the problem has not come back
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2009 Volkswagen Jetta Question: P0304 code on start up, car started to stumble and engine check light came on,
Answer #1patrick mannion from Greg Solow's Engine Room, July 20, 2011, 22:14Master
P0304 is a misfire in number 4 cylinder. Perhaps bad fuel of something caught under an injector that has cleared itself. If the light continues to stay out ignore it but if it comes back on you will have to have the problem investigates.
Answer #2chinasinoy July 21, 2011, 00:28Journeyman
for the p0304, the possible solution:
If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.
this tool can help you reset the code.
If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.