P0303 – Cylinder 3 Misfire Detected means the engine control unit has identified that cylinder 3 is not firing as it should. A misfire in a single cylinder can cause rough idle, hesitation, poor fuel economy, and potential catalytic converter damage if ignored. The fault may be related to ignition components, fuel delivery, air metering issues, or mechanical problems. This guide explains what P0303 means, the symptoms, causes, diagnostic steps, and the best repair solutions to restore proper engine performance.
What Does P0303 Mean?
P0303 is a generic OBD-II code that indicates the ECU has detected a misfire occurring specifically in cylinder 3. The ECU monitors crankshaft speed and compares cylinder power contribution. If cylinder 3 produces insufficient torque or irregular combustion events, the ECU registers P0303.
Misfires must be addressed quickly because unburned fuel can enter the exhaust and overheat the catalytic converter, leading to expensive repairs if left unresolved.
Quick Reference
- OBD-II Family: P-Code (Powertrain)
- Scope: Generic
- System: Ignition / Fuel / Air / Mechanical
- Difficulty Level: Easy–Moderate
- Estimated Repair Cost: €50–€350+
- Last Updated: 5 December 2025
Real-World Example / Field Notes
A compact SUV came in with a rough idle and flashing check engine light under acceleration. Misfire counters showed cylinder 3 consistently failing. The spark plug was heavily worn and showed signs of carbon fouling. Installing a new plug and coil restored smooth performance.
In another case, a sedan misfired only when cold. The injector for cylinder 3 was partially clogged, causing a lean misfire until engine temperature stabilized. Professional injector cleaning corrected the imbalance and eliminated P0303.
Symptoms of P0303 – Cylinder 3 Misfire Detected
- Rough idle: Noticeable shaking or vibration at low RPM.
- Loss of power: Hesitation or stumbling during acceleration.
- Poor fuel economy: Misfires reduce combustion efficiency.
- Check engine light: May flash during severe misfires.
- Fuel smell: Unburned fuel may enter the exhaust.
- Stalling: Severe misfires can cause intermittent stalling.
Common Causes of P0303
Most Common Causes
- Worn or fouled spark plug in cylinder 3
- Faulty ignition coil or coil boot
- Clogged or leaking fuel injector (cylinder 3)
- Vacuum leak near cylinder 3 intake runner
- Contaminated or failing mass airflow (MAF) sensor
Less Common Causes
- Low compression from valve or piston wear
- Failing camshaft or crankshaft position sensor
- Fuel pressure too low due to pump or regulator issues
- ECU driver malfunction (rare)
- Carbon buildup affecting cylinder 3 combustion
Diagnosis: Step-By-Step Guide
Diagnosing P0303 requires a structured approach to ignition, fuel delivery, airflow, and mechanical condition.
Tools You’ll Need
OBD-II scanner with misfire counters, spark tester, fuel pressure gauge, compression tester, multimeter, smoke machine, and standard hand tools.
- Review freeze-frame data: Identify engine load, RPM, and temperature when the misfire occurred.
- Inspect cylinder 3 spark plug: Look for wear, oil fouling, cracks, or carbon buildup.
- Swap ignition coils: Move coil 3 to another cylinder. If the misfire follows, the coil is faulty.
- Check fuel injector performance: Use a balance test or swap injectors to verify spray pattern and flow.
- Check for vacuum leaks: Smoke-test the intake area near cylinder 3.
- Clean or test the MAF sensor: Under-reporting airflow can cause lean misfires.
- Evaluate fuel trims: Lean trims suggest air leaks; rich trims may indicate injector issues.
- Measure fuel pressure: A weak pump or regulator reduces fuel delivery to all cylinders.
- Perform compression test: Low compression confirms mechanical issues affecting cylinder 3.
- Check cam/crank timing: Incorrect timing or sensor faults may cause intermittent misfires.
Pro Tip: If the misfire appears only under load or high RPM, suspect ignition coil breakdown — heat and voltage stress expose weak coils first.
Possible Fixes & Repair Costs
- Replace spark plug (cylinder 3): €20–€60
- Replace ignition coil: €50–€120
- Clean or replace fuel injector: €120–€250
- Repair vacuum leak: €50–€150
- Clean or replace MAF sensor: €20–€150
- Repair mechanical issues (valves/piston): €300–€900+
Can I Still Drive With P0303?
Driving with P0303 is not recommended. While the vehicle may run, misfires send unburned fuel into the catalytic converter, causing overheating and severe damage. If the check engine light is flashing, stop driving immediately to avoid costly repairs.
Related Codes
- P0399 – Ignition Coil “R” Primary/Secondary Circuit
- P0364 – Reserved
- P0363 – Misfire Detected Fueling Disabled
- P0362 – Ignition Coil “L” Primary/Secondary Circuit
- P0361 – Ignition Coil “K” Primary/Secondary Circuit
- P0360 – Ignition Coil “J” Primary/Secondary Circuit
- P0359 – Ignition Coil “I” Primary/Secondary Circuit
- P0358 – Ignition Coil “H” Primary/Secondary Circuit
- P0357 – Ignition Coil “G” Primary/Secondary Circuit
- P0356 – Ignition Coil “F” Primary/Secondary Circuit
Key Takeaways
- P0303 means cylinder 3 is misfiring and needs immediate attention.
- Most common causes include spark plugs, coils, injectors, or vacuum leaks.
- A flashing MIL indicates serious catalyst-damaging misfires.
- Early diagnosis prevents costly repairs and restores smooth operation.
FAQ
What is the most common cause of P0303?
Failing ignition coils and worn spark plugs are the most common causes of cylinder-specific misfires.
Can a clogged injector cause P0303?
Yes. If injector 3 has restricted flow or stuck pintles, it can cause a lean or inconsistent misfire.
Is it safe to drive with a cylinder 3 misfire?
No. Misfires can damage the catalytic converter and create unsafe driving conditions.
Can a vacuum leak affect only cylinder 3?
Yes. A leak near the intake runner for that cylinder can create a localized lean condition.
Should I replace all coils or just the faulty one?
You can replace only the failed coil, but on high-mileage engines, replacing coils in sets may prevent future failures.