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Figuring out why your Check Engine Light is on means deciphering OBD-II (or “OBD2”) codes, and that can be kind of annoying. That’s why Car Bibles did it for you! You’re welcome, now let’s get into solving your issue.

OBD2 P0303: What It Means

This code reads as misfire detected cylinder #3.” While self explanatory, it could also be accompanied by other misfire codes. If it’s on its own, then this will be pretty simple to figure out!

Likely Symptoms 

Here’s what you may experience due to the P0303 code:

  • Check Engine Light.
  • Rough engine running. If the cylinder is misfiring badly enough, you will notice a rough idle and rough acceleration.
  • Decreased fuel economy. The engine will try to protect itself and enter limp mode, running a safe tune that won’t hurt the engine.
  • Decreased power. With the engine potentially down a cylinder, it won’t be able to make full power.

Probable Causes

Here’s what could be causing the issue:

  • Bad spark plug. Generally, a spark plug goes bad and causes misfires. A simple fix. 
  • Bad ignition coil. If changing plugs doesn’t work, the ignition coil is probably bad. The ignition coil is responsible for firing the spark plug, and on modern cars, each spark plug has an ignition coil. If you car has individual ignition coils, try moving the suspected bad coil to another cylinder, and see if the misfire follows it.
  • Bad or clogged fuel injector. The cylinder needs air, fuel, and spare to fire, so if there isn’t any fuel, it will cause a misfire. Most of the time, the injector is simply clogged.
  • Burnt valve. Uncommon, but it can happen. A burnt valve leaks compression and causes misfires. An engine also needs to compress air to combust the fuel mixture, so no compression means no ignition, means misfire. Sometimes, the misfire can be so bad, that it burns the valve.
  • Low compression. Same thing, low compression means the engine will misfire. 

What Part Is Potentially Affected?

If a misfire goes on for too long, it can foul spark plugs, burn valves, and cause low compression from unburnt fuel flooding the cylinders. Fuel is a solvent, and washes oil away, causing metal-on-metal contact, ruining the cylinder.

Possible Fixes

Here are the most common fixes to remedy the P0303 code:

  • Replace spark plugs. Do a quick spark plug replacement, and see if the misfire goes away.
  • Replace ignition coil. This is the most common cause of a misfire. It can be pricey, so to lower costs, move the suspected bad ignition coil around and see if the misfire follows it. If it does, replace that ignition coil.
  • Compression test. Inexpensive compression testers are everywhere now, or your local shop can do it for you. Verifying that the compression is good or bad, will go a long way to finding a repair. If the compression is bad, then you will likely have to do major engine work.

Finding The Parts You Need

Now that you’ve figured out what’s wrong with your hooptie, let’s talk about where you’re gonna find that part’s replacement.

There are plenty of places you can buy auto parts from, but Car Bibles gets paid if you click this Advance Auto link so that’s the one we’re serving up. Advance Auto Parts also has delivery, curbside pickup, and a host of helpful diagrams to aid your repair!

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