#OBD Codes
P0304 – Cylinder #4 misfire detected
Technical definition
Cyl. 4 misfire detected
What does P0304 mean?
The diagnostic trouble code P0304 means there’s a misfire in cylinder number 4 of your engine.
A cylinder misfire happens when the air and fuel mixture inside the cylinder don't burn properly (or don't burn at all) during the combustion process. This incomplete combustion can occur for various reasons – problems with the ignition system, fuel system, or even mechanical components.
To identify which cylinder is misfiring, you first need to know how your engine numbers its cylinders. Each manufacturer has its own numbering system, but the count typically starts at the front of the engine near the drive belt.
Misfires can be intermittent or continuous. Intermittent misfires are harder to spot and diagnose without a diagnostic tool, whereas continuous misfires are usually obvious and can feel like the engine is running roughly.
A misfire can cause your check engine light to switch on, reduce your engine's power, and even increase emissions, which is sometimes enough to cause your vehicle to fail an emissions test.
How serious is the P0304?
The P0304 code requires immediate attention because failing to address it early can lead to bigger problems later on.
When a cylinder misfires, your engine burns fuel inefficiently. That means worse fuel economy, more harmful emissions, and even the potential for damage to components that are very expensive to replace.
One of the most vulnerable parts is the catalytic converter. When a cylinder misfires, unburned fuel can make its way into the exhaust system, where it ignites from the high temperatures. This generates intense heat that can melt and clog the ceramic core of the catalytic converter, leaving you with a hefty repair bill.
Misfires can also cause engine knocking or pinging, which stresses components like pistons and valves. If not addressed, this can lead to severe engine damage over time.
Can I still drive with P0304?
Technically, yes, you might still be able to drive – but it’s not a good idea. Misfires can cause rough idling, poor acceleration, and reduced fuel efficiency, making driving both uncomfortable and risky. Imagine trying to merge onto a highway or pass another vehicle with sluggish acceleration.
In some cases, your car might even switch into “limp mode.” This is a safety feature that limits engine power to prevent further damage. While it’s handy in emergencies, it’s not ideal for long drives – just enough to get you to the nearest repair shop.
The sooner you address the issue, the better. Fixing the problem quickly can save you from costly repairs and ensure your vehicle keeps running smoothly.
Common symptoms of P0304
Here are the common signs of a cylinder #4 misfire:
- Check engine light
- Rough idling
- Poor acceleration
- Unusual engine noises, such as knocking or pinging
- Reduced fuel efficiency
- Engine stalling
Common causes of P0304
The P0304 code usually points to these issues:
Fuel system issues
- Dirty fuel filter
- Clogged fuel injector on cylinder no. 4
- Faulty fuel pump
- Fuel rail issues
Ignition system issues
- Faulty spark plug or spark plug wire on cylinder no. 4
- Worn or damaged ignition coil on cylinder no. 4
- Incorrect ignition timing
- Faulty distributor cap (on older vehicles)
- Air intake issues
- Vacuum leak in the intake manifold or vacuum hoses
Mechanical issues
- Damaged intake or exhaust valve on cylinder no. 4
- Damage on cylinder no. 4 walls
- Worn piston rings in cylinder no. 4
Circuit issues
- Crankshaft or camshaft position sensor problems
- Faulty oxygen (O2) sensor
Other issues
- Poor fuel quality
- Incorrect engine chip tuning
- Malfunctioning EGR valve or clogged EGR passages
How to troubleshoot P0304
Here’s how you can pinpoint and fix a P0304 code accurately:
Scan fault codes
Start by using an OBD scan tool to look for any other related trouble codes, which may provide additional clues to the underlying problem – misfire in cylinder 4. These faults include:
- P0011-P0025: Camshaft timing-related codes, which indicate problems with the camshaft position or timing, such as a faulty camshaft position sensor or issues with the variable valve timing (VVT) system
- P0100-P0104: Mass air flow (MAF) sensor- related codes that indicate sensor problems affecting the air-fuel mixture
- P0130-P0167: O2 sensor- related codes that might signal problems with the sensors responsible for monitoring the air-fuel ratio
- P0171 and P0174: Indicate that the engine is running too lean (too much air or not enough fuel). These fault codes narrow the possible problems to those related to the air/fuel ratio.
- P0300-P0312: These codes mean misfiring cylinders. The last two digits indicate the cylinder (e.g., P0301 for cylinder 1), while P0300 indicates multiple cylinders misfiring. This helps identify if the issue is cylinder-specific (e.g., bad spark plug) or affects all cylinders (e.g., low fuel pressure).
- P0335-P0339: Crankshaft position sensor trouble codes that could affect ignition timing
- P0351-P0362: Ignition coil issues, which might mean one or more coils aren’t delivering spark properly
Clear the codes and test drive
Use your scan tool to clear the fault codes.
- If the fault codes immediately reappear: This indicates the issue is ongoing. Skip the test drive and continue troubleshooting other potential causes.
- If the codes clear successfully: Drive your car for about 15 minutes to fully warm up the engine and observe its behavior.
After the test drive, perform another scan to see if the code reappears. If it does, this confirms the problem persists and requires further investigation.
Check freeze frame data
The freeze frame data provides a snapshot of your car's engine conditions at the exact moment when a fault code occurred. This data may include parameters such as engine load, temperature, RPM, and other factors that help you understand the conditions that triggered the code.
Why does it matter?
It can show whether the misfire happened while idling, accelerating, under high load, or with the engine running during specific conditions.
