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.Google+
Car Problem Reports
Scion tC Check Engine Light Due to Defective VVT-i Controller
Scion tC Problem
Average Mileage: 111,855 mi (32,000 mi - 170,000 mi)
Visitor, 2006 Scion tC, 170,000 mi
My car turn on the engine light when the gas cap is loose. I had to change the cap and the light turn off about 2 weeks later. Now I had the same problem... I check the gas cap again... it looks good.. but the light still on. Don't know what to do now...
Rough idle, high acceleration noise. Need to repair, just happened.
Visitor, 2005 Scion tC, 32,000 mi
dash lights have been on for quite some time. Had a mechanic look at it and was told the camshaft gear assembly needs to be replaced at about $700.00.