. When by-pass valve closes, exhaust gases pass into the HC adsorber where the gases are stored until the temperature of front Three-Way Catalytic Converter (TWC) increases. This prevents hydrocarbons from being delivered from vehicle when TWC temperature is low. After TWC becomes warm, the HCAC VSV closes and vacuum to by-pass valve is shut off, causing by-pass valve open. When rear TWC becomes warm, exhaust emissions are cleaned by the TWC. The HCAC VSV may also be operated during a deceleration condition when HC adsorber is warm in order to scavenge the hydrocarbons that remain in the HC adsorber. DTC is set if abnormality exists in the engine vacuum for HCAC system. Possible causes are:

• Defective vacuum hose or line.
• Defective HCAC VSV.
• Defective vacuum sensor.
• Defective check valve.