On higher mileage vehicles, an engine misfire may develop and/or the Check Engine Light may illuminate due to a failed ignition coil. It is not uncommon to replace all the coils when the first one fails in order to prevent return trips to the repair shop.
2007 Toyota Avalon Problem Reports
RepairPal Verified 2007 Toyota Avalon Problem Reports
The rear Oxygen sensors are a critical element in fuel control (unlike other vehicles) and can cause an emissions test failure for high CO and HCs. The sensing element cracks and and reports an erroneous steady state lean condition which confuses the computer, which in turn makes the fuel mixture overly rich.