Powertrain Codes

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P0304

Cylinder 4 Misfire Detected

★★★★★ Common diagnostic code

P0304 means cylinder 4 is not contributing power consistently. The fault may be caused by ignition, fuel delivery, compression, vacuum leak, or wiring issues affecting that cylinder.

Atlas Quality75% · Good
Editorial StatusGood
SystemUnderstanding Fuel Delivery Systems
SeverityHigh
Can I keep driving?Use caution and diagnose before replacing parts.
Typical repair cost$80-$1,200+
Typical labor0.5-3.0 hours

Atlas Academy

Learn the System

Before diagnosing this code, it helps to understand the system behind it.

Read Understanding Fuel Delivery Systems

What P0304 Means

P0304 means cylinder 4 is not contributing power consistently. The fault may be caused by ignition, fuel delivery, compression, vacuum leak, or wiring issues affecting that cylinder.

Before You Replace Anything

Record the evidence first. Clearing codes too early can erase the clues that point to the real fault.

✓ Save freeze-frame data✓ Check for additional trouble codes✓ Look for technical service bulletins✓ Verify battery voltage✓ Verify engine oil level and coolant level✓ Perform a visual inspection✓ Do not clear the codes yet

Common Symptoms

⚠ Check Engine Light⚠ Flashing Check Engine Light⚠ Rough idle⚠ Engine shaking⚠ Loss of power⚠ Poor fuel economy⚠ Possible fuel smell

Most Common Causes

Worn spark plug90%
Faulty ignition coil78%
Fuel injector fault66%
Vacuum leak54%
Low compression42%
Fuel pressure problem30%

Diagnostic Confidence

Spark plug88%
Ignition coil78%
Fuel injector68%
Plug wire58%
Vacuum hose48%

Use this as a starting point, not a replacement for testing.

Recommended Diagnostic Workflow

1Save freeze-frame data
2Check misfire counters
3Inspect spark plugs and coils
4Swap coil or plug if practical
5Check injector operation
6Check for vacuum leaks
7Verify fuel pressure
8Perform compression or leakdown test if misfire remains

Mechanic's Tips

Tip #1

Use cylinder 4 as the starting point, but confirm whether the fault follows the coil, plug, injector, or remains with the cylinder before replacing parts.

Common Mistakes to Avoid

✕ Replacing parts before testing power, ground, and signal circuits✕ Ignoring related codes and freeze-frame data✕ Skipping the visual inspection✕ Clearing codes before confirming the repair

Common Repairs

Replace worn spark plug

Success Rate: ★★★☆☆

Average Cost: $80-$1,200+

Labor: 0.5-3.0 hours

Replace confirmed faulty ignition coil

Success Rate: ★★★☆☆

Average Cost: $80-$1,200+

Labor: 0.5-3.0 hours

Repair injector or wiring issue

Success Rate: ★★★☆☆

Average Cost: $80-$1,200+

Labor: 0.5-3.0 hours

Repair vacuum leak

Success Rate: ★★★☆☆

Average Cost: $80-$1,200+

Labor: 0.5-3.0 hours

Parts Commonly Replaced

Spark plugIgnition coilFuel injectorPlug wireVacuum hoseCoil connector

Tools Used During Diagnosis

OBD-II scan toolDigital multimeterBasic hand toolsManufacturer service information

Related Repairs

Diagnostic workflowComponent testingWiring inspectionRepair estimate checklist

How to Explain This to Your Customer

Cylinder 4 is misfiring, meaning that cylinder is not burning fuel correctly. Continued driving with a flashing Check Engine Light can damage the catalytic converter.

Manufacturer Notes

Generic OBD-II information. Exact diagnostic procedures, component locations, drive cycles, and known failures vary by make, model, year, engine, transmission, and calibration.

Technician Notes

Use cylinder 4 as the starting point, but confirm whether the fault follows the coil, plug, injector, or remains with the cylinder before replacing parts.

Related Codes

P0300 P0301 P0302 P0303 P0305 P0306 P0204 P0354

FAQ

Can I drive with P0304?

It depends on symptoms. If the vehicle runs normally, short trips may be possible, but flashing warning lights, overheating, severe misfire, poor shifting, or strong fuel odor should be diagnosed immediately.

Will P0304 fail emissions?

Usually yes if the Check Engine Light is on or readiness monitors are incomplete.

What should I check first for P0304?

Start with freeze-frame data, related codes, and the simple visual checks listed in this chapter before replacing parts.

Does P0304 always mean the listed part is bad?

No. The code points to a system or circuit. Testing should confirm the actual cause before replacing parts.

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Chapter Status

Quality Score75%
Editorial StatusGood
Last ReviewedPending review
Template Version1.3.4
Related Academy GuideUnderstanding Fuel Delivery Systems

Editorial Readiness

Reference Chapter Checklist

✓ Academy linked○ Quality Score ≥95✓ FAQ complete✓ Customer explanation✓ Internal links○ Reviewed

Why Trust This Guide?

✓ Updated regularly✓ Written for working technicians✓ Practical diagnostic process✓ Built for mobile mechanics and independent shops✓ Maintained by PistonCMS

Built by mechanics. Improved by mechanics.

This guide is maintained for working mechanics, mobile technicians, fleet maintenance professionals, and independent repair shops.