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P0446 Diagnostic & Repair Guide for 1999 Lexus ES30-0: EVAP Vent Control Circuit Issue

Model: 1999 Lexus ES300 Fault Code: P0446 Posted: 2018-07-17 15:29

Hello, I'm experiencing a persistent DTC P0446 code on my 1999 Lexus ES300. The code was previously flagged as 'pending' by an OBD-II scanner, but recently the Check Engine Light came back on. Last week, I replaced all six coil packs and installed factory-spec, pre-gapped platinum spark plugs after detecting misfire codes—those went away, and the vehicle ran smoothly at that time. However, today the engine light returned with a P0446 code: 'Evaporative Emission Control System - Vent Control Circuit.' I attempted several OBD-II drive cycles as recommended online (e.g., Toyota/Lexus-specific drive cycles), but my scanner reports incomplete or interrupted sequences. I have access to a Chilton's manual covering 1996–2002 Toyota/Lexus models. Can anyone recommend a clear, step-by-step diagnosis process? Are there any common fixes for this code? Is the repair straightforward (e.g., replacing a vacuum solenoid or vent valve)? What are the short- and long-term consequences of ignoring this issue? Thanks in advance! Rick

Related fault codes
P0446
Comments (5)
Anonymous 2018-07-17 16:12

See attached troubleshooting guide. There's also a TSB (Technical Service Bulletin) for a new design Vacuum Switching Valve (VSV). Your local dealer can verify if it applies to your vehicle based on VIN and build date. Addressing this issue is essential to prevent emission failures or system malfunctions.

Anonymous 2018-07-17 17:23

Hi Kev2, thanks for the quick reply! I'm hoping the root cause is either the EVAP Vent Valve/Solenoid or the EVAP Purge Solenoid. There are so many acronyms—'VSV' was mentioned but I couldn't find a definition in my search. Is 'VSV' short for Vacuum Switching Valve? Are there other terms like 'Vent Solenoid Valve'? Thanks again! Rick

Anonymous 2018-07-17 17:28

Found it: VSV stands for 'Vacuum Switching Valve'. That makes sense in the context of EVAP system control.

Anonymous 2018-07-19 11:00

Are the EVAP Vent Control Valve and Evaporative Canister Purge Valve two separate components, or are they essentially one part? I've seen references to both—could they be related or interchangeable in this system? Thanks for clarification!

Anonymous 2019-08-03 12:20

Rick, I had the exact same issue after replacing coils and plugs. I eventually resolved it by replacing the EVAP Vent Control Solenoid (VSV) and completing a full drive cycle. The code cleared, and the Check Engine Light stayed off. If you're planning to have your son drive back to college, this fix is worth doing before that happens.