P0303 1996 Pont Grand Prix

possible causes for code P0300
you've already change coil and plug....

P0300 could be bad #3 plug or coil... switch those also and see if code change..

if it persist try the next thing..

I would try trading fuel injector 4 to 5 and putting 5 into 4 and see which code you get.....

• Faulty spark plug (s)
• Faulty ignition coil (s)
• Clogged or faulty fuel injector (s)
• Intake air leak
• Fuel injectors harness is open or shorted
• Fuel Injectors circuit poor electrical connection
• Ignition coils harness is open or shorted
• Ignition coils circuit poor electrical connection
• Insufficient cylinders compression
• Incorrect fuel pressure

code P0305 is simmilar....

• Faulty spark plug 5
• Clogged or faulty fuel injector 5
• Faulty ignition coil 5
• Fuel injector 5 harness is open or shorted
• Fuel injector 5 circuit poor electrical connection
• Ignition coil 5 harness is open or shorted
• Ignition coil 5 circuit poor electrical connection
• Insufficient cylinder 5 compression
• Incorrect fuel pressure
• Intake air leak

I don't know liter size of engine? I don't know exactly what testing you have done? I need more info?
You have dis ignition? Engine cranking, do you have spark at the plugs, proper fuel pressure and fuel injector pulse?
The purple wire with white tracer from ignition module to computer has a dc pulse with engine cranking? With bypass ignition, engine cranking, it's the ignition module that controls spark, during cranking rpms. Did you check voltage and ground for ignition module. With dis ignition, there is a crank sensor? I'm not saying replace anything. I would have replaced the --spark plugs and plug wires, Possibly the ignition coils.
I just don't know exactly what testing you have done?
If you have already done everything I suggested, then, pardon my post. Just trying to help.

P1401 Differential Press Feedback EGR Circuit High Input
Your EGR port is plugged. Replace your EGR valve, and clean the ports
A code P0302 may mean that one or more of the following has happened:

• Faulty spark plug or wire
• Faulty coil (pack)
• Faulty oxygen sensor(s)
• Faulty fuel injector
• Burned exhaust valve
• Faulty catalytic converter(s)
• Running out of fuel
• Poor compression
• Defective computer
• A code P0305 may mean that one or more of the following has happened:
  
  • Faulty spark plug or wire
  • Faulty coil (pack)
  • Faulty oxygen sensor(s)
  • Faulty fuel injector
  • Burned exhaust valve
  • Faulty catalytic converter(s)
  • Running out of fuel
  • Poor compression
  • Defective computer
  • A code P0301 may mean that one or more of the following has happened:
    
    • Faulty spark plug or wire
    • Faulty coil (pack)
    • Faulty oxygen sensor(s)
    • Faulty fuel injector
    • Burned exhaust valve
    • Faulty catalytic converter(s)
    • Running out of fuel
    • Poor compression
    • Defective computer
    • A code P0302 may mean that one or more of the following has happened:
      
      • Faulty spark plug or wire
      • Faulty coil (pack)
      • Faulty oxygen sensor(s)
      • Faulty fuel injector
      • Burned exhaust valve
      • Faulty catalytic converter(s)
      • Running out of fuel
      • Poor compression
      • Defective computer
      • p1409
      • EGR Vacuum System Leak - Read Our Article on EGR Valve Codes For Help With This Oldsmobile Check Engine Light Code

How old are the spark plugs and wires. GM vehicles are great runners until something goes wrong with the typical spark or fuel delivery issues. Wires or fouled plugs may not set off the computer, it will just run crappy.

or you getting the right fuel pressure? check fluid filter, check pressure regulator, check spark plugs to see if they are soaking wet if they or wet take them all out and pull fuel pump relay out turn engine over and blow all gas out the plug holes. install plugs and relay and try start again , check engine compression, check coolant senor ,check cam sensor

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.

It could very well be a bad coil pack,or you have low compression in that cylinder.If the compression is good then go with the coil pack,if not you may need a head gasket or an intake manifold gasket.

Sound like an air / vacuum leak to me.
Re-check for an air leak on #2 cylinder inlet manifold.
Use a propane torch (unlit, of course) to flood propane gas around inlet manifold/head flange & gasket area.
If there is any change in idle speed during this test you have a vacuum leak.

The Specific code "P0303" is stating that the Third Cylinder or Cylinder number 3 Is mis-firing, thanks to all that you have done this has narrowed it down to two "confirmed" possible problems;

The first is that the Exhaust VALVE itself may be "Burned" and the Second may be a Poor condition or "Faulty" Catalytic converter(s).

This is only a possible cause but please keep in mind that on some certain models (not too sure about this specific model) there are multiple O2 Sensors located in different spots. (usually in or next to the preliminary and primary catalytic converts)

Also one of my experiences lead me to beleive that the Ignition Control Module (ICM) may be faulty causing the injector to not provide proper Fuel to the Cylinder.which will more often than not cause the computer chip to sense the misfiring as opposed to the bad signal from the ICM.

Hope it helps you out, and good luck.

I would inspect the connecter and wires for the #3 cylinder fuel injector for damage.