Ignition & misfire · Powertrain · generic (SAE-defined)
P0303: Cylinder 3 Misfire Detected
P0303 means cylinder 3 is misfiring often enough for the engine computer to log it, typically due to a spark, fuel, or compression problem in that cylinder.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0303 sets
The powertrain control module (PCM) measures crankshaft acceleration after each expected firing event. When cylinder 3 repeatedly fails to add power, the PCM counts misfires for that cylinder and sets P0303 once a calibrated threshold is crossed. Thresholds are set by each manufacturer.
Common causes of P0303
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Worn or incorrectly gapped cylinder 3 spark plug
- Weak ignition coil or damaged coil boot on cylinder 3
- Restricted or stuck cylinder 3 fuel injector
- Oil leaking into the cylinder 3 spark plug tube
- Leaking intake manifold gasket near cylinder 3
- Low compression in cylinder 3
P0303 symptoms
- Check engine light on or flashing
- Rough running, especially at idle
- Hesitation or jerking under load
- Lower fuel mileage
How P0303 is diagnosed
- Check freeze frame data and other stored codes.
- Pull the cylinder 3 coil and inspect the boot and plug well for oil or cracks.
- Swap the cylinder 3 coil with another and see if the misfire follows.
- Inspect the spark plug for wear, gap, and deposits.
- Check injector function and wiring for cylinder 3.
- Test compression if spark and fuel are confirmed good.
Typical fixes for P0303
- Replace the spark plug and coil boot
- Replace the cylinder 3 ignition coil
- Replace leaking valve cover or spark plug tube seals
- Clean or replace the cylinder 3 injector
- Repair intake gasket leaks
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
P0303 FAQ
What does code P0303 mean?
P0303 means cylinder 3 is misfiring often enough for the engine computer to log it, typically due to a spark, fuel, or compression problem in that cylinder. The standard definition is "Cylinder 3 Misfire Detected."
Can I drive with a P0303 code?
You can usually drive carefully to a repair shop while the check engine light is steady. Once it begins flashing, the misfire is severe enough to harm the catalytic converter, and driving should stop soon.
Why is there oil on the cylinder 3 spark plug?
Oil in the plug well usually comes from leaking valve cover or spark plug tube seals and can short the coil boot. Oil on the firing tip of the plug, however, may point to internal wear.
Should all spark plugs be replaced for a P0303?
If the plugs are near their service interval, replacing the full set is common practice so all cylinders fire evenly.
Keep researching
Engine guides by brand (known issues, specs)
More ignition & misfire codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.