P0300 – Random/multiple cylinder(s) -misfire detected

Trouble CodeFault LocationProbable Cause
P0300 Random/multiple cylinder(s) -misfire detected Spark plug(s), HT lead(s), injector(s), ignition coil(s), low compression, wiring

What Does P0300 Mean?

Cars built after 1996 will set a diagnostic trouble code in response to engine misfires.  The ECM misfire detection monitor is sensitive enough to distinguish individual cylinder misfires.   If one and only one cylinder misfire is detected, the code will be between P0301 – P0312 – depending on the cylinder affected.  A P0300 code is a “random misfire” which means multiple cylinders have misfired.

Troubleshooting P0300

Your common misfire is caused by either:

(1)  Spark problem

(2)  Compression problem

(3)  Timing Problem

(4)  Fuel Problem

It is a good idea before testing spark, compression, timing, or fuel to collect as much data about the car’s condition including: fuel trims, timing, oxygen sensors, engine coolant temperature, etc.  If your code is P0300, which indicates multiple cylinder misfires, connecting a scan tool that can read live data from the car will help diagnose which cylinders are misfiring.  Misfire codes can be set by failed sensors like oxygen sensors.  Be on your toes!  Approach a misfire systematically.

1. Spark Problem

When troubleshooting a misfire I like to begin with the quickest test – the spark plugs.  Inline spark testers are the best way to determine if your spark plug is bad.  The spark tester will light up if the spark plug works.   An inline spark tester costs less than $10.   For an intermittent misfire, there may be damage to the spark plug wiring.  Check the wiring insulation also.  Bosch Auto has excellent photos describing the cause of various damaged spark plugs. Compare Your Spark Plug: SparkPlugFaces

2. Compression Problem

Engine compression problems are often accompanied by audible changes from a normal engine.  If your engine sounds funny and you have a misfire code, a compression problem could be the cause.  Let’s take a look at the second step in diagnosing a misfire – compression testing.

Testing the compression of a cylinder is easy.  Attach your pressure gauge to the test cylinder and obtain a pressure reading by starting the engine.  A good cylinder reading is generally around 140 PSI.  Make sure you disable the ignition coil, fuel injectors, and spark plugs before testing the cylinder so that the car does not start during the test.  Also, make sure your battery is fully charged or the pressure readings will be low.  For more information, Linn Benton Community college has an excellent article on compression testing: http://www.linnbenton.edu/auto/perform/compress.html

A relative compression test should be performed on the engine to verify what a good cylinder reading is for your car.  The relative compression test is performed by testing one of the non-misfiring cylinders.  The low compression reading for the two cylinders should be within 70% of the high reading.

3.  Timing Problem

Important: Timing problems are not likely if only one cylinder is misfiring.  A timing problem cannot cause an intermittent misfire.

The best way to test your engine for a timing problem is the following:

  1. Remove cylinder # 1 spark plug
  2. Obtain access to engine crankshaft
  3. Rotate crankshaft clockwise by attaching the proper size socket to the crankshaft bolt
  4. Rotate until you reach top dead center (top dead center occurs when the piston is at its peak position within the cylinder) on the compression stroke of cylinder # 1
  5. Check distributor for timing with the crankshaft

Timing misalignment results when a tooth is jumped on either the camshaft and/or the crankshaft.

4.    Fuel Problem

Fuel Injection Return System

First, I will describe the most common fuel system – the fuel injection return system.  The components of the fuel system are the fuel pressure regulator, feed line, the rail, injectors, tank, return line and fuel pump.

Fuel Injection Return System

The fuel pump causes fuel to flow from the tank to the rail.  The rail feeds the fuel injectors.  The fuel injectors deliver fuel to the intake manifold or to the engine cylinders (direct injection).  The amount of fuel delivered by the injectors is controlled by the amount of time the injectors are held open not by fuel pressure.  Most cars have a standard fuel pressure of around 55 PSI.  Keeping the fuel pressure constant is the job of the fuel pressure regulator.  The fuel pressure regulator maintains the pressure by restricting the fuel path after the rail.

Fuel Pressure Regulator

The fuel pressure regulator senses the vacuum from the engine and restricts the flow of fuel according to the amount of vacuum.  At idle the vacuum is high.  At open throttle the vacuum is low which increases the fuel pressure.

Returnless Fuel Injection System

Newer cars have returnless fuel injection systems.  The returnless system does not have a return fuel line to the fuel tank.  Underhood Services has an excellent article on returnless fuel injection systems: http://www.underhoodservice.com/point-of-no-return-returnless-fuel-injection-systems/

Static Pressure Test

The first test you should perform on the fuel system is a static pressure test.  For this test you need to connect a fuel pressure gauge to measure fuel pressure.  Once the gauge is connected you will turn on the car but do not start the engine.  The pressure gauge should maintain a steady pressure (the car’s priming pressure) for a few minutes.  If the pressure does not change, then there are no fuel leaks in the injectors, the fuel pressure regulator, the fuel pump check valve, or an external fuel leak.  If the pressure does drop then further testing is necessary to find the leak.