For example, if the misfire occurred during high RPMs, it might suggest fuel delivery issues like a failing pump or clogged injectors. If it happened at idle, the cause could be an ignition problem or vacuum leak.
Analyze misfire counter live data readings
Live data tools, like OBDeleven, give you real-time insights into your engine’s performance. Specifically, the misfire counter tracks how often misfires occur in each cylinder. For Volkswagen Group vehicles, these values are easily accessible in the engine control unit’s details.
OBDeleven live data feature
What to look for:
- Misfire frequency: If cylinder 4 shows a higher misfire count than others, it pinpoints the affected area.
- Driving conditions: Misfires occurring during high engine load could indicate insufficient fuel pressure, while misfires at idle may point to ignition or mechanical issues.
Pro tip: Use live data to test specific scenarios. For example, monitor the misfire counter while accelerating hard or idling to recreate the conditions that trigger the code. To learn more about how to interpret live data effectively, check out our blog post on How to read OBD live data.
Check other live data parameters
Other related live data parameters can also help identify potential causes of the P0304 code. Consider looking at:
- Fuel trim readings (short-term and long-term). Fuel trim readings show how the engine adjusts the air-fuel mixture. High positive values mean it's too lean (not enough fuel), and high negative values mean it's too rich (too much fuel). Normal values usually range between -5% and +5%.
- Ignition timing. Monitor the ignition timing angle to ensure it remains within the appropriate range, which depends on various factors. You can learn more about ignition timing here.
- Mass air flow (MAF) sensor readings. Make sure the MAF sensor is providing accurate airflow readings. Abnormally high or low readings may indicate sensor issues or intake air leaks.
- Oxygen sensor voltage. Check the upstream O2 sensor (closest to the engine) for proper operation, which typically fluctuates between 0.1 and 0.9 volts. Consistently low or high readings can indicate air-fuel mixture problems.
- Engine speed (RPM). Consistent and stable RPM readings indicate engine works properly. Fluctuations or sudden drops in RPM can signal misfiring or broader engine performance issues.
Inspect and replace spark plugs regularly
Check the spark plugs’ condition and replace them if worn out or damaged. Changing plugs is an important part of regular vehicle maintenance, as in normal conditions, spark plugs last from 50,000 to 160,000 kilometers (30,000 to 100,000 miles).
If all the spark plugs look good, check the spark plug boots and spark plug wires. The rubber tends to deteriorate over time, resulting in cracks in the insulation.
Test the ignition coil packs
To test the coil pack, you'll need a multimeter. Follow these steps:
- Set the multimeter measure resistance (Ω)
- Test the resistance between the coil terminals
- Compare the reading to your vehicle’s specified range (found in the manual)
If the reading is zero or falls outside the recommended range, the coil pack may need replacing. Faulty ignition coils can cause misfires, reduced engine performance, and increased emissions.
Pro tip: Not all ignition coils are the same. You can check how to test different types of ignition coils in this video.
Check the fuel system
A misfire might also stem from a fuel delivery issue. Using a fuel pressure gauge, you can test whether the fuel pressure meets your car’s specifications. Low pressure could point to problems with the fuel pump, filter, or fuel injectors. Addressing these issues promptly can prevent further damage and ensure a balanced air-fuel mixture.
Inspect for vacuum leaks
Inspect the intake manifold, hoses, valve cover, and gasket for cracks, holes, or loose connections. A smoke machine can help you find hard-to-spot leaks – wherever smoke escapes, there’s a problem.
Another method is to spray a small amount of carburetor cleaner around suspect areas while the engine runs. If the engine's idle speed changes, this might mean a vacuum is leaking.
Perform a compression test
Finally, check the engine’s compression to rule out mechanical issues like bent valves, blown head gaskets, or worn piston rings.
Here’s how to perform a compression test:
- Remove all spark plugs.
- Insert a compression gauge into one cylinder at a time.
- Crank the engine and record the pressure reading for each cylinder.
Once you’ve tested all the cylinders, compare the results to the manufacturer’s specifications:
- Low compression in particular cylinder often points to issues like a bent valve or a blown head gasket.
- Low compression in all cylinders may indicate worn piston rings.
Engine compression measurement. Source: autozone.com
FAQ
How much does it cost to fix a P0304 code?
Fixing a P0304 error code can range from as little as $25 for a new set of spark plugs to several thousand dollars for major repairs, such as an engine rebuild required due to worn piston rings or bent valves. The exact cost depends on the underlying cause and the extent of the damage.
Can a clogged fuel filter or dirty air filter cause a P0304 misfire code?
Yes, a clogged fuel filter can restrict fuel flow to the engine, causing the vehicle to run lean. Similarly, a dirty air filter can restrict airflow and make the air-fuel mixture too rich, which can prevent proper ignition and cause misfires.
How do camshaft and crankshaft position sensors affect misfires?
Camshaft and crankshaft position sensors provide critical information about their position and speed to the powertrain control module (PCM). When these sensors malfunction, it can lead to improper fuel injection and ignition timing.
Can a bad catalytic converter cause a P0304 code?
A bad catalytic converter rarely causes a misfire. Still, if partially blocked, it can restrict exhaust flow, trap old gases in the cylinder, and disrupt combustion. It may also cause incorrect readings from the post-cat oxygen sensor. However, catalytic converter damage is more often a result of prolonged misfires than the cause.
Disclaimer: Advice, how-to guides, and car care tips on our blog are intended as helpful resources for general maintenance and repairs. While we strive for accuracy, the information is provided to the best of our knowledge and should be used at your own discretion and risk.
Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.
Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.