# P0303: Cylinder 3 Misfire Detected **Verdict:** Drive to a shop this week Ignition and misfire - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$1,500 - **Urgency:** This week - **Most likely:** Spark plug or ignition coil on cylinder 3 - **DIY?:** Steps 1-3 ## Quick answer P0303 means the engine computer detected misfires in cylinder 3. Most often the cylinder 3 coil or spark plug is failing. Carbon tracking on a coil boot or a cracked plug insulator is a common and easy-to-miss cause, especially after wet weather. ## What does P0303 mean? Cylinder 3's location varies more than most. On inline four-cylinder engines it is the third cylinder from the timing end, one of the two center cylinders. On three-cylinder engines, such as small Ford, Mitsubishi and Kia engines, it is the last cylinder. On GM and Chrysler V8s it is second on the driver's side, and on Toyota and Honda transverse V6s it sits on the rear bank near the firewall, where access is tighter. The engine computer counts brief crankshaft slowdowns and assigns each one to a cylinder by firing order. P0303 means cylinder 3 is missing often enough to raise emissions or, at a higher rate, to threaten the catalytic converter, in which case the light flashes. The milder version usually must appear on two trips before the light stays on. A misfire that worsens in rain or high humidity points to spark leaking through a cracked coil boot or plug insulator. ## What are the symptoms of P0303? - Check engine light on or flashing - Rough running that may be worse in damp weather - Jerking under acceleration - Engine shake at idle ## What causes P0303? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Failing ignition coil or cracked coil boot** (most common). Coil boots harden with heat and can crack or develop carbon tracks, thin black lines where spark leaks to the engine. Moisture makes the leak worse, so a misfire that appears on rainy or humid days, or after a car wash, points here. On some engines the boot is sold separately, making the repair cheaper than a full coil. 2. **Worn or cracked spark plug** (common). A crack in the plug's white porcelain insulator, often from a dropped plug or overtightening, lets spark escape before it reaches the electrode. Worn electrodes and heavy carbon have the same effect. Inspect the cylinder 3 plug under bright light for hairline cracks and dark streaks on the porcelain, and compare its gap with the cylinder 2 or 4 plug. 3. **Clogged or failing cylinder 3 injector** (less common). A clogged injector delivers less fuel than commanded, so cylinder 3 runs lean and misfires, mostly at idle and light throttle. A leaking injector floods it instead and can cause a rough cold start. Engines that sit for long periods are more prone to varnish. Swapping the cylinder 3 injector with a neighbor shows whether the misfire follows it. 4. **Vacuum or intake leak near cylinder 3** (rare). A leaking intake manifold gasket at the cylinder 3 port, or a cracked plastic runner, adds unmetered air to that one cylinder. It runs lean and misfires mostly at idle, smoothing out as rpm rises. Engines with plastic intake manifolds that have been through many heat cycles are more prone. A smoke test of the intake system finds the leak. 5. **Low compression in cylinder 3** (rare). A burnt or tight valve, worn piston rings or a head gasket leak lets cylinder 3 lose pressure, and new ignition parts will not fix it. The misfire tends to be constant at all speeds. Engines that have overheated or burn oil are most at risk. A compression test, then a leak-down test, shows a reading well below the others. ## How do you diagnose P0303? 1. Find cylinder 3 on your engine using a cylinder numbering diagram. 2. Remove the cylinder 3 coil and inspect the boot for carbon tracks, cracks or moisture. 3. Swap the coil with a neighboring cylinder, clear codes and drive; if the misfire moves, replace the coil. 4. Pull and inspect the cylinder 3 spark plug for cracks or heavy wear. 5. Test the injector and compression if the misfire stays on cylinder 3. ## How do you fix P0303, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace the cylinder 3 ignition coil or boot | $100-$350 | Some engines allow just the boot to be replaced. | | Replace spark plugs | $120-$450 | Do all cylinders at once. | | Replace the cylinder 3 injector | $250-$700 | After testing confirms it. | Full price breakdowns: [ignition coil replacement cost](/cost/ignition-coil-replacement), [spark plug replacement cost](/cost/spark-plug-replacement). ## Is it safe to drive with P0303? > **Drive to a shop this week.