random engine misfire
Auto Repair Library, Auto Parts, Accessories, Tools, Manuals & Books, Car BLOG, Links, Index, CarleySoftware

P0300 random misfire dtc scan tool



If your CHECK ENGINE light is on and you have found a P0300 diagnostic trouble code (DTC) by plugging a scan tool or code reader into your vehicles diagnostic connector, your engine has a "random misfire" problem.

A random misfire code can be set on newer vehicles with OBD II onboard diagnostics when multiple misfires occur randomly in multiple cylinders. The cause is typically a vacuum leak in the intake manifold, throttle body or vacuum plumbing, a defective Exhaust Gas Recirculation (EGR) valve that is leaking exhaust into the intake manifold, or even bad gasoline. Less common causes include bad spark plug wires, worn or fouled spark plugs, a weak ignition coil, dirty fuel injectors, low fuel pressure, or weak valve springs. If a misfire is occurring in only one or two cylinders, you will usually find a misfire code for that specific cylinder rather than a random misfire code.

If the engine is misfiring but no codes have yet been set, and you have a scan tool that can access Mode 06 data (the raw misfire counts that are being tracked by the OBD II system), use your scan tool to look at the Mode 06 cylinder misfire counts. If you see one or more cylinders that are showing misfire counts, focus your investigation on those cylinders. The OBD II system usually will NOT set a misfire code until the rate of misfires exceeds about two percent.

The exact criteria for setting misfire codes will vary from one vehicle manufacturer to another, and from one model year to the next, but the chart below gives an example of what will trigger various misfire codes on General Motors engines:

general motors misfire code chart

A random P0300 misfire code or cylinder specific misfire codes (P030X) will be set when the OBD II system detects misfires that may increase emissions 1.5X the legal limit, or if the misfire rate is high enough to cause possible damage to the catalytic converter. The latter is worse and will usually cause the Check Engine light to flash while the misfire is occurring.


Start by checking engine intake vacuum. Connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and note the vacuum reading.

On most engines, intake vacuum should be steady between 16 and 22 inches. A lower reading usually indicates a vacuum leak, but it might also indicate an exhaust backpressure problem (such as a plugged catalytic converter) , worn valve guides or weak valve springs. A vacuum reading that gradually drops while the engine is idling almost always points to an exhaust restriction. An oscillating vacuum reading usually indicates a leaky valve or worn valve guides.

diagnostic checks
The first step in diagnosing a misfire
is to use a scan tool to check for misfire codes.

If the engine is experiencing any of the following symptoms, a vacuum leak is probably causing the P0300 random misfire DTC:

Too fast an idle speed. The powertrain control module (PCM) will maintain normal idle speed and compensate for a small vacuum leak by closing down the throttle body air bypass. But if the leak is too large and the idle control system cannot compensate for the extra air, the engine may idle too fast. Common leak paths include the throttle body gaskets, intake manifold gasket, any of the engine vacuum fittings, hoses or vacuum-operated accessories (such as the power brake booster or EVAP canister purge valve). It is even possible that leaky O-rings around the fuel injectors may be allowing air to leak past the seals. Another overlooked item can be a worn throttle shaft.

A rough idle or stalling. A large vacuum leak can lean the air/fuel mixture out to such an extent that an engine will not idle at all. An EGR valve that is stuck open at idle can have the same effect as a vacuum leak. So too can a loose positive crankcase ventilation (PCV) hose, a leaky PCV valve or the wrong PCV valve (one that flows too much air for the application). If somebody replaced the PCV valve recently, they may have installed the wrong PCV valve. The rough idle in all of these cases is caused by "lean misfire." The fuel mixture is too lean to ignite reliably so it often misfires and fails to ignite at all. Lean misfire will show up as elevated hydrocarbon (HC) readings in the exhaust, which may be enough to cause a vehicle to fail an emissions test.

For more information about diagnosing lean codes, see Lean Codes (P0171 & P0174)

Hesitation or misfiring when accelerating. This may be due to a vacuum leak, but it can also be caused by dirty fuel injectors, a weak fuel pump (low fuel pressure) or a faulty fuel pressure regulator. It can also be caused my ignition misfire due to worn or fouled spark plugs, bad spark plug wires, or weak ignition coil.

The important thing to keep in mind about vacuum leaks is that they have the most noticeable effect at idle. At part and full throttle, more air is entering the engine through the throttle opening so a vacuum leak has less effect on the air/fuel ratio.


Start by visually inspecting all the vacuum hoses and connections. Look for disconnected, loose or cracked hoses, broken fittings or other obvious problems. Also, listen to the engine while it is idling (outdoors only or with proper exhaust ventilation, NEVER indoors!) Vacuum leaks will often make a sucking or whistling noise.

Looking at fuel trim values with a scan tool is another technique you can use to diagnose vacuum leaks and other problems. For more information on this subject, see What Is Fuel Trim?

