DIYAUTOFIXESMANUALS

Diagnostic Trouble Code Database

P0303Severe (Stop & Fix)

Cylinder 3 Misfire Detected

System:Ignition & Combustion•Classification:Powertrain (OBD-II Generic)•Severity Impact:Active cylinder 3 combustion failure. Continuous driving under heavy load causes catalytic converter overheating.
Direct Technical Definition (Fast Answer)

Code P0303 indicates that cylinder 3 is failing to contribute combustion energy during crankshaft rotation. Most commonly caused by a worn spark plug, cracked ignition coil boot, or fuel delivery shortfall at cylinder 3.

Estimated Repair Time20 – 45 minutes
DIY Parts Cost$15 – $65
Shop Repair Quote$180 – $380
Average DIY Savings$165 – $315
The 80/20 Workshop Rule

Swap cylinder 3's ignition coil with cylinder 4. If the code shifts to P0304, replace the ignition coil. If it stays on cylinder 3, replace the spark plug.

Estimated DIY Cost
$15 – $65
Tools & DIY parts
Dealership Shop Quote
$180 – $380
Labor ($180/hr) + parts
Net Money Saved
$165 – $315
In your driveway today

Interactive Symptom Pinpoint Troubleshooter

Answer 2 diagnostic tests to isolate the exact component failure for P0303.

DIAGNOSTIC TEST 1 OF 1Step 1

Did the misfire code move to cylinder 4 after swapping the ignition coil?

Live Scanner & Multimeter Benchmark Values

Compare your live OBD-II PID sensor data against OEM factory operating specs for P0303.

Monitored Parameter / Sensor PIDHealthy OEM Operating RangeFault Threshold (Triggers P0303)
Cylinder 3 Ignition Coil Primary Resistance
0.5 – 1.8 Ohms
Open loop (OL) confirms burned coil winding
Cylinder 3 Compression Ratio (WOT Cranking)
140 – 185 PSI (within 10% of others)
<100 PSI indicates burnt exhaust valve or cylinder wall scoring

Symptoms Associated with P0303

Drivers typically observe the following symptoms when the ECM flags P0303:

Check engine light illuminated or flashing
Engine shaking noticeably at stoplights
Loss of power when accelerating
Gasoline odor from exhaust

Common Root Causes & Statistical Probability Matrix

Statistical breakdown of what triggers this code based on automotive repair shop data.

Failed ignition coil pack on cylinder 3
High ProbabilityDifficulty: Easy
Relative Frequency: 75%Estimated Parts Cost: Low
Worn spark plug with excessive gap
High ProbabilityDifficulty: Easy
Relative Frequency: 75%Estimated Parts Cost: Low
Faulty fuel injector or wiring harness pigtail
Medium ProbabilityDifficulty: Moderate
Relative Frequency: 45%Estimated Parts Cost: Medium

Step-by-Step Diagnostic & Repair Procedure

Follow this logical order to isolate and repair P0303 before spending money on costly parts.

Checklist Progress: 0 of 3 steps complete

Step 1: Swap Coil

Swap coil 3 with coil 4. Test drive with OBD2 scanner.

Step 2: Check Plug

Inspect cylinder 3 plug condition and gap.

Step 3: Check Compression (if misfire persists)

Perform screw-in compression test on cylinder 3.

Vehicle-Specific Failure Trends & Technical Bulletins

Real workshop failure patterns reported by mechanics across popular makes and models for P0303.

Subaru Outback / Forester (2.5L EJ25 / FB25)

Common Failure Point:Tight clearance against frame rail causes technicians to leave cylinder 3 spark plug unchanged during tune-ups.
Verified Workshop Fix:Use a wobble extension and thin-wall 5/8 socket to install fresh NGK Laser Iridium plug.

Jeep Cherokee / Wrangler (3.6L Pentastar V6)

Common Failure Point:Needle roller bearings in cylinder 3 intake rocker arm fail, wearing camshaft lobe flat.
Verified Workshop Fix:Listen for distinct ticking noise; replace rocker arm and camshaft before valve damage occurs.

Dave Miller, ASE Master Tech

ASE L1 Certified Master Reviewer

ASE Certified Master Automobile Technician (A1-A8) & L1 Advanced Engine Performance • 24 years automotive diagnostic experience

“Always verify sensor voltages and harness grounds before purchasing replacement electronic components.”

Editorial review and field diagnostic verification: Updated October 2026

Frequently Asked Questions About P0303

Do not drive if the check engine light is blinking. If steady, drive gently to a repair garage.