An illuminated Check Engine Light could be caused by a defective VVT-i controller, which changes cam timing for optimal power. Updated parts are available and the repair may be covered under the powertrain warranty.
Car problem reports
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Most reported 2005 Scion tC problems
Check Engine Light Illuminated
Problem: Diagnostic Showed Error Code 420: "Catalytic Converter Efficiency Below Threshold (Bank 1)
Solution: Free Replacement by Dealer Under California 7yr/70k or Federal 8yr/80k Warranty Covering Emissions Parts