A faster technique for finding vacuum leaks is to get a bottle of propane and attach a length of rubber hose to the gas valve. Open the valve so you have a steady flow of gas. Then hold the hose near suspected leak points while the engine is idling. If there is a leak, propane will be siphoned in through the leak. The resulting "correction" in the engine air/fuel ratio should cause a noticeable change in idle speed and/or smoothness (Note: you may have to temporarily disconnect the idle speed control motor while doing this test). Use caution because propane is highly flammable and can be ignited by a spark or flame.

A "smoke machine" can also be used to find small leaks. A smoke machine feeds artificial smoke into the intake manifold (engine OFF). The smoke may also contain ultraviolet dye to make it more visible when illuminated with UV light. If you see smoke coming out of a hose, fitting, connection, manifold gasket or a crack in the manifold, you have found the leak. Note: Plastic intake manifolds can crack, often in places that are difficult to see. The most vulnerable places are plastic vacuum ports on the manifold.

A less effective method for finding a vacuum leak is to lightly pressurize the intake manifold with about three (3) lbs. of regulated air. Use an adjustable regulator on the air supply and feed air into the intake manifold through a vacuum port. DO NOT use any more pressure than this! Plastic intake manifolds can be easily damaged by excessive pressure. With the engine off and the intake manifold pressurized, spray soapy water on suspected leaks. If you see bubbles, you have found the leak.

Propane can also be used in conjunction with an infrared exhaust analyzer. Vacuum leaks almost always cause carbon monoxide (CO) readings to drop, and fluctuating hydrocarbon (HC) readings in the exhaust. The exhaust analyzer can (1) tell you if there is indeed a leak, and (2) where the leak is using the propane procedure described above. The trick here is to look for a rise in CO and a drop in the HC readings while directing propane at suspected leak points on the engine.

Two types of vacuum leaks can be diagnosed with an exhaust analyzer. The first kind is a general vacuum leak (PCV hose, brake booster, etc.) that leans out the mixture and causes a very low CO reading and only a slightly higher fluctuating HC reading. The O2 reading will also be high. The second kind of vacuum leak is a "point" leak that affects only one or two cylinders (a leaky manifold gasket or a crack or porosity leak in one of the manifold runners). This will be indicated by a normal or low CO reading combined with high fluctuating HC readings. O2 will again be high.

It is important to note that an overly lean idle mixture will also cause a fluctuating HC reading the same as a vacuum leak. To tell one from the other, there is a simple "trick" you can use. Momentarily enrich the idle mixture to 1.5 to 2.0% CO by directing propane into the throttle body. If the engine smooths out and HC drops and remains stable, the problem is a lean idle condition. Check for dirty injectors, low fuel pressure or a defective fuel pressure regulator. If HC still fluctuates, however, the engine is still too lean in one or more cylinders indicating a vacuum leak.

If you suspect a vacuum leak in a particular vacuum circuit, use a hand vacuum pump to isolate the fault by applying vacuum to the hoses and connections until you find the one that does not hold vacuum. On some vehicles, there may be a vacuum reservoir, a small metal canister that holds vacuum. These can sometimes leak.

exhaust gas recirculation valve


The EGR valve should remain CLOSED when the engine is COLD and IDLING. When the engine is warm and is accelerating or working hard under a load, the EGR valve should OPEN. With vacuum-operated EGR valves, intake vacuum pulls the valve open when conditions are right. Some EGR valves also sense exhaust backpressure and do not open until backpressure reaches a certain level. On others, the EGR valve is electronic and is controlled by the PCM. No vacuum is used to operate the valve.

When the EGR valve opens, it allows a small amount of exhaust gas to be siphoned back into the intake manifold. This dilutes the air/fuel mixture, cools combustion and reduces the formation of oxides of nitrogen (NOx). If the EGR valve fails to close when EGR is not required, or if it leaks because of carbon buildup on the valve stem or valve seat, exhaust will be sucked into the intake manifold all the time creating a lean fuel mixture. This will cause lean misfire and set a random misfire code. Cleaning the EGR valve seat and stem may eliminate the sticking problem. If this fails, replace the EGR valve.

On some engines, this kind of problem may not appear until the engine warms up. The EGR valve may remain closed while the engine is cold, but it starts to leak when the engine is warm. This may be due to a weak spring inside the EGR valve. The fix here is to replace the EGR valve.

The operation of the EGR valve can be checked several ways. One is to observe the valve stem (if possible) while goosing the throttle. No movement would indicate a problem with the EGR valve or its vacuum supply. Another is to apply vacuum directly to the EGR valve with a hand pump with the engine running at fast idle. This should pull open the EGR valve and cause a momentary drop in rpm. A third method is to replace the EGR valve and see if that cures the random misfire problem.

compression gauge


Remove the spark plugs and use a compression tester to check compression. A cranking compression test is done with the ignition disabled and the throttle held open. On most engines, cranking compression should be 140 to 160 pounds per square inch (PSI) or higher in every cylinder. If low, the cylinder may have a burned exhaust valve or leaky head gasket, or weak or broken valve spring. A "wet" compression test can then be performed to determine if the problem is the rings, or the valves or head gasket. Squirt a few drops of oil into the cylinder through the spark plug hole to temporarily seal the rings. Repeat the cranking compression test. If the readings are now higher, the problem is worn rings. If there is no change in the readings, the engine has a valve problem or a leaking head gasket.

