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.
2004 Toyota Highlander Problem Reports
Most Reported 2004 Toyota Highlander Problem Reports
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When changing the air filter, it is easy to knock off a hose on the air filter housing, which is part of the emission control system. If the vehicle is driven with this hose disconnected, the Check Engine Light will illuminate.