P0304 on Ford: Causes, Symptoms, and What to Fix First

Quick Answer

P0304 is a Ford diagnostic trouble code (DTC) signaling a misfire in cylinder 4. The most likely causes (ordered by cost and ease of repair) are: a worn spark plug, faulty ignition coil, clogged fuel injector, vacuum leak, or internal engine damage. To fix it:

1. Confirm the code with an OBD2 scanner and check for related codes.
2. Replace the cylinder 4 spark plug (cost: $10–$30 for parts).
3. Test the ignition coil by swapping it with a neighboring cylinder (cost: $30–$100 for a replacement coil).
4. Inspect and clean/replace the fuel injector (cost: $50–$150 for parts, $100–$200 labor if professional).
5. Perform a compression test to rule out internal issues (cost: $100–$300+ for professional repair if needed).

Symptoms of P0304 include rough idle, reduced engine power, poor fuel economy, a flashing check engine light (indicating severe misfire), and occasional stalling.

Before You Start

Before beginning diagnostics, ensure:

• You have a reliable OBD2 scanner to confirm the P0304 code and monitor live data (e.g., misfire counts, fuel trim).
• Disconnect the negative battery terminal to avoid electrical shocks or accidental airbag deployment.
• Let the engine cool completely to prevent burns.
• Note your Ford’s year, model, and engine type (e.g., 2018 Ford F-150 3.5L EcoBoost) as cylinder numbering and component specs vary.

Tools and Parts

Required Tools

• OBD2 scanner (compatible with Ford vehicles)
• Spark plug socket (size varies by engine—usually 13/16” or 5/8”)
• Torque wrench
• Compression tester
• Digital multimeter
• Flathead and Phillips screwdrivers
• Socket set
• Vacuum gauge (optional)
• Fuel injector cleaner (e.g., Sea Foam, optional)

Recommended Parts

• OEM or aftermarket spark plug (match your Ford’s engine specs; brands like NGK, Motorcraft, or Bosch work well)
• Ignition coil (if faulty; OEM Motorcraft coils are preferred for long-term reliability)
• Fuel injector (if clogged/faulty)
• Spark plug wires (for pre-2000 Ford models with distributor systems)
• Replacement vacuum hose (if a leak is found)

Where to Buy: OEM parts from Ford dealerships offer perfect fitment; reputable aftermarket options are available at AutoZone, Amazon, or RockAuto for cost savings.

Step-by-Step Diagnostic & Fix Path

Follow this order to prioritize low-cost, easy repairs first:

1. Confirm the P0304 Code & Monitor Live Data

• Plug your OBD2 scanner into the port under the dashboard (driver’s side).
• Retrieve the P0304 code and check for related codes (e.g., P0171 for lean fuel, which can worsen misfires).
• Monitor live data while idling and accelerating: Look for consistent misfire counts on cylinder 4. Misfires only under load point to fuel or ignition component issues.

2. Inspect & Replace Cylinder 4 Spark Plug

• Locate cylinder 4 (verify with your Ford service manual: inline 4 engines = rearmost cylinder; V6/V8 = passenger-side bank, second from the front for most models).
• Remove the ignition coil (coil-on-plug models) or spark plug wire (older models) from cylinder 4.
• Use a spark plug socket to remove the plug. Inspect for worn electrodes, carbon buildup, oil fouling, or a cracked insulator.
• Replace with a new spark plug, torquing to the manufacturer’s spec (typically 20–30 ft-lbs; always verify from your service manual).
• Reinstall the coil/wire, clear the code with the scanner, and test drive to check for misfires.

3. Test the Ignition Coil

• If the spark plug replacement didn’t fix the issue, swap the cylinder 4 coil with a neighboring cylinder (e.g., cylinder 3 or 5).
• Clear the code and test drive. If the misfire code changes to P0303 or P0305, the original cylinder 4 coil is faulty.
• Replace the faulty coil, torquing mounting bolts to 8–12 ft-lbs (verify spec). Clear the code and retest.

4. Check Fuel Injector Functionality

• Use a multimeter to test the injector’s resistance (should match service manual specs—usually 12–16 ohms for most Ford injectors).
• If resistance is out of range, replace the injector.
• If resistance is normal, add fuel injector cleaner to the gas tank and drive 50+ miles. If misfires persist, remove and clean the injector or replace it.
• Note: High-pressure EcoBoost models may require specialized tools (e.g., injector pullers) for replacement; refer to your service manual for steps.

5. Perform Compression Test

• Remove all spark plugs from the engine.
• Disable the fuel system (unplug the fuel pump relay) and ignition (disconnect coil packs).
• Thread the compression tester into cylinder 4’s spark plug hole.
• Crank the engine for 3–5 seconds and record the reading. Normal compression is 120–175 psi; readings below 100 psi indicate internal engine issues (worn rings, blown head gasket, valve damage).
• Compare readings with other cylinders. A 20+ psi difference confirms a cylinder-specific problem.

Common Mistakes to Avoid

• Misidentifying cylinder 4: Working on the wrong cylinder wastes time and money. Always confirm numbering with your Ford service manual.
• Over-torquing spark plugs: This can crack the cylinder head or damage the plug. Use a torque wrench and follow specs.
• Skipping live data monitoring: Misfires that only occur under load may be missed with a static code check.
• Ignoring vacuum leaks: Small leaks around the intake manifold or hoses can cause lean fuel conditions leading to misfires. Use a vacuum gauge or smoke tester to identify leaks.

Fitment or Diagnostic Notes

• Ford Cylinder Numbering: For V6 engines (e.g., 3.5L EcoBoost), cylinder 4 is on the passenger-side bank, second from the front. For inline 4 engines (e.g., 2.0L Focus), it’s the rearmost cylinder.
• EcoBoost Models: Direct-injected engines often develop carbon buildup on intake valves, which can cause misfires. If other tests pass, consider professional walnut blasting.
• Pre-2000 Ford Models: Distributor systems may have worn caps/rotors or faulty spark plug wires contributing to misfires—add these to your diagnostic path.

Safety Precautions

• Always disconnect the negative battery terminal before working on ignition or electrical components to avoid shocks.
• Wear safety glasses to protect against flying debris when removing spark plugs or coils.
• Do not work on a hot engine to prevent burns.
• When handling fuel injectors, avoid open flames or sparks—fuel vapor is flammable.
• Use caution when cranking the engine for compression tests; keep hands clear of moving parts.

When To Stop and Call a Professional

• Compression test shows low or uneven readings (indicates internal engine damage like a blown head gasket or worn rings).
• Fuel injector replacement requires specialized tools or access to high-pressure fuel systems (common on EcoBoost models).
• Electrical issues (e.g., faulty wiring harness, PCM problems) that can’t be diagnosed with a basic multimeter.
• The misfire persists after replacing the spark plug, coil, and injector—this may point to a faulty camshaft position sensor or engine timing problem.