Possible causes of low compression include leaking head gasket, burned exhaust valve, bent intake or exhaust valve, weak or broken valve spring, or worn camshaft lobes.


Random misfire caused by a loss of spark or a weak spark may be due to fouled spark plugs, plug gap too wide, bad plug wire, weak ignition coil, cracks or carbon tracks in an ignition coil or rotor

Remove and inspect the spark plugs. If oil or carbon fouled, the engine is probably sucking oil past worn valve guides and seals. Replacing the spark plug will only temporarily solve the misfire problem.

If the spark plug is normal, check the plug wire and boot for damage or looseness. Measure resistance end to end. Replace the wire if resistance exceeds specifications (about 50,000 ohms/foot). If the spark plug wire is loose, damaged, burned, chaffed or exceeds resistance specifications, replace it. On coil-on-plug (COP) ignition systems, there is no spark plug wire so inspect the coil sheath for cracks or carbon tracks.

If ignition and compression are both okay, focus next on the injectors. Use a test light or voltmeter to see if the injector is receiving voltage when the key is on, and that the injector solenoid clicks when the injector circuit is grounded. If the injector is functioning electrically, it may be clogged with fuel varnish deposits. If cleaning cannot restore normal fuel delivery, the injector must be replaced.

Misfire in two "paired" cylinders in an engine that has a distributorless ignition system (DIS) would usually tell you the DIS system has a bad coil.


Random misfires can occur if the fuel injectors fail to delivery their normal dose of fuel to the engine. This creates a lean air/fuel mixture and increases the risk of lean misfire. The underlying cause may be dirty fuel injectors (a buildup of varnish deposits in the injector nozzles that may occur over time from using low detergent gasoline or frequent short trip stop-and-go driving), or low fuel pressure caused by a weak fuel pump, defective fuel pressure regulator or dirty fuel filter.

One way to check for dirty injectors is to use a "pressure drop test." This requires a fuel pressure gauge, connecting the gauge to the fuel supply rail and comparing the drop in pressure when each injector is energized for a few seconds with the key on engine off. If an injector shows a significantly lower pressure drop than the others, it is restricted and should be cleaned or replaced.

One quick fix here would be to clean the injectors. A can or two of concentrated fuel injector cleaner added to the fuel tank (follow the product's directions) may remove some or all of the deposits that are restricting fuel delivery. Off-car injector cleaning on a special injector cleaning machine usually gives the best results,

Check fuel pressure. If it is not within specifications, there is a problem that will require further diagnosis. The fault usually be a weak fuel pump, defective fuel pressure regulator or a dirty fuel filter

Don't overlook the possibility of "bad" gasoline. Gasoline contaminated with water or excessive amounts of alcohol will run lean and cause misfires.

To read more about misfire detection Click Here

scan tool companion information


A scan tool or scanner software can help you diagnose a random misfire When the OBD II system detects a misfire problem and sets a P0300 series code, the LAST TWO DIGITS of the code will correspond to the number of the misfiring cylinder (P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0309, P0310, P0311, P0312). A code P0304, for example, would tell you cylinder #4 is misfiring. If you only find a code P0300. Refer to the vehicle service literature to determine how the cylinders are numbered as this will vary from one engine to another. If the misfire is random and jumps from cylinder to cylinder, the OBD II system will set a P0300 random misfire code.

Regardless of what kind of misfire problem the engine is having, you can use a scan tool to look at various engine performance data perimeters (PIDs). These are the system inputs that the PCM uses to control the engine. They include sensor inputs such as throttle position, engine temperature, mass air flow, manifold absolute pressure, open or closed loop status, oxygen sensor voltage values and so on. Professional grade scan tools can access all of this information plus special onboard diagnostic tests that are not available with a typical do-it-yourself or inexpensive entry level scan tools, or scanner software that lacks bidirectional communication.

You can use a scan tool to read and clear codes, to read sensor data, and to verify the vehicle is ready to pass an OBD II plug-in emissions test.

For a list of scan tool suppliers, Click Here.

engine misfireMore Driveability & Emissions Related Articles:

Misfire Diagnosis

Misfire Diagnosis Chrysler 3.5L V6

Finding & Fixing Vacuum Leaks

What Is Fuel Trim?

Lean Codes (P0171 & P0174)

Don't Neglect the Spark Plugs

Why Spark Plugs Still Need To Be Replaced

Check Engine Light OnBoard Diagnostics

More on Check Engine Lights & Diagnostics

Scan Tool Diagnostics

Decoding Onboard Diagnostics


Understanding OBD II Driveability & Emissions Problems

OBD II Diagnostic Tips

Mode 06 Diagnostics

To More Technical Info Click Here to See More Carley Automotive Technical Articles

Additional Auto Repair Resources:

CarleySoftware Quick Reference & Training Guides

Link to CarleySoftware

Mitchell 1 DIY eautorepair manuals

Mitchell Repair Information for the Do-it-Yourselfer


Privacy Policy