On the 2007 Toyota Sienna, The P0304 is pointing to a misfire on Cylinder #4. This is the middle cylinder on the side of the engine closest to the front of the vehicle. More commonly, this is due to a faulty Spark Plug, Ignition Coil or Fuel Injector issue. So, best place to start is by inspecting the spark plug. Look for any signs of cracks in the body and/or a worn diode. If there are any doubts, replace all the spark plugs. If the plug checks ok, you can test the coil with the use of a spark tester. Or, move it to another cylinder and see if the code follows (ie. P0304 is now P0302). If so, replace the coil (this is the most common). If the coil checks ok, test the fuel injector. With the engine running, unplug the injector. The engine should respond. If not, the injector pulse should be checked with the use of a noid light. If pulse is good, replace the injector. If the injector checks ok, then ultimately, a cylinder compression test will need to be done.
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