** Drive gently to a shop if the light is steady. If it flashes, pull over safely and do not keep driving, because the catalytic converter can be damaged quickly. ## Will P0303 fail an emissions test? With P0303 setting the check engine light, the car will not pass an OBD-based emissions inspection. If the misfire has gone on for a while, the shop may also find catalyst efficiency codes once it is fixed. After the repair and code clearing, the evaporative, catalyst and oxygen sensor monitors must rerun, which takes normal driving over several days. ## After the repair: clearing P0303 After replacing a coil or boot, clear the code and test in the conditions that caused trouble, including a damp morning if the misfire was weather-related. A scan tool showing zero cylinder 3 misfire counts at idle and on acceleration is the best sign. If counts creep up again, move on to the injector and compression checks. ## How to prevent P0303 - Inspect coil boots for black carbon tracks or cracks whenever the spark plugs come out. - Avoid pressure-washing the engine bay directly over the coils and spark plug wells. - Keep hood seals and cowl drains clear so rainwater does not drip onto the coils. - Replace spark plugs as a set, gapped and torqued to the maker's specification. - Handle new plugs carefully; a dropped plug can have a hidden crack in its insulator. ## Codes often seen with P0303 - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected - [P0302](/codes/p0302): Cylinder 2 Misfire Detected - [P0304](/codes/p0304): Cylinder 4 Misfire Detected - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) ## Frequently asked questions ### Why does my P0303 misfire get worse in the rain? Moisture lets spark leak through a cracked coil boot or plug insulator to the engine instead of jumping the plug gap. That is a classic sign of a worn coil or boot. Replacing the coil or boot usually fixes it, and inspecting the plug for cracks at the same time is wise. ### Can I fix P0303 myself? Often yes, if the coil or plug is the cause and you can reach them. On most four-cylinder engines the coil is held by one bolt. On Toyota and Honda V6s, cylinder 3 is on the rear bank and may need cowl or intake parts removed. Injector and compression problems are shop work. ### Will a P0303 code clear on its own? It may go away if the misfire stops for a few trips, but the cause is usually still there. The light will come back when the misfire returns, often on the next damp day. Fix the coil, plug or injector to stop it for good rather than clearing it and hoping. ### Where is cylinder 3? On an inline four-cylinder, cylinder 3 is third from the timing belt or chain end, next to cylinder 4. On three-cylinder engines it is the last one. On GM and Chrysler V8s it is second on the driver's side, and on Toyota and Honda transverse V6s it is on the rear bank. ### How much does it cost to fix P0303? Most P0303 repairs cost $100 to $350 for a coil or boot, or $120 to $450 for a set of spark plugs at an independent shop. An injector runs $250 to $700. Rear-bank access on some V6s adds labor, and compression problems are the main path to a bill over $1,000. ### Can I replace just the coil boot? On some engines, yes. Some makers and aftermarket suppliers sell the rubber boot separately, and replacing a carbon-tracked boot can cure the misfire for much less than a full coil. If the coil body is cracked, or the misfire returns after a new boot, replace the whole coil. ### What does a carbon track look like? A carbon track is a thin, dark line or burn mark running along the inside of a coil boot or down the plug's porcelain. It shows where spark has escaped to the engine instead of jumping the plug gap. Once a track forms the part keeps leaking, so cleaning it is no lasting fix. ### Do cylinders 2 and 3 share a coil? On many older four-cylinder engines with a waste-spark coil pack, yes, cylinders 2 and 3 fire from the same coil. A bad coil then usually misfires both, so P0303 alone points more to the plug, plug wire or injector. Most newer engines have a separate coil on each plug instead. ### Can I drive with P0303? Gentle driving to a shop is usually fine with a steady light. If the light flashes, pull over when safe and avoid hard acceleration, because the misfire is overheating the catalytic converter. A weather-related cylinder 3 misfire can get much worse in heavy rain, so avoid long trips until it is fixed. ### Will P0303 pass inspection? No, a vehicle with P0303 and the check engine light on fails an OBD-based emissions inspection. After the repair, clear the code and drive a few days of mixed city and highway trips. Use a scan tool to confirm the readiness monitors have completed before going back, so you are not turned away. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) ## Other ignition and misfire codes - [P0300 Random/Multiple Cylinder Misfire Detected](https://diagxcar.