System: Powertrain (P-Code) Coverage: SAE Standard (Generic) Fault Family: misfire_single_cylinder

Code P0303: Cylinder 3 Misfire Detected

Diagnostic & Technical Explanation

Code P0303 (Cylinder 3 Misfire Detected) sets when the ECM's misfire monitor isolates repeated combustion events lost on one specific cylinder (or, for the unspecified variant, a single cylinder it cannot pin down). The diagnosis path is identical regardless of which cylinder number is reported: the fault lives in that cylinder's ignition, fuel delivery, or mechanical sealing, most commonly a fouled spark plug or failing coil, a clogged or leaking injector, or low compression from a burnt valve or worn rings.

Probability-Ranked Repair Procedures (4 Ranked Fixes)

Rank #1

Swap spark plug and ignition coil to confirm the cylinder

Easy DIY

Parts Estimate: $15.0–$45.0 USD Labor Estimate: 0.6 Shop Hours

1. Remove the coil and plug from the misfiring cylinder
2. Inspect the plug for fouling, wear, or an incorrect gap
3. Swap the coil with a known-good adjacent cylinder's coil
4. Clear the code and see if the misfire follows the coil or stays with the cylinder
5. Replace the plug and/or coil based on the result

Required Replacement Parts: 🔧 Spark plug (single) · 🔧 Ignition coil-on-plug unit

Rank #2

Test and clean or replace the cylinder's fuel injector

Moderate DIY

Parts Estimate: $40.0–$180.0 USD Labor Estimate: 1.0 Shop Hours

1. Perform an injector balance or ohms test on the affected cylinder
2. Swap the injector with a neighboring cylinder to see if the misfire follows
3. Clean the injector on an ultrasonic bench or replace it
4. Reinstall with new O-rings and clear codes

Required Replacement Parts: 🔧 Fuel injector (single cylinder) · 🔧 Injector O-ring service kit

Rank #3

Run a cylinder compression or leak-down test

Moderate DIY

Parts Estimate: $20.0–$80.0 USD Labor Estimate: 1.5 Shop Hours

1. Disable fuel and ignition for a dry compression test on all cylinders
2. Compare the flagged cylinder's reading against its neighbors
3. Add oil and retest (wet test) to isolate rings vs. valves
4. Follow up with a leak-down test to confirm the exact leak path

Required Replacement Parts: 🔧 Compression tester kit · 🔧 Cylinder leak-down tester

Rank #4

Inspect and replace a leaking or damaged cylinder head gasket/valve

Professional Required

Parts Estimate: $150.0–$900.0 USD Labor Estimate: 4.5 Shop Hours

1. Confirm the leak-down result points to a head gasket or valve seat
2. Remove the valve cover/head as required for access
3. Replace the head gasket or recondition the valve seat
4. Reassemble to torque spec and verify compression returns to spec

Required Replacement Parts: 🔧 Cylinder head gasket set · 🔧 Valve seat/guide service kit