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p0300 this comes usually when one of multiple cylinder are misfiring which actually means that the cylinder is not burning fuel adequately or not at all. there are many reasons for this as its one of the most wide scope code in automotive line it can start from the tip the air filter to the end the exhaust.its bit easy to diagnose if accompanied with other codes if not then the fault could be anywhere in anyform it's not that you can't fix it but it's a long procedure to narrow it down if left alone its a very nasty thing which can create many troubles like drivability issues rough idle,high fuel consumption, weaken the engine mounts, ruin the catalytic converter,choke the catalytic converter which in turn will ruin the rings and fail the engine and or run the catalytic converter so hot that it could lead to a fire hazard. the fix The spark plugs first off all is to pull the codes and see what actually is going on get some live reads from the diagnosis machine if its a advanced car if not then start from the basics like check the spark plugs if their ceramics been burnt at the base then it is time to change them the air air filter should be inspected and replaced if necessary air metering sensor should be checked and verified if in specs the throttle hose should be inspected for air leaks also the intake hoses and the intake manifold itself should not be ruled out too they too get bent or develop cracks that can be checked by spraying starting fluid and or carefully listen you can hear a hissing noise sometimes the ignition coils/plug wires then check the ignition coils for any electricity escape burn marks it'll show up like rainbow color (if distributor less) just loosen up the ig coil and raise it a bit while the engine is running you will hear the ticks ticks at uniform speed if not then switch off and exchange with the one which does tick tick that way you can narrow it down to the ig coils or the wiring of that particular coil harness which can be corrected looking up the wiring diagram of the particular make and model if still it remain in the particular cylinder then the coil driver of the particular cylinder located in the ecm is bad if the car is equipped with the distributor then wait for it to be dark or darken the place nicely and then open the bonnet and observe the plug wires for any lightning escape through the wires be careful though as its gonna sting real bad because of very high current the injectors/fuel pump it could be that the injectors are not working or its wiring could be bad that too can be checked exchanging the injectors place and again hearing the ticks of the injector using mechanics stethoscope if still not then the wiring of that particular injector needs to be looked at by using the wiring diagram also its important to remove them all and get them serviced at a good place so that you can know that the spray patterns and the spray quantity is good if still the problem persists in that same cylinder then the injector driver in the ecm is the problem for some cars which have direct injection injectors you wont be able to do this as removing injectors in those means you have to put back new ones the fuel pump pressure can be checked by fixing a gauge at the fuel rail and or a T joint should be put between the fuel rail and the feed pipe. please bear in mind that if the car was standing for a long time then its better to empty the tank and start with fresh fuel. the timing components the timing belts or chains could skip a teeth or two and the engine will run out of timing and result in misfire like behaviour you always have to check the camshaft ,crankshaft and the balancers (if equipped) that they alin at the right timing marks (if present) you can look up the FSM's of particular engine's the engine cylinders to check these you have to remove the fuel pump relay so no raw fuel will contaminate the catalytic converters ( if equipped ) and stick a gauge in the spark plugs place and note down each cylinders pressure also (generally) be aware that no two cylinders pressure should be more the 20 psi apart. they should be above 80 to have a start condition for specific engine pressure please look in the manual (to pin point ring and valve or leaking gasket problem apart you have to put oil in the cylinders to take the reading) the exhaust sometimes with a direct hit it crumbles or sometimes because of running hot the catalytic converter melts and chokes the pipe if it is in the right place of the pipe. it can stall or run very different then you are accustomed to as there could be very high back pressure which could alter the firing cycle the waiver those with heart conditions stay away from ignition components as very high voltage risk is involved. the work explained above is very dangerous to attempt on your own. there are many additional steps which have not been mentioned here but is not limited to like cover removal or the intake removal,timing cover removal which maybe required multiple times to complete the procedure you will always need the help of someone who is professionally trained to handle this types of work. the risks involved can be of electrocution, burning (yourself or the car or the premise where you are performing these exercises ) amputation, respiratory illness, blindness