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P0300 Diagnostic Code - Random Misfire Technical Description Random/Multiple Cylinder Misfire Detected What does that mean? Basically this means that the the car's computer has detected that not all of the engine's cylinders are firing properly. A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why. Symptoms Symptoms may include: the engine may be harder to start the engine may stumble / stumble, and/or hesitate other symptoms may also be present Causes A code P0300 may mean that one or more of the following has happened: Faulty spark plugs or wires Faulty coil (pack) Faulty oxygen sensor(s) Faulty fuel injector(s) Burned exhaust valve Faulty catalytic converter(s) Stuck/blocked/leaking EGR valve / passages Faulty camshaft position sensor Defective computer Possible Solutions If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back. 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. Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open. It is helpful to have access to a factory service manual and an advanced scan tool to properly diagnose a random misfire DTC. P0300 Diagnostics Video Here is a helpful video that may assist you in learning more, diagnosing, and repairing your P0300 DTC. This video is intended for auto repair professionals but could be helpful to DIYers. It shows use of an advanced OBD-II scan tool during diagnostic steps:
first go over the plugs and wires again. Make sure plugs are gapped correctly, wire boots have good wire connection in the boot top, and the boot is seated correctly on the plug stem. ensure plug wires are soundly seated/connect at the other end. If you still get the misfire code -- get the engine diagnosed -- a sensor or a computer module may be faulty.
The plugs and crank sensor could have done it, but the TPS and idle air control would have nothing to do with a random miss. A more likely cause is the ignition module or a fuel problem. You may have to wait until the problem is bad enough to kill the engine or you have more codes to check out.
Up to this point in time, mechanic 1 has only done some maintenance work on the car. Things that would be done from time to time to maintain the car. Since you have lost trust in this mechanic, I would go to a different place and have them diagnose the codes. If the second place gives you the same recommendation you can decide what to do. You can tell the second place you are getting a second opinion, or not.
No, it's not time to replace your PCM. It's time to properly diagnose and repair the problem. Most Oxygen sensor codes are not the fault of the oxygen sensor. codes P0132 and P0152 mean that both of your upstream O2 sensor circuits are shorted to voltage. It does not mean that the O2 sensors themselves are bad. The code P0300 is a random misfire code. This does not mean change the spark plugs. This means that several or all of the cylinders are misfiring. This could be due to spark plugs, but you will most likely get a single cylinder code (like P0301 or Po302 etc.) if the spark plugs are getting worn out. This random misfire code is most likely being caused by your shorted oxygen sensor circuits. If they are shoted to voltage they will cause your fuel to run extremely lean (vey little fuel) to the point that it is causing the cylinders to misfire. Code P0300 can have seveal different causes, including a fuel pump getting worn out, am manifold vacuum leak, an intemittent crank sensor signal, etc. When other codes are being set at the same time as a P0300, the other codes need to be diagnosed and repaired first.
Make sure you're using the recommended spark plugs. When my car did that, I had put Bosch Platinum Plus-4 plugs in and got that same problem. Turns out the plugs are too hot and cause it to fire prematurely and kick the misfire code.