A vehicle with the Northstar engine is brought into the shop with much reduced power shortly after start-up. All three oxygen sensors report a lean mixture, though the spark plugs are all black with...

A vehicle with the Northstar engine is brought into the shop with much reduced power shortly after start-up. All three oxygen sensors report a lean mixture, though the spark plugs are all black with carbon. Even before you pull the OBD II codes, what kinds of problems could cause these symptoms?