What are the typical symptoms of P0303?

Miss that worsens under load and lightens at steady cruise Lean trim swing when unmetered air causes the miss Sharp popping or spitting sound at the intake or tailpipe Vibration through the pedals arriving in bursts Fuel odour at the tailpipe after prolonged idling

What causes code P0303?

Weak spark from a failing coil or a gap opened by plug wear Vacuum leak at the runner or injector O-ring starving cylinder 3 Injector for cylinder 3 clogged or spraying an uneven pattern Carbon tracking inside the plug boot bleeding spark to ground Burnt exhaust valve or a worn cam lobe on cylinder 3

What recommended checks apply to P0303?

Watch firing kV on cylinder 3 under load to split spark from fuel Pressurize the intake and listen at the runner and injector base Compare injector current ramps across cylinders on a scope Inspect the boot and plug well for arc tracks and oil Run a leak-down on cylinder 3 if spark and fuel test clean

P0303Cars & SUVs

Easy

P0303Cylinder 3 misfire detected

Q: What does code P0303 mean?

P0303 means the monitor pinned the combustion fault on cylinder 3 rather than spreading it across the engine. Because a lean single-cylinder miss and an ignition-side miss feel nearly identical from the driver's seat, the fastest split is to watch the spark event under load: observe firing kV on that cylinder with a scope or an in-line tester while the engine pulls in gear. Voltage that flares high points to plug gap, a cracked boot, or an open secondary; normal kV with a persistent miss shifts the search to fuel and air. Check the intake runner and injector O-ring for a leak that starves only this cylinder, then compare injector current ramps side by side. Postponing the repair pushes raw fuel into the converter, which is a far larger bill than a coil.

Content reviewed: August 2026Content reviewed by a heavy-duty technician. About our editorial approach

OK to drive

— Unburned fuel from a persistent miss overheats the catalytic converter: treat a flashing light as stop-driving and a steady light as a repair within days.

What does this severity mean?

General guidance only — not a substitute for a professional inspection or official roadside / out-of-service criteria (e.g. CVSA OOSC) in your jurisdiction.

Part of system: Electrical sensing & powertrain inputs · Wire gauge & ampacity tool

Means the monitor pinned the combustion fault on cylinder 3 rather than spreading it across the engine. Because a lean single-cylinder miss and an ignition-side miss feel nearly identical from the driver's seat, the fastest split is to watch the spark event under load: observe firing kV on that cylinder with a scope or an in-line tester while the engine pulls in gear. Voltage that flares high points to plug gap, a cracked boot, or an open secondary; normal kV with a persistent miss shifts the search to fuel and air. Check the intake runner and injector O-ring for a leak that starves only this cylinder, then compare injector current ramps side by side. Postponing the repair pushes raw fuel into the converter, which is a far larger bill than a coil.

Parts that may be involved

Coil and boot assembly for cylinder 3
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Spark plugs matched to the engine specification
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Injector seals or O-rings if a leak is found at the base
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Vehicles often affected

Frequent on V6 layouts where cylinder 3 sits on the rear bank and is serviced last, and on engines with plastic intakes whose gaskets shrink with age.

Typical symptoms

Miss that worsens under load and lightens at steady cruise
Lean trim swing when unmetered air causes the miss
Sharp popping or spitting sound at the intake or tailpipe
Vibration through the pedals arriving in bursts
Fuel odour at the tailpipe after prolonged idling

Likely causes

Weak spark from a failing coil or a gap opened by plug wear
Vacuum leak at the runner or injector O-ring starving cylinder 3
Injector for cylinder 3 clogged or spraying an uneven pattern
Carbon tracking inside the plug boot bleeding spark to ground
Burnt exhaust valve or a worn cam lobe on cylinder 3

Recommended checks

1
Watch firing kV on cylinder 3 under load to split spark from fuel
2
Pressurize the intake and listen at the runner and injector base
3
Compare injector current ramps across cylinders on a scope
4
Inspect the boot and plug well for arc tracks and oil
5
Run a leak-down on cylinder 3 if spark and fuel test clean

DIY vs professional

DIY: plug and boot inspection, a basic smoke test with a rented tool. Pro: scope-based kV and injector diagnostics, cylinder head work.

Content reviewed by a heavy-duty technician. About our editorial approach