Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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 to 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.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.