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This is actually a "known problem" and Toyota has issed a Technical Service Bulletin:
Title:DIAGNOSTIC TROUBLE CODE P0446
Models:'99 Avalon, Camry & Solara
June 13, 1999
IntroductionUnder certain driving conditions, some 1999 Model Year Avalon, Camry and Solaras will exhibit a false M.I.L. "ON" DTC P0446. A Vacuum Control Valve Assembly has been developed to prevent this condition.
This repair is covered under the Toyota Basic Warranty. This warranty is in effect for 36 months or 36,000 miles, whichever occurs first, from the vehicle's in-service date.
Coverage is extended to 36 months or 50,000 miles, whichever occurs first, in the states of California and Massachusetts under the California Emission Warranty