Generic OBDII P Codes: P0300-P0399

Learn the code definitions and the most common causes for Generic OBDII P Codes P0300-P0399

This post lists the most common trouble codes in the OBDII P Codes P0300-P0399 category. These are Ignition system and Misfire-related trouble codes. Start by reading the most common codes and causes and then refer to the code chart at the end of the post for official code definitions.

I’ll start by discussing the most common Trouble codes in this group, along with the most common causes of those codes.

How to find your P Codes P0300-P0399 definition

There are 134 300-series codes. There are several ways to find the code definition in the chart below.
1) Enter the trouble code in the search box. That’s the fastest way
2) If you’d like to scroll all the codes, change the Number of entries to show on the page in “Show Entries” box. Click NEXT or PREVIOUS at the bottom right of the table.

This graphic shows the three ways you can navigate the refrigerant chart: 1) Increase the number of entries, enter the year of your vehicle in the search box, or scroll the chart using the next/previous buttons at the bottom of the chart

The most common trouble codes in the P0300-P0399 category

P0300 random misfire is a common trouble code

A P0300 code means the computer has detected a random misfire or multiple misfires have been detected with no pattern as to which cylinder is misfiring more than others. It’s possible to have this code if you’re driving on equally worn spark plugs, but that’s not a common cause.

Most common cause of a P0300 code

The most common cause of a P0300 code is an air-fuel mixture problem that affects all cylinders. This can be caused by a fuel pump issue or an air leak—anything that affects all cylinders.

Diagnosing a P0300 random misfire code

Start by checking for other trouble codes. For example, a P0171 indicating a lean condition can also be accompanied by a P0300 code. In that case, you should check for an air leak between the air filter box and the throttle body, vacuum leaks, or exhaust leaks. In addition, you should check the long-term fuel trims. If the fuel pump isn’t outputting enough fuel, you’ll see a very high long-term fuel trim number, usually above 10 percent. If you see a high fuel trims you should perform a fuel pump pressure and volume test.

P0301 thru P0306 specific cylinder misfire codes are also common

Notice the condition of the side and center electrodes on a worn spark plug versus a new spark plug

These codes indicate a misfire problem in certain cylinders. The most common causes for individual cylinder misfire codes are: 1) a damaged or worn spark plug or a worn or damaged ignition coil or spark plug wire.

Diagnose a P0301 thru P0306 code

This image shows what damaged ignition coils look like. The high voltage has fired right through the plastic or rubber structure to reach ground. This condition is most often caused by a worn out spark plug. If the spark can’t jump the gap on the spark plug it will fire right through the side of the coil.

Start by removing the spark plug wire ignition coil, and spark plug for the specific cylinder. Examine the condition of the spark plug wire and the ignition coil. Look for signs of fire-through tha look like cuts or burn marks in the cable or the ignition coil boot. If the misfire code persists after replacing the spark plug or coil, the problem is often mechanical. Perform a compression test for that cylinder.

I have written an article on how to diagnose a specific cylinder misfire using a step-by-step guide. Find the article here.

P0325 thru P0334 knock sensor codes are common

These codes refer to the knock sensor. They detect a knock condition and report to the ECM. The ECM. The knock sensors by purposely inducing an air-fuel condition that will result in a knock. If the ECM sees a knock report, it knows the sensor is working.

You can test a knock sensor yourself by connecting it to a multimeter and hitting the block next to the sensor using a ball peen hammer. I have shown a step-by-step guide for testing the knock sensor here.

P0341 -P0349 are camshaft position sensor codes are also common

The camshaft position sensor tells the ECM the exact location of the camshaft so it can coordinate ignition timing. Camshaft position sensors can fail on their own, but the most common cause of failure is corrosion in the electrical connector or wire harness damage. In addition it’s common for aftermarket camshaft position sensors to fail early. I understand that OE sensors are more expensive, but they’re also far more reliable in the long run.

Diagnosing a P0341 -P0349 code

Start by removing the electrical connector to the camshaft position sensor and check for corrosion or harness damage. If the wiring is good and there’s no corrosion, some sensors can be tested using a multimeter. Some of the sensors on light metal cars can only be tested using a scope. If you don’t have access to those tools, your only option may be to replace the part.

P0359 -P0362 Ignition Coil Primary/Secondary Circuit Malfunction are common trouble codes

These codes indicate a problem with a specific ignition coil. Some carmakers like Toyota have built-in sensors to detect if the ignition coil has actually fired.

Diagnosing an Ignition Coil Primary/Secondary Circuit Malfunction

Place the induction pickup on top of the ignition coil and run the engine. The neon light will flash every time the ignition coil fires

Start your diagnosis by checking for power to all ignition coils. In most vehicles, power is provided to all the ignition coils, and either the ignition module or the ECM provides ground that causes the coil to fire. As you check for power, check the condition of the wiring harness. Rodents love to chew on the wires to the ignition coils because their insulation is soy-based.

On many V-style engines, it can be difficult to check for secondary circuit issues because you have to remove the intake manifold to access the ignition coils. That prevents you from running the engine. However, if you can fit a neon ignition coil tester over the coil, You can sometimes detect ignition coil failure.

P0385-P0394 Crankshaft Position Sensor codes are also common

Crankshaft position sensor codes can be caused by corrosion in the electrical connector, wiring harness issues, or damage to the tone wheel or the harmonic balancer.

Diagnosing a crankshaft position sensor code

Start by checking for corrosion in the electrical connector and then check the condition of the wires in the wiring harness. Some sensors can be tested using a multimeter, but others require a scope. For example, camshaft position sensors and aftermarket sensors are not as reliable as OE sensors. They tend to fail far more often.