Powertrain Ignition medium

P0304 Code — Cylinder 4 Misfire Detected (Causes & How to Fix)

Fix OBD-II code P0304: Cylinder 4 Misfire Detected. Step-by-step diagnosis and repair guide.

⏱️ Time30-60 minutes
📋 Severitymedium
🚗 Applies ToToyota, Honda, Ford, Chevrolet
⚠️ DTC CodeP0304
P0304 — Powertrain Ignition diagnostic guide

What Does DTC P0304 Mean?

P0304 on your Powertrain Ignition system means: The engine control module has sensed that cylinder 4 is not completing its combustion cycle properly. This usually means the spark isn’t firing, the fuel isn’t being delivered, or the compression is too low for that cylinder.

How to Diagnose and Fix P0304 — Step by Step

  1. 1

    Quick Visual Scan

    Inspect spark plug wires, ignition coil, and fuel injector for obvious damage or loose connections. Verify the gas cap is tight and the battery terminals are clean.

  2. 2

    Read Freeze‑Frame Data

    Pull the freeze‑frame snapshot for the P0304 event. Note engine speed, load, and fuel trim values at the moment the misfire was logged.

  3. 3

    Spark Plug Inspection

    Remove cylinder 4’s spark plug. Look for fouling, carbon buildup, or electrode wear. Measure resistance with a multimeter; a healthy plug typically reads a few kilo‑ohms.

  4. 4

    Ignition Coil Test

    Swap the coil for cylinder 4 with another cylinder’s coil. Clear the code and see if the misfire follows the coil. If it does, replace the faulty coil.

  5. 5

    Fuel Injector Function Check

    Listen for a consistent click on cylinder 4’s injector using a mechanic’s stethoscope. If the click is weak or absent, try a short‑pulse test with a scan tool or replace the injector.

  6. 6

    Compression Test

    Perform a dry compression test on cylinder 4 and compare it to the other cylinders. A reading more than 10‑15% lower indicates internal mechanical issues.

  7. 7

    Check for Vacuum Leaks

    Spray carburetor cleaner or a propane torch around the intake manifold near cylinder 4 while the engine idles. A change in RPM signals a leak.

When to Call a Professional Mechanic

If the compression test shows low pressure, if the injector fails the short‑pulse test, or if the misfire persists after replacing plugs, coil, and injector, have a professional inspect the head gasket, camshaft, or timing chain.

Diagnostic Flowchart

Follow this visual guide to systematically diagnose P0304:

Quick Check – 60‑Second Diagnosis

Your check‑engine light is flashing, and the OBD‑II scanner shows P0304 – Cylinder 4 Misfire Detected. Before you open the hood, do a rapid visual scan:

  1. Gas cap – make sure it clicks tight. A loose cap can throw off fuel pressure and trigger a misfire code.
  2. Battery terminals – clean any corrosion; a weak battery can cause low coil voltage.
  3. Spark plug wire/coil connector – look for cracked boots or loose pins.
  4. Engine oil level – low oil can cause excessive wear on piston rings, leading to compression loss.

If anything looks out of place, fix it, clear the code with your scanner, and take the car for a short test drive. If the code returns, move on to the detailed steps below.

Safety note: Always disconnect the negative battery terminal before removing ignition components. The engine and exhaust can stay hot for several minutes after shutdown.

Symptoms You’ll Notice While Driving

A cylinder‑4 misfire creates a distinct set of driver‑visible cues:

| Symptom | What It Means | |

Real-World Diagnostic Scenario: P0304

When a technician encounters P0304 in the shop, the diagnostic approach differs significantly from other codes. Unlike generic fuel system codes, P0304 requires checking specific subsystems in a particular order to avoid misdiagnosis. Here’s what experienced mechanics look for first, based on thousands of documented repair cases.

Several manufacturers have issued TSBs specifically addressing the root causes of P0304. Checking for applicable TSBs can save significant diagnostic time and may indicate a known factory defect covered under extended warranty.

Note: TSB availability varies by manufacturer and model year. Check with your dealer or an online TSB database for your specific vehicle.

Frequently Asked Questions

Q: What is the average repair cost for a P0304 code, and does it vary by vehicle make?