If the car passes the static pressure test you can then start the engine.  A good test to perform is to increase the engine RPMs and measure the fuel pressure response.  The fuel pressure should not drop at the increased engine RPM.  If the pressure does drop then the air fuel mixture will be lean and your car will likely have a P0171 or P0174 troublecode.

Low Fuel Pressure

Fuel systems can exhibit low pressure because of a fuel pump, wiring, or restriction/filter problem.  If the fuel pressure to the rail is good, there can be problems with the fuel delivery through the injectors.

  • Testing a Fuel Pump:
  1. Test wiring with voltage drop from battery to the fuel pump.
  2. To eliminate wiring as a cause of the low fuel pressure can connect 12 Volt power source directly to the fuel pump and test fuel pressure.  If the fuel pressure is the same whether the fuel pump is connected to an external power source or to the car’s battery (through the wiring) then the wiring can be eliminated.  The fuel pump or a restriction is the cause.
  • Testing a Fuel Filter:
  1. Test fuel pressure before and after the fuel filter.  If the pressure is low before and after the fuel filter the fuel pump is bad.  A higher pressure before the fuel filter indicates a restriction in the fuel line or fuel filter.  Replace the fuel filter and retest.
  • Testing a Fuel Pressure Regulator:
  1. Snap Throttle Test – Throttle should be positioned fully open and then released.  The fuel pressure will initially increase to the priming pressure and then decrease below the idle fuel pressure.  Shortly after the low pressure reading, the fuel pressure regulator will cause the pressure to quickly stabilize at the idle fuel pressure reading.  If your car’s pressure readings follow the priming-low-idle pattern your fuel pressure regulator is functioning properly.
  2. Remove vacuum hose on the fuel pressure regulator -At idle the fuel pressure should increase when the vacuum hose is removed from the fuel pressure regulator.  The pressure should be similar to the priming fuel pressure found when the engine was turned off.  (When the engine is started, vacuum is applied to the fuel system through the fuel pressure regulator causing the fuel pressure to decrease.)

Common Symptoms & Causes of a Faulty Fuel Pressure Regulator

  1. Symptom – Engine is running rich (Negative long term fuel trim)
  • Cause – Leak in the vacuum hose to fuel pressure regulator
  • Tests –
  1. Does the engine only run rich at idle?  An engine that only runs rich at idle is a strong indication that there is a vacuum hose leak.
  2. Does the fuel trim/fuel pressure respond to the direct application of vacuum at the fuel pressure regulator?  If so, the fuel pressure regulator is functioning and the hose is causing the malfunction.  If there is no response to the application of vacuum, then most likely the diaphragm of the fuel pressure regulator has failed and the fuel pressure regulator needs replacement.


  1. Symptom – Engine is consistently running lean and you have low fuel pressure
  • Cause – Bad fuel pressure regulator
  • Tests –
  1. Pinch off the fuel return line at a rubber section of the fuel line.  The pressure should increase indicating your fuel pressure regulator is not restricting fuel flow to the rail and injectors.  If the fuel pressure is still low you will have to check the fuel filter, pump, and fuel lines.  If the fuel pressure increases but does not hold then there is a leak in the fuel system.  To test for an injector leak or leak at an internal fuel pressure regulator you should pinch off the fuel feed line and the return line with the engine off.  If the pressure does not hold there is a leak in the injectors, fuel rail, or the fuel pressure regulator.


High Fuel Pressure

Common Symptoms & Causes of a Faulty Fuel Pressure Regulator

  1. Symptom – Fuel pressure is high
  • Cause – Some fuel pressure regulators have internal filters that can clog.

Fuel Pressure Regulator Filter

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[…] Re: P0300 help Have a look here. http://www.troublecodes.net/pcodes/p0300/ […]


P0300 replace plugs fuel filter and regulator map sensor runs good for awhile then miss fire smell like gas need help


What vehicle are you running? How was it running before you did the tune-up?
Some vehicles don’t like aftermarket spark plugs or even certain branded spark plugs – did you use properly-gapped spark plugs recommended by the manufacturer? Does it run fine when cold and then go wonky when it gets hot? Are the spark plug wires or coils in good condition? If you have an EGR, how does the car run if you disconnect it? You could try rapping on the EGR with a hammer to see if it’ll close – maybe stuck open.
I know that misfire codes can be difficult to address. Try and go through the article one step at a time and you should come across the fault.