One problem related to check engine light on has been reported for the 2012 Toyota Prius. The most recently reported issues are listed below.
Engine misfiring at start. While at intersection in city and turning left, accelerating and triggering the combustion engine, misfiring causing jerking and lack of acceleration almost causing a stalled car in oncoming traffic. Check engine light on. Service says misfiring caused by faulty head gasket. Appears to be symptomatic of gen 3 Prius.