The average repair cost for a P0304 code can range from $200 to $1,000, depending on the underlying cause and the vehicle’s make and model. For example, if the problem is a faulty spark plug, the repair cost may be on the lower end, around $200-$300. However, if the issue is a more complex problem like a cylinder head or engine block issue, the cost can be significantly higher, up to $1,000 or more. Additionally, some vehicle makes, such as Ford or Chevrolet, may have more affordable repair options compared to luxury brands like BMW or Mercedes.

Q: Is it safe to drive with a P0304 code, and what are the potential risks?

Driving with a P0304 code can be done for a short period, but it’s not recommended to continue driving for an extended time. The code indicates a misfire in cylinder 4, which can cause poor engine performance, reduced fuel efficiency, and increased emissions. Prolonged driving with this code can lead to more severe problems, such as catalytic converter damage or engine damage, which can result in more costly repairs. It’s essential to address the issue as soon as possible to avoid any further damage.

Q: What is the most common cause of a P0304 code in Toyota vehicles?

In Toyota vehicles, the most common cause of a P0304 code is a faulty spark plug or spark plug wire. Toyota’s ignition system is known for its reliability, but spark plug wear and tear can still occur over time. Replacing the spark plug or spark plug wire can often resolve the issue, but it’s crucial to inspect the spark plug well and the surrounding area for any signs of damage or wear.

Q: Will a P0304 code cause my vehicle to fail an emissions test?

Yes, a P0304 code can cause your vehicle to fail an emissions test. The code indicates a misfire in cylinder 4, which can lead to increased emissions and poor engine performance. Most emissions tests check for excessive emissions, and a misfiring engine can produce higher levels of pollutants, such as hydrocarbons and carbon monoxide. To pass an emissions test, it’s essential to diagnose and repair the underlying cause of the P0304 code.

Q: How long does it take to repair a P0304 code, and can I do it myself?

The repair time for a P0304 code can vary depending on the underlying cause and the individual’s level of mechanical expertise. If the issue is a simple spark plug replacement, the repair time can be as short as 30 minutes to an hour. However, if the problem is more complex, such as a cylinder head or engine block issue, the repair time can take several hours or even days. While it’s possible to diagnose and repair a P0304 code yourself, it’s recommended to seek the help of a professional mechanic if you’re not experienced with automotive repairs.

Q: What’s the difference between a P0304 code and a P0300 code, and how do I diagnose the issue?

A P0304 code specifically indicates a misfire in cylinder 4, while a P0300 code is a more general code that indicates a random or multiple cylinder misfire. To diagnose the issue, you’ll need to use a scan tool to monitor the engine’s performance and identify the specific cylinder that’s misfiring. You can also use a spark tester to check the spark plug wire and spark plug for any signs of weakness or damage. Additionally, checking the fuel injector and fuel pressure can help determine if the issue is fuel-related.

Q: Can a P0304 code be caused by a low fuel pressure issue, and how do I check the fuel pressure?

Yes, a P0304 code can be caused by a low fuel pressure issue. If the fuel pressure is too low, the engine may not be able to get the proper amount of fuel, leading to a misfire in cylinder 4. To check the fuel pressure, you’ll need a fuel pressure gauge, which can be connected to the fuel rail or fuel filter. The recommended fuel pressure for most vehicles is between 35-45 psi, but this can vary depending on the vehicle make and model. If the fuel pressure is low, you may need to replace the fuel pump, fuel filter, or fuel pressure regulator.

Q: Will clearing the P0304 code with a scan tool resolve the issue, or is further repair needed?

Clearing the P0304 code with a scan tool will only temporarily resolve the issue, as it doesn’t address the underlying cause of the problem. If you clear the code without making any repairs, the code will likely return, and you may experience continued engine performance issues. To permanently resolve the issue, you’ll need to diagnose and repair the underlying cause of the P0304 code, whether it’s a faulty spark plug, spark plug wire, or another issue. It’s essential to use a scan tool to monitor the engine’s performance and identify the root cause of the problem.

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Published: · Updated: · By FixCarCodes Team

This guide is for informational purposes only. Always consult your vehicle's service manual and consider hiring a certified automotive mechanic for complex repairs. FixCarCodes guides are AI-assisted and reviewed for accuracy, but vehicles vary — verify part numbers and procedures for your specific make and model. Learn about our editorial process.