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Re: my car is misfiring what could be causing this...
Probably a coil or the connector that goes from the coil to the spark plug.. unplug one coil wire at a time and listen for RPM loss.. The cylinder with the least RPM drop off could be your problem. Change that coil to another cylinder .. see if the problem travels with it.. if it does you have found the problem..Remove the coils and check the connectors for cracks and carbon tracking.. if it does not travel check the spark plug. This is a large problem.. missing cylinders leads to bad cats.. a very very expensive problem.
Look to rockauto.com for parts.
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misfires are more from faulty injectors / leaking injectors so check the codes as the misfire code will indicate the problem cylinder
exampl code p0300 is random misfires and codes p0301 through to p 0308 is the cylinder ie p 0303 is no 3 cylinder
plug wire , plug or a valve. usually plug wire will cause a miss a valve if on intake side will cause it to misfire sometime back into the intake. the plug can sometimes fire and sometimes not so simple replacement would fix. compression check would tell if it is valve issue.
The cheapest and easiest way to remove a possible cause is to put a set of new, good quality spark plug wires in there. Bad wires, especially where they snap onto the top of the plug, can cause misfires. Do not mix up the firing order, replace one wire at a time.
P0300 is a random engine misfire code. P 1781 is a transmission code, but it refers to a malfunctioning engine speed signal (crankshaft angle sensor). A bad crankshaft angle sensor or camshaft angle sensor can cause the P0300 code. A large list of items can cause a P0300 code, but since you have both, I'd be checking the crankshaft angle sensor and the related circuit. Sometimes, the mounting screws loosen up, which can cause the sensor to not function properly, all the time. Get this fixed, because over a period of 4 to 6 months, this can destroy the catalytic converter in the exhaust system ($$$$) Charlie
I would start with the cylinder 4 spark plug and make sure it look good and is firing correctly. After that I would look either at the final coil, ignition module and then o2 sensor. These problems are so hard to diagnose and are trial and error so much of the time.