P0303moderate severityGenerally driveableP0303 Code: Cylinder 3 Misfire Detected
Written in plain English and reviewed by the eli5cars editorial team · Last reviewed June 2026
What it means in plain English
P0303 is the same problem as P0301 — a cylinder misfiring — but cylinder 3 is the one that's stumbling. The engine isn't burning fuel cleanly in that cylinder, causing rough idle, loss of power, and sometimes a noticeable vibration. Because the ECM has pinpointed the exact cylinder, you know exactly where to start looking.
Most likely causes — ranked
#1 Worn or fouled spark plug in cylinder 3
most likelyFix: Locate cylinder 3 on your specific engine (on most 4-cylinder transverse engines, counting from the timing chain/belt end: cyl 1–2–3–4; on inline engines from the front). Remove and inspect the plug. If it is oil-fouled, black, or the electrode is worn flat, replace the full set — plugs should always be replaced together.
#2 Failed ignition coil for cylinder 3
likelyFix: Swap the cylinder 3 coil with cylinder 1 or 4. Clear the code and run through a drive cycle. If the code changes to P0301 or P0304, the coil is confirmed bad — replace it. If P0303 stays, the coil is fine and the problem is the plug, injector, or mechanical.
#3 Clogged or dead fuel injector on cylinder 3
possibleFix: Add a bottle of high-quality fuel injector cleaner (Chevron Techron, BG44K) and drive a full tank. If the misfire clears, a dirty injector was the likely culprit. If it persists, the injector needs professional cleaning or replacement.
#4 Low compression in cylinder 3 (worn piston rings or valve seat damage)
possibleFix: A compression test on all four cylinders identifies a mechanically worn cylinder. Cylinder 3 reading more than 15–20 psi below others suggests internal wear. This is typically a shop repair; cost depends heavily on root cause (rings vs. valves vs. head gasket).
Check engine light on? Get a free code scan
If your check engine light is on, most auto-parts stores (AutoZone, O'Reilly, Advance) will read the code for FREE — no purchase required, just walk in. Bring the code back here and look it up at eli5cars.com/obd2 for the plain-English explanation.
Note: a scan only helps when there's an active fault code. For purely mechanical symptoms (noises, vibrations, leaks), a scan may show nothing — the suspects listed above are your starting point.
Want to read the codes yourself?
First, the free option: AutoZone, O’Reilly, and Advance Auto Parts read your codes for free — no purchase needed. That’s the cheapest way to get the code to look up here. If you’d rather have your own so you can check anytime (and clear a code after a fix), the Veepeak OBDCheck BLE is an inexpensive, reliable plug-in reader that pairs with a free phone app and works on any car sold since 1996.
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Frequently asked questions
Can I drive with a P0303 cylinder 3 misfire?
With a solid check engine light and mild symptoms, you can drive carefully to the auto parts store for a free scan and to pick up spark plugs. A flashing check engine light means pull over immediately — severe misfires destroy catalytic converters.
I got both P0300 and P0303. Which do I fix first?
Start with cylinder 3 — P0303 identifies where the misfire is happening. Fix cylinder 3 (new plug, check the coil) and see if P0300 also clears. Often P0300 is set because the cylinder 3 misfire was frequent enough to look random to the ECM.
Where is cylinder 3 on my engine?
Cylinder numbering varies by engine layout. On most 4-cylinder front-wheel-drive cars (transverse mount), cylinders run 1–2–3–4 from the timing chain/belt end. On a V6, cylinder 3 may be on Bank 1 or 2 depending on the manufacturer. Search 'cylinder 3 location [your year make model]' or check the factory service manual.
How much does it cost to fix a P0303?
Spark plug only: $10 DIY, $80–$120 at a shop. Ignition coil: $35 DIY, $130–$200 at a shop. Injector: $50–$80 DIY (if accessible), $250–$400 at a shop. Compression issue: $1,500–$4,000 depending on root cause.