I have a 2000 pontiac grand am 2.4 it has a code p0300 i have changed coil packs ,spark plugs,sprayed propane to check for vacume leak it was fine ,checked compression ,and it is still having a rough idle and does not sound like it is going to stay running and i can drive it down the road i had it to the floor and the rpms was at 5 and i never got over sixty mph it is missing really bad a year ago i replaced fuel pump ,catilac converter ,both oxygen sensors,fuel filter, crank module and the spark plug boots, and the white thing that covers coil packs what could be wong please help

Definition of Diagnostic Trouble Code P0300 Category: Powertrain Definition: Random/Multiple Cylinder Misfire Detected

P0300 Diagnostic Code - Random/Multiple Cylinder Misfire Detected

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 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 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.


Hope helps, and keep us updated.

Hi there,
Have you tried vacuum leak test?
You have already changed the most common problematic parts for your code.
What you would need is a common shop propane torch.
How you use.
1. Start your engine.
2.now take the propane torch (Don't light it!!!) crack open the valve.
3. point the valve at each vacuum fitting on the intake manifold.
what your doing is if there is a leak, the propane will be sucked into it.
This will cause the ideal to rise. When it rises you have found your leak.
Also check where the intake meets the head.
If you have a leak it will cause the effected cylinder to run very lean (to much air to fuel mixture) and cause your misfired cylinder.
I hope this help. Give feed back if this helps anyone. Fixya

try switching one of the other coil packs with one of the bad ones. if this works then there you go. another way of telling if it is a coil pack is to disconnect the spark plug of the affected coil pack. if its a strong coil then you will hear the spark as well as see it. compare it to another plug boot that has the good spark. it sounds to me that it is a coil pack. i just changed one about 2 months ago with the same problem as yours.

i work for autozone as a tech first question did the obd2 give codes if so what are they the person representing autozone might be green ..
are you getting a single misfire of random misfire
if you are single and you did what you said then do this get a can of spray carb cleaner put the red tip in it start the car and let it idle spray the carb cleaner around the intake and all the vacum lines
if your engine rears up you have a vacum leak eather in the rubber or in the intake manifold
if you had a multi cylinder misfire change the crank sensor that will smooth the engine out
if it runs like a dog check the cam sensor if you car has one ( some use the crank sensor also as a cam sensor
make sure you disconnect the battery before doing anything (mamdatory) after your done re connect the battery and start the car wait 5 min to see if the check engine light comes back on then do another obd2 check at autozone get someone who is well schooled in ignition systems like me if you get another code let me know what the code is and i'll try to help with the explanation

A code “multiple misfire” 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 EGR valve / passages
•Faulty camshaft position sensor
•Defective computer

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.

They may not be firing. I would check spark from coils. You may have a bad coil pack. Do check for any vacuum lines. You can go to autzone.com and register for full free online repair manual

this vehicle has coil packs for the ignition correct? have you ran the engine at night with all surrounding lights outs so its really dark. spray a mist of water on the coil packs and spark plug wires to see if you have spark/voltage leaks jumping out from them. sometimes the tiny sparks will be so fine and skinny you can barely see them so it needs to be dark. hope this helps your trouble shooting. a coil pack could have such a fine crack for the voltage to leak from that you cant see it.

try unplug the up streem O2 sensor drive the car for a while then check the code if its not p0300 change the O2 sensor

Im surprised that didn't fix it. On the Quad4 engine its usually a cracked coil housing.
Have you checked:

• fuel injectors / fuel pressure
• coolant temp sensor
• compression - to check for bad head gasket
• ignition control module
• EGR system - make sure the valve isn't stuck
• crank position sensor.

The crankshaft position system variation compensating values are stored in the PCM non-volatile memory after a learn procedure has been performed. If the actual crankshaft position system variation is not within the crankshaft position system variation compensating values stored in the PCM, DTC P0300 may set.