P0303 – Meaning, Causes, Symptoms, & Fixes

Code P0303 Definition

Cylinder 3 misfire detected.

  • Commonly associated with codes: P0300

What Does P0303 Mean?

P0303 indicates that cylinder number 3 is experiencing misfires. A misfire occurs when an insufficient amount of fuel is burning in a cylinder. The efficient burning of fuel is essential to engine operation as the combustion of fuel is what provides the energy to power the engine. A misfire from one or more cylinders can be caused by many reasons from a faulty ignition system, fuel system or internal engine failure. When P0303 occurs it should be fixed immediately as long term driving with engine misfires could cause consequential damage to your engine.  Many times P0303 occurs when there are worn out spark plugs, spark plug wires, or a faulty ignition coil.

P0303 Symptoms

  • Check Engine Light is on
  • Check Engine Light flashing
  • Engine runs rough and shaking
  • Lack of power from the engine
  • Fuel smell from the exhaust
  • Hesitations/Jerking when accelerating

P0303 Causes (* = Most Common)

  • Faulty or worn spark plugs *
  • Faulty spark plug wires or coils *
  • Distributor failure *
  • Faulty fuel injector *
  • Vacuum leak
  • Low fuel pressure
  • Camshaft sensor defective
  • Crankshaft sensor defective
  • Engine timing off
  • Leaking head gasket
  • Low engine compression
  • Poor quality fuel

Code P0303 Severity – Severe    

P0303 should be repaired immediately. Ignoring this error could result in ignition failure, catalytic converter damage, and unsafe/dangerous conditions while operating the vehicle.

Code P0303 Common Diagnosis Mistakes

Loose fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked.

Code P0303 Diagnosis Steps

Tools Needed to Diagnose:

How To Diagnose P0303

  1. Use FIXD to scan your vehicle to verify P0303 is the only code present. If other codes are present, they must be addressed first.
  2. Check for loose connectors at the ignition coils or for damaged wiring. Look for loose engine ground wires as well. These can cause random misfire conditions. Tighten or connect where necessary.
  3. Check the condition of your spark plugs and spark plug wires. If your vehicle is equipped with individual coil packs instead of spark plug wires, start by removing the cylinder 3 coil and swapping it with the cylinder number 4 coil. If the misfire moved to cylinder number 4, then you have determined that the coil pack is faulty and needs replacement. Replace spark plugs and spark plug wires/ignition coil packs if needed and recheck for misfires.
    1. How to identify a fouled spark plug
    2. How to test spark plug wires
    3. How to gap spark plugs
    4. How to replace spark plugs
    5. How to swap ignition coils
  4. If you have determined that your ignition system is operating correctly, there may be a problem within your fuel system that is causing the random misfires. The following should be checked to ensure the engine is getting the proper amount of fuel:
    1. Check fuel pressure. Low fuel pressure can cause intermittent misfires on multiple cylinders. When the pressure is below the specification, the engine does not receive the proper amount of fuel and will start to lean misfire. The fuel pump or fuel pressure regulator could be the source of the low fuel pressure.
      1. How to check fuel pressure
    2. Check that the fuel injectors are functioning properly and activating. Random misfires can be a sign of faulty or clogged fuel injectors that need to be replaced. Also check that the fuel injector wiring is not damaged and is connected properly.
      1. How to check fuel injectors by ear
      2. How to check fuel injectors with a digital multimeter
  5. If the ignition system and fuel system checks out you may want to perform an engine compression test and leakdown test to see if there are any mechanical problems causing your misfire. Some common mechanical problems that cause misfire can be:
    1. Broken valve spring
    2. Broken piston ring
    3. Worn valve guides
    4. Burned valve
    5. Timing chain or belt skipped tooth and engine is off time.
Was this post helpful?
Let us know if you found this information useful!

Comments 7


August 8, 2018

Hi after fixing IAVC i started seeing P0303 & P1399, i replaced complete tune up system ( spark cable/plugs/router) can you help what else to fix

Paul Wittmann

August 20, 2018

Greatest device on the market.

FIXD Marketing

September 3, 2018

Thanks Paul! We appreciate it!


November 9, 2018

I have a 07 grand Prix it keep say reduce engine power I have changed the map sensor,throttle body,gas pedal


November 29, 2018

Everything was fine until this morning when I started the vehicle. I bought gasoline last night at a station I dont generally go to and bought ethanol which I dont usually do. Today I showed a P0303 and I think it is bad gas causing it but unfortunately I have a full tank now. wondering if I put some octane boost or some stuff to rid gas of moisture would help


December 8, 2018

While driving my 1998 Navigator, the check engine light started to flash then stayed steady.
I ran a scan and found out I have a misfire in cylinder 3. I have the device turn off the light and then I rerun the scan- no issues. I reran the scan four additional times and still no issues and no check engine light.
I do not understand the reason for the issue but this little device keeps my blood pressure low!

FIXD Marketing

December 9, 2018

Glad we could help Michael! Always a bonus when we can contribute to the health of the people who use FIXD!