OBD2 Code P0300: What It Means

Figuring out why your Check Engine Light is on means deciphering OBD-II (or “OBD2”) codes, and that can be kind … Continued

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 Code P0300: What It Means

The P0300 code indicates that there is a random misfire, or misfires, occurring. A misfire happens when a car’s cylinder isn’t firing properly. This one is a tad tricky; if there were a 1, 2, 3, 4, etc. in place of the last 0 in this code, it would be more specific and tell you which cylinder is experiencing the misfire. Because of this, it might take a little longer to figure out.

Likely Symptoms

Here’s what you may experience due to P0300 code:

  • Hard start. The engine might be hard to start.
  • Stumbling. The engine might stumble and run awkwardly.

Probable Causes

Here’s what could be causing the issue:

  • Faulty spark plugs or plug wires. The spark plugs might be at the end of their life, and no longer lighting enough of a fire inside the cylinder. Or, the wires themselves might be cashed.
  • Faulty coil, coil pack, or distributor cap. Depending upon what your ignition system is comprised of, the source of this lack of electricity could be coming from a few different parts.
  • Faulty oxygen sensor. The oxygen sensor might not be accurately relaying the exhaust gas’ makeup to the ECU.
  • Faulty fuel injector. Fuel injectors that are worn out or clogged won’t spray enough fuel into the cylinder, creating enough of a burn.
  • Faulty EGR valve. A clogged or faulty EGR valve won’t effectively re-circulate exhaust gases into the engine.

What Part Is Potentially Affected?

Essentially, whatever your ignition system is comprised of will determine what needs to be looked at.

On older cars with spark plugs, spark plug wires, distributor caps, a coil wire, and coil, there’s a lot to sift through and troubleshoot. On newer cars with spark plugs and coil packs, there’s significantly less, so there is much less to troubleshoot.

Possible Fixes

Here are the most common fixes to remedy P0300 code:

  • Inspect and replace necessary spark plugs. Pulling out and seeing which spark plugs are cashed and in need of replacement is one of the easiest and cheapest first steps, just be sure to remember which coil pack went where.
  • Replace coil packs, or swap them around. Replacing all of the coil packs might not be necessary, though could be a sort of “while you’re in there” fix if the car has higher mileage, or you’re near a service interval that requires their replacement. Or, try swapping the coil packs around to see if a code does occur that tells you which cylinder is the source of the misfire. Just be sure to not mix up the wires.
  • Replace wires. Replacing spark plug or coil pack wires can be an inexpensive and easy fix.
  • Inspect and replace the distributor cap. If the car utilizes this, it can be easy to remove and inspect, and then replace. Just don’t mix up which wire goes where.
  • Clean the EGR valve. Cleaning out the EGR Valve with carb cleaner, or any other suitable solvent, like replacing spark plugs, is one of the easiest and cheapest methods.
  • Inspect and/or replace the oxygen sensor. Looking for frayed wires attached to the oxygen sensor, un-plugging it and seeing how the car runs, or just straight up replacing it are all ways of remedying this potential issue. I recommend making this the last line item of your troubleshooting.

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!

Peter Nelson

Peter NelsonPeter Nelson has been wrenching on and playing with cars since he started driving them quickly between the cones at Chicagoland autocross events in his late teens. Nowadays, he can be found wringing out his Mazda2 at tracks all over California. His writing background includes Winding Road, Donut Media, and Autolist.com. He's also an avid cyclist and '80s/'90s action film connoisseur. Contact the author here.