The engine control module (ECM) is responsible to moment-by-moment operation of the engine. Using multiple sensors and actuators, the ECM ensures the engine is running at its most-efficient. For the driver, this means optimal power output and minimal emissions. The ECM constantly monitors engine running conditions, adjusting output depending on ever-changing conditions. If the ECM detects a problem, such as a cylinder misfire, it will set a diagnostic trouble code (DTC) in memory and illuminate the check engine light (CEL).
DTC P0300 describes a random cylinder misfire or multiple cylinder misfire, depending on the vehicle. The main idea that separates P0300 from a specific misfire DTC, such as PO301 or P0305, is that the ECM cannot determine which cylinder is misfiring or has determined that multiple cylinders are misfiring.
If random or multiple cylinders are misfiring, you’re sure to note some common symptoms that point to it, such as hard starting, rough idle, stalling under load, poor engine performance, and increased fuel consumption. In some cases, the engine may not idle at all, but only run with the foot on the accelerator. If the misfire is severe enough, the CEL may blink, indicating possible damage to the catalytic converters.
In addition to noting these common random misfire symptoms, noting operating conditions, such as weather, traffic, road, or demand, may help when it comes to troubleshooting DTC P0300.
Whereas diagnosing a specific cylinder misfire might lead to you focus on a single cylinder’s operating characteristics, diagnosing a random cylinder misfire should lead you to consider problems that affect all cylinders or one bank. Remember, at its most basic, efficient engine operation depends on the precise mixing and timing of air, fuel, and spark, so you’ll need to look at things like air intake, vacuum leaks, fuel pressure, fuel flow, etc.
Air – Check the air intake hose, after the mass air flow (MAF) sensor, for leaks or loose connections. Also, check vacuum hoses for leaks. There may be many vacuum lines, going to things like EVAP purge, vacuum brake booster, power-steering idle up switch, EGR actuator, fuel pressure regulator, etc. Small vacuum leaks typically lead to fuel trim problems, but a big vacuum leak can lead to a random misfire. Engine valve timing is usually one of the last things to check, which could lead to a multiple cylinder misfire.
Fuel – Instead of focusing on individual fuel injectors, focus on system-wide fuel issues, such as the fuel pressure regulator, fuel lines, and fuel filter. Check for pinched or kinked fuel lines, which can restrict fuel delivery. Check fuses and relays to ensure the fuel pump stays on while the engine is running. Checking for a restricted fuel filter might be more difficult, and usually isn’t considered on low-mileage vehicles.
Spark – This may not apply to vehicles with individual Coil-on-Plug (COP) ignition coils, but more to vehicles that have one or two ignition coils and spark plug wires. Use a spark tester to test the spark plug wires, and remove spark plugs for inspection.
DTCs – Concurrent DTCs can also help you close in on the problem. Check for any DTC that refers to air flow, air pressure, fuel pressure, or ignition coils. Knock sensor DTCs may affect one bank on a V6 or V8, and variable valve timing (VVT) or engine timing DTCs may indicate valve timing, spark timing, compression, or oil pressure problems.
There are a great many things that can cause a random cylinder misfire, and it would be impossible to list them all, but here are some of the most common:
Faulty Ignition Coil – COP engines don’t usually have this problem, but a central coil or one that fires a pair of cylinders could fail, affecting just those cylinders, leading to a “multiple” cylinder misfire.
Worn Spark Plugs – Individual spark plugs wouldn’t typically cause individual cylinder misfires, but high-mileage or poorly-maintained vehicles might simply need a new set of plugs to restore spark strength and definition.
Worn Spark Plug Wires – This is usually something that comes up more in wet weather, but worn spark plug wires may have miniscule cracks in the insulation. Humidity or wetness can cause short circuits and cylinder misfires that seem intermittent or random but paying attention to the weather might clue you in on this one.
Stuck EGR Valve – The EGR valve is usually most-open at highway speeds, but a stuck EGR valve may fail to close completely when the vehicle returns to local driving speeds. A concurrent EGR-related DTC might reveal this problem, or simply tapping on the EGR valve with a hammer might loosen it so it closes, but replacing it is the only real solution.
Faulty Sensors – Because the ECM uses camshaft and crankshaft position sensors (CKP & CMP) for cylinder misfire detection, faults here could lead to false misfire DTCs. Check for loosely mounted sensors and concurrent CMP or CKP sensor DTCs.
Contaminated Fuel – Aside from accidentally filling a gas engine with diesel or vice-versa, contaminated fuel can cause a couple of problems. Certain fluid contamination might simply be non-flammable and not burn in the cylinder. Flushing the fuel system should help with this, and be sure to always use fuel stabilizer on stored vehicles and fuel. Particulate contamination isn’t common, but can stuff a fuel filter or the fuel injectors very quickly, starving the engine for fuel. Flushing the fuel system and replacing clogged components can get expensive.
Stuck Idle Air Control Valve – Carbon can cause the idle air control (IAC) valve to stick, and usually results in hard starting, rough idle, or a no-start condition. Try starting the engine with the accelerator slightly depressed. Also, check for concurrent IAC-related DTCs.
Skipped Timing Belt – Anything from belt stretch to mice to gremlins can cause timing belt skipping, usually just one or two teeth. The engine will still run, but compression, valve timing, and spark timing will be off. Check that the timing marks line up correctly at top dead center.
Low Oil Pressure – Most VVT systems rely on oil pressure to operate, but low oil pressure may prevent them from working at all. On V6 or V8 engines, faulty valve timing may result in one bank of random misfires, while it would affect all of an i4, i5, or i6 engine. Check oil level and VVT screens for blockage.
There are perhaps a dozen other things that might be added to this list, but we’ve tried to keep it as close to the most-common problems as possible.