P0303: Cylinder 3 Misfire Detected
Title: SAE J2012 generic definition. Manufacturer-specific definitions and enhanced codes can differ; check service information for the exact vehicle.
What P0303 means
The ECM counted repeated misfires on cylinder 3. This page focuses on separating fuel-side faults from ignition faults.
What sets it
Set by the crankshaft-speed misfire counter for cylinder 3 passing its calibrated threshold. On engines where cylinder 3 is hard to reach, the injector side is often skipped, which is why it gets the focus here.
Symptoms
- Steady miss at idle that may smooth out at higher rpm
- Hesitation under load
- Fuel-trim changes on the bank that contains cylinder 3 (V engines)
Is it safe to drive?
General guidance, not a guarantee: Minimize driving if the light flashes; get it diagnosed soon if it stays steady. If in doubt, have it towed and diagnosed.
Common causes
Listed roughly from most to least common in general practice. The real order varies by vehicle, mileage and history.
- Coil or plug on cylinder 3: check these first because they are quick to test
- Injector restricted, leaking or not pulsing: often the cause when coil and plug swaps don't move the misfire
- Injector wiring or connector
- Intake leak near cylinder 3
- Compression loss
Test before you replace: diagnostic sequence
General guidance only. Always follow the vehicle manufacturer's service information, specifications and safety procedures. Any figures below are general guidelines that defer to the manufacturer's specification.
- Confirm cylinder numbering for this engine in the service information.
- Rule out ignition quickly: swap the coil and plug to another cylinder and see whether the misfire follows.
- Check the injector for pulse with a noid light, or better, a scope or current probe that shows the injector's electrical signature compared with the other cylinders.
- Check the injector circuit for power and the driver control at the connector, following the wiring diagram.
- Where the manufacturer's procedure supports it, run an injector balance or contribution test with the scan tool or a fuel-pressure gauge.
- Look at fuel trims for the bank that contains cylinder 3. A restricted injector may make that bank read slightly lean.
- If fuel and spark are both confirmed, test compression and leak-down on cylinder 3.
- Verify the repair with misfire counters.
Bench note: If the plug comes out dry and clean while its neighbors are tan, suspect fuel delivery to that cylinder before ignition.
Related codes
FAQ
How do I know if it's the injector or the coil?
Swap the coil first. If the misfire stays on cylinder 3, check injector pulse and do a balance test. A dry plug on that cylinder is a strong hint that fuel isn't getting there.
Can a clogged injector be cleaned?
Sometimes. Professional off-car cleaning and flow testing exists, but replacement is common. Confirm the injector is the fault first.
Does the location of cylinder 3 matter?
Only for access. Cylinder numbering varies by engine, so confirm it before testing.
Sources
- SAE J2012, Diagnostic Trouble Code Definitions (standard page; code titles only)
- California Code of Regulations, 13 CCR 1968.2, OBD II requirements (via Cornell LII)
- US EPA, "What To Do With the Check Engine Light" (EPA420-F-00-041)
- Generic code title cross-check: obd-codes.com generic code list
Written with AI assistance and reviewed for accuracy; not a substitute for OEM service information. Shop Docs Studio is not affiliated with SAE, EPA, CARB or any vehicle manufacturer.