Engine Code p0303 Means, Causes, Fixes (6-Step Guide)

Are you wondering how to solve that pesky P0303 code?

You've come to the right place!

In this guide, you're going to learn:

  • What the P0303 code means
  • How to diagnose the P0303 code
  • How to go about solving the P0303 code

And much more!

Engine Code p0303 Means, Causes, Fixes (6-Step Guide)

This trouble code says that a cylinder 3 misfire was detected in the vehicle! 

Well, that's no good. Read further to educate yourself on why this error code showed up on the OBD-II scanner, and what you can do to fix a cylinder misfire.

So what's the next step for fixing your engine problems? Stay tuned to learn about engine code p0303 means, causes, fixes, and more in this DIY article. 

Here are 6 simple steps to help you pin down this p0303 third cylinder misfire issue

What Does The p0303 Code Mean?

The p0303 code is an engine misfire code, where the cylinder is specifically misfiring on the third cylinder. 

What Caused The p0303 code (Possible Symptoms)?

When your engine misfires it will behave badly, and there are many different causes of misfiring in today's OBD-II engines.

Here are some symptoms of a misfiring engine:

  • Less power and acceleration
  • The engine will rumble distinctly
  • Decreased fuel economy
  • Check engine light illuminated on the dash
  • Inconsistent rpm

In the simplest and most common of cases, your cylinder is misfiring because of a failed ignition coil or bad spark plug.

Is the p0303 the only misfire code? Or is there misfiring on all the other cylinders also?

This will help you narrow down your diagnostics even further to find the solution to the misfire.

Are there other codes set off also, like a p0340 camshaft sensor code or a p0101 mass airflow sensor code? Look at all the details on the OBD, and don't leave anything out of the question. 

Here are the possible causes of the p0303 code:

  • failed ignition coil pack
  • faulty spark plug
  • vacuum leak
  • failed mass airflow sensor
  • clogged catalytic converter
  • failed crankshaft position sensor
  • ignition timing off
  • faulty fuel injector
  • bad air/fuel mixture
  • low fuel pressure/bad fuel pump
  • leaky intake manifold gasket
  • problem with vehicle's PCM module(a rare occurrence)
  • distributor cap 
  • EGR valve stuck open
  • bad battery
  • faulty spark plug wires
  • bad fuel filter

Read More >> What Do You Do About The Service Engine Soon Light?

How To Diagnose A p0303 Code?

Want to know the cause of the misfire?

For proper diagnosis of the p0303 code, you must first determine which cylinder is cylinder three. 

For example, if you have a transverse-mounted four-cylinder engine(front-wheel drive), the third cylinder from the left is the third cylinder. 

In cases where this isn't the only code being thrown the situation is a little more complicated. 

It's more of an easy fix when the third cylinder is the specific cylinder that is misfiring, and all others are functioning properly. 

Don't forget to go through all the fuses and make sure they are all still good in any situation like this one!

Step 1 - Identify Ignition System: Trade Ignition Coils/Check Distributor Cap

Inspect ignition components.

The first step and easiest diagnosis for this code if it is the only code stored in the PCM  is to simply switch the third cylinder ignition coil with one of the coils from another cylinder. 

Make sure the car is turned off when working with the ignition coils.

Ignition coils are very easy to remove. 

First, carefully remove the electrical connector. Rubbing alcohol in a spray bottle helps for removing electrical connectors without breaking them; just simply spray some alcohol on the connector before removing.

Next, unbolt the ignition coil and remove it from the valve cover. 

Trade the ignition coil with one from a cylinder that isn't misfiring. 

Again, this is a foolproof way to easily diagnose the problem if ONLY the p303 code is stored. If other cylinders are misfiring also, this doesn't work.

  • Inspect Distributor Cap

Older carbureted vehicles misfire when the distributor cap or distributor rotor goes bad. Remove and inspect the distributor cap and rotor for damage.

Step 2 - Inspect Spark Plug On Cylinder Number 3

Again, if cylinder 3 is the only cylinder that is misfiring, it's a good idea to remove the spark plug, inspect it, re-gap it, and reinstall it. Either that or replace the plug.

