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Check Engine Light Code P0300 through P0312: Engine or Cylinder Misfire

January 1, 1970

Check Engine Light Series: Article #2

Did you know there are more than 1,400 reasons that can cause your vehicle’s “Check Engine” or “Service Engine Soon” warning light to turn on?The light is part of your vehicle’s on-board diagnostics (OBD-II) system and can indicate anything from a quick fix like a loose gas cap to a major problem, such as engine misfire.The primary purpose of the OBD-II system is engine management with a secondary focus on emissions reduction. Your car is programmed to alert you any time the engine control computer detects that a sensor or system isn’t performing the way it’s supposed to.Here’s another example of a problem that can trigger the light…

Trouble Code: P0300 (Random Cylinder Misfire) through P0312 (Cylinder 12 Misfire)

When your vehicle's onboard computer records an engine misfire code, it can be extremely helpful or terribly misleading. From P0300 (Random Cylinder Misfire) to P0312 (Cylinder 12 Misfire) and every code in between, misfire codes will lead you to the right neighborhood but not to the exact house – in a manner of speaking.Most vehicles produced today are equipped with coil-over ignition systems. Each cylinder still has a spark plug. Each cylinder also has its own ignition coil and coil/spark plug wire or boot, depending upon design. When the engine is started, the car battery supplies the ignition coils with voltage. The powertrain control module (PCM) then supplies a ground pulse at the appropriate instant, causing the coil to emit a high-intensity spark that travels through the spark plug and creates an explosion in the cylinder.When your PCM records an engine misfire code, the technician's ability to isolate the cause of the malfunction will depend upon diagnostic skill and understanding of the vehicle characteristics. Here are a few of the factors we consider when facing an engine misfire code:

  • We must consider the engine, cylinder head and ignition system design in order to map a diagnostic strategy.
  • While there are some instances when a technician may lay his/her hands immediately upon ignition coils, spark plugs, and spark plug cables the moment the hood is opened, much more frequently, there are parts which must be removed prior to beginning a diagnosis. In some cases, the upper intake must be removed to gain access to ignition system components. When this situation is encountered, we must carefully consider the mileage and specific maintenance records of the affected vehicle.

The Importance of the Regularly Scheduled Maintenance Tune Up

If the vehicle has more than 100,000 miles and has never had a "tune up," it is not uncommon to recommend a full tune-up complete with ignition coil replacement. The goal is to save you money and frustration in the future, and this is especially true if we must remove the upper intake to access your ignition components. After all, the labor will be the same to change a single ignition coil, boot and plug or to change them all.Today’s coil-over ignition vehicle doesn’t generally need a tune-up until the odometer reaches 100,000 miles. Because of the higher temperatures and increased workload placed on these modern ignition components, a complete ignition tune-up normally consists of spark plug, boot and coil replacement.Even in rare instances when my customers have declined a complete ignition tune-up and opted to replace only the defective coil/s, it seems that less than 20,000 miles will elapse before other coils begin to fail. This can lead to added expense, inconvenience and frustration. Usually, by the time the third (or fourth) coil has failed, the customer agrees that it would’ve been better to have replaced them all at one time, despite the added cost of the parts. Some clients hesitate to replace all of the coils at once because, at $100 or so each, the cost adds up quickly. But keep in mind that cars of yesteryear demanded a tune-up every 30,000-miles or less. They also required distributor caps, rotor buttons, points, cumbersome spark plug wires, etc. By the time you factor-in inflation and labor, it all works out about equally.Misfire Code ComplicationsThe design of the spark plug galleys also factors in. Many coil-over ignition systems feature spark plug galleys which are deep within the cylinder head, valve cover or intake manifold. In these situations, oil and coolant leaks must also be considered. Engine fluids which leak into spark plug galleys will saturate rubber boots and cause the high-intensity spark to ground prematurely (inside the galley). It is not uncommon to see the entire spark plug galley full of oil or coolant. This will leave parts of the ignition coil, the rubber boot and upper part of the spark plug totally submerged in fluid. This eliminates spark in the combustion chamber and leads to a stored misfire code.When this occurs, the oil or coolant leak simply must be repaired before the ignition components are replaced. Oil in the plug galley is typically caused by a leaking valve cover gasket or grommet. Valve cover replacement and a thorough clean-up usually rectifies the situation. Coolant in the spark plug galley may be from a leaking heater hose or intake manifold gasket. Either way, the leak will need to be repaired before new tune-up parts are installed. Sometimes my valued customers will ask, “How bad is the leak?” I reply, “There is no good leak.” In this case, even a slight leak will contaminate rubber parts and accumulate over time until the misfire returns.Keep in mind that a P0300 through P0312 Misfire Code does NOT guarantee that an ignition component is defective. A careful and complete diagnosis must be performed before replacing parts at random. Misfire codes may also be caused by engine vacuum leaks, insufficient fuel delivery, partially clogged EGR passages, even sticking lifters and valves (especially on GM cars with variable displacement systems).Note that only a misfire code will cause a flashing Service Engine Soon light. This indicates an urgent condition. If the misfire is so evident that the Service Engine Soon lamp is flashing (usually under acceleration), it means that catalytic converter damage is imminent. It can easily cost a couple thousand dollars to replace a failing catalytic converter. If your Check Engine Light begins to flash, we urge you to get the vehicle in for diagnosis and repairs ASAP.Common Causes of Misfire Codes

  • Defective ignition coil
  • Bad spark plug wire or boot
  • Faulty spark plug
  • Oil or moisture on high-energy ignition components

Less Common Causes

  • Bad fuel injector
  • Clogged EGR ports (inside intake manifold)
  • Low engine compression
  • Wiring or connector issues

What To Do When Your ‘Check Engine’ Light Comes OnWhen your Service Engine Soon light turns on, you won’t know whether it’s because of a P0300 – P0312 or a completely different issue. The first thing we advise is confirming that it is the ‘Service Engine’ or ‘Check Engine’ light. If you’re not sure, consult your owner’s manual.If it is the engine light, see if the gas cap is to blame. Check to make sure the gas cap is tight and that there’s no rust or debris preventing it from sealing. If that all checks out and the light doesn’t turn off within the next several miles, you’ll want to schedule an appointment with Auto Lab for further testing. We’ll be able to retrieve the code(s) and perform pinpoint testing to zero in on the actual culprit. From there, we can provide an accurate cost estimate for the repair.See our detailed guide on how to respond when a dashboard warning light turns on.

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