P0303 - Cylinder 3 Misfire Detected

P0303 means cylinder 3 is misfiring often enough for the ECM to flag it. Good diagnosis confirms whether the problem follows the plug or coil, stays with the injector, or points to compression loss, leak-down failure, or a small coolant seep on that cylinder.

Treat the code as the starting point: compare symptoms, likely causes, and repair paths before pricing parts or moving into an estimate.

Cylinder 3 Misfire Diagnostic Path

Treat P0303 as a focused cylinder 3 fault until spark, fuel, air, and compression evidence points to the repair.

Inspection Priority

• Inspect the spark plug before replacing the coil
• Swap coil or plug only when the test can prove whether the misfire moves
• Check injector command, fuel delivery, and compression if the fault stays on the same cylinder

Estimate Guidance

• Quote ignition parts after plug or coil testing supports the fault.
• Use injector or compression diagnosis when swap testing does not move the misfire.
• Check catalyst-risk history before pricing downstream converter work.

Use the blueprint links to verify the likely system, then continue the estimate with the same vehicle and code context.

Code Overview

P0303 means cylinder 3 is misfiring often enough for the ECM to flag it. Good diagnosis confirms whether the problem follows the plug or coil, stays with the injector, or points to compression loss, leak-down failure, or a small coolant seep on that cylinder.

Common Causes

• Ignition coil breakdown under load or heat soak
• Spark plug fouling, worn electrodes, incorrect gap, or oil contamination
• Injector leakage causing cold-start misfire or fuel-washed plug tips
• Low compression, valve sealing problems, or worn valvetrain components
• Small head gasket seep causing overnight coolant intrusion into one cylinder
• Vacuum leaks, intake runner leaks, or PCV leaks affecting cylinder balance

Symptoms

• Noticeable shake at idle
• Hesitation or stumble under load
• Reduced fuel economy while the misfire is active

Diagnostic Steps

1. If the misfire happens only on cold start, inspect for injector leakage, coolant seep, or valve sealing issues before replacing coils only.
2. If the misfire appears under acceleration or load, inspect ignition coil output, plug gap, plug condition, and coil boots for breakdown.
3. If the same-cylinder misfire stays after a coil swap, move toward injector testing, compression testing, and leak-down testing.
4. If coolant loss appears with an overnight rough start, head gasket seep or coolant intrusion suspicion increases.
5. If the misfire repeats with clean plugs and no obvious ignition fault, compression and leak-down testing become important.
6. If multiple cylinders misfire with positive fuel trims, inspect vacuum leaks, intake leaks, PCV leaks, and shared fuel delivery.

Diagnostic Insight

P0303 should be narrowed by condition and movement: whether cylinder 3 misfires cold, under load, or remains fixed after plug and coil swaps determines whether ignition, injector, compression, or head gasket testing comes next.

• If the misfire follows a cylinder 3 coil swap, ignition coil replacement becomes a stronger path; if it follows the plug, inspect fouling, gap, carbon tracking, and plug condition first.
• Cold-start misfire that improves warm can point to injector leakdown, plug fouling, valve sealing, or small head gasket coolant seep rather than a simple coil fault.
• Load-related misfire keeps coil breakdown, plug gap, plug-wire or boot tracking, and spark leakage high on the list.
• If the fault stays on cylinder 3 after plug and coil swaps, injector balance, contribution testing, compression testing, and leak-down testing become important.
• Repeated same-cylinder misfire after ignition parts replacement should raise suspicion for fuel injector, compression, valve sealing, or coolant intrusion issues.

Repair Difficulty

Moderate

General difficulty estimate for the most common repair path.

Likely Repairs & Cost Guides

Use symptoms, scan data, and quick checks to confirm the likely repair path before pricing parts. The estimator helps compare repair paths before replacing anything unnecessarily.

Related OBD Codes

Browse nearby code-family pages when the same fault pattern overlaps across systems, such as misfire, lean fuel trim, EVAP sealing, cooling, or charging faults.

Next Steps

Move from code lookup to diagnosis, then estimate the likely repair only after symptoms, causes, and checks point to the same path.