Carefully remove the spark plug with a spark plug socket, and inspect it for fouling. Fouling can cause a misfire. 

Step 3 - Electrical Tests: Is The Coil Pack Getting Voltage?

We are now tracing the problem back from the spark plugs. Turn the ignition on and see if the coil pack electrical connector is getting it's healthy 12V of electricity with a multimeter.

If it's not, visually inspect the wiring harness for damage like burns, and rat chewing.

Seriously, rodents hive up in your engine bays because it's warm; and they'll chew at the electrical cables. 

Test the battery with a battery tester to verify its correct function.

Step 4 - Check For Vacuum Air Leaks

One common cause of a misfire is unmetered air finding its way into the combustion chamber from one of the engines vacuum powered components. 

You can try spraying soapy water onto the intake manifold to check it for leaks, performing a smoke test on all parts that carry air, etc.

Visually inspect the engine's hoses. Sometimes vacuum hoses simply come unplugged because there is no fastener on them. 

Step 5 - Inspect Head Gasket/Overheating Problem

When the head gasket(s) blow out, the engine overheats because of loss coolant and oil and this can cause an overheating problem. 

Or vice versa, overheating can cause a coolant leak.

Inspect your head gaskets for leakage. This includes checking the oil and coolant levels. 

Step 6 - Check Ignition Timing/Timing Belt

Some vehicles have a timing belt that has to be serviced every 60-100 thousand miles.

Timing belt wear can make the timing slip out, where you'll be handling random misfiring on all cylinders.

This is more of an advanced problem because it involves removing some covers and the timing belt.

Read More >> What Does The P0420 Engine Code Mean?

Did you know: Some of the most common causes of cylinder misfire or malfunction are contaminated fluid, damaged seals, cylinder pressure, and piston damage.

How Do I Fix A p303 Code?

Great question!

When problems like a misfire happen in your vehicle it requires you to search for the problem to find a proper solution. Here are the reasons why your vehicle could be misfiring on its third cylinder:

Potential Fix 1 - Replacing the Third Cylinder Ignition Coil/Distributor cap

Replacing the ignition coil is about the easiest way out of this check engine light pickle. 

Don't want to pay a lot of money? Buy one on Amazon or even pull one from a salvage yard.

Potential Fix 2 - Replace Spark Plugs

Replacing the spark plugs is a must-do when maintenance, and even if the misfire problem isn't the spark plugs, after all, your vehicle could be due for replacement anyway.

Treat your car like it's yourself; because it is, it's an extension of you.

Potential Fix 3 - Perform A Vacuum Test

Replace the intake manifold/gasket according to the vehicle's service manual. Torque the bolts down in the correct order.

Use your vehicle's service manual to check all vacuum line connections. Fix any broken vacuum lines and reassure that everything is connected correctly.

Potential Fix 4 - Electrical Repair

Replace fuses/battery if necessary.

Repair any electrical wires

Check your vehicle manufacturer for recall campaigns calling for the replacement of the PCM due to this code.

Potential Fix 4 - Compression test

The next step for diagnosing an engine misfire would be measuring the PSI of the cylinder with compression tester or leak down tester. 

You can find a compression tester at your local auto parts store or on Amazon for relatively cheap.

Simply install the tester into the third cylinder like you would the spark plug, unplug the fuse to the fuel pump so the engine doesn't start up, and crank the engine over as if you were going to start it. 

If the cylinder tests for low compression (anywhere below 100 PSI), you can then conclude that the engine has either a leaky valve or a bad piston ring, and needs rebuilding. There is no easy fix to this problem. 

Reset the check engine light with a scan tool or by simply unplugging the negative battery terminal for a few minutes.

About Your Mechanic

About Ryan Nichols

Ryan here! My pro mechanic career began as a technician for Mercedes-Benz. After two years of that great experience, I went out on my own in both the automotive and construction fields. I've since pulled my share of salvage yard parts and fixed more vehicles than I can count.

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