P0304 medium Severity

P0304: Cylinder 4 Misfire Detected

Rough idle, hesitation, loss of power, reduced fuel economy, potential catalytic converter damage

Overview

P0304 indicates a misfire in cylinder number four detected by the engine control unit; common causes include spark plug, ignition coil, or fuel delivery issues.

Common Symptoms

  • Check engine light illuminated or flashing
  • Rough idle or engine vibration
  • Loss of engine power or hesitation during acceleration
  • Poor fuel economy
  • Strong smell of unburned fuel

Most Common Causes

Faulty spark plug in cylinder 4 high

Worn, fouled or damaged spark plug often causes misfire specific to one cylinder

Faulty ignition coil or coil pack for cylinder 4 high

Ignition coil failure prevents proper spark delivery

Clogged or malfunctioning fuel injector on cylinder 4 medium

Restricted or failed injector disrupts fuel delivery

Vacuum leak near cylinder 4 intake runner medium

Unmetered air can create lean condition causing misfire

Low compression in cylinder 4 medium

Mechanical issues (valve, piston ring wear) can cause incomplete combustion

Diagnostic Steps

  1. Connect an OBD-II scanner and verify P0304 code, check for related codes
    Expected: P0304 present, note freeze-frame data and any additional misfire or sensor codes
    Interpretation: Confirms misfire in cylinder 4 and helps identify concurrent issues
  2. Inspect spark plug in cylinder 4
    Expected: Check for wear, fouling, carbon buildup
    Interpretation: Bad plug indicates need for replacement
  3. Swap ignition coil from cylinder 4 to another cylinder
    Expected: If misfire follows the coil, coil is faulty
    Interpretation: Isolates ignition coil failure
  4. Test fuel injector function or swap injector
    Expected: Misfire shift indicates injector issue
    Interpretation: Identifies fuel delivery problems
  5. Perform vacuum leak test around intake manifold near cylinder 4
    Expected: Smoke or pressure test reveals leaks
    Interpretation: Confirms air intake leak affecting mixture
  6. Conduct compression test on cylinder 4
    Expected: Compare compression to other cylinders
    Interpretation: Low compression indicates mechanical engine issue

Repair Solutions

Replace spark plug in cylinder 4 easy
Estimated Cost: $10 - $60

Basic maintenance often resolves misfire if plug worn

Replace ignition coil for cylinder 4 medium
Estimated Cost: $50 - $120

Common fix if coil is defective

Clean or replace fuel injector for cylinder 4 medium
Estimated Cost: $100 - $250

Restores proper fuel delivery

Repair vacuum leaks medium
Estimated Cost: $50 - $200

Seals unmetered air entry affecting air-fuel mixture

Engine mechanical repair for low compression hard
Estimated Cost: $300 - $900

Requires internal engine work if compression issue present

Common Misdiagnoses

Avoid these common diagnostic mistakes:

  • Assuming random misfire without checking specific cylinder causes
  • Replacing parts (e.g., fuel injector) without verifying ignition issues
  • Blaming oxygen sensor instead of localized misfire cause

Frequently Asked Questions

Technical definition of P0304 OBD-II Code

Technically, the P0304 code represents Cylinder 4 Misfire Detected. It usually points to a fault within the Ignition.

How serious is the P0304 code?

You should treat P0304 as a medium issue. Rough idle, hesitation, loss of power, reduced fuel economy, potential catalytic converter damage

What happens when the P0304 code is active?

The most typical signs are: Check engine light illuminated or flashing, Rough idle or engine vibration, Loss of engine power or hesitation during acceleration.

What parts fail when P0304 is set?

This code is frequently triggered by: Faulty spark plug in cylinder 4, Faulty ignition coil or coil pack for cylinder 4, Clogged or malfunctioning fuel injector on cylinder 4.

How much does it cost to fix P0304?

It depends on what needs to be fixed. Replace spark plug in cylinder 4 repairs are cheaper (~$60), while Engine mechanical repair for low compression repairs are on the higher end (~$900).

Is P0304 a generic or manufacturer-specific code?

The P0304 code is a generic powertrain code, meaning it applies to almost all makes and models, such as Honda, Toyota, VW, Ford, and Chevy.

User Comments (0)

Share your experience or ask a question about this code.

Be the first to share your experience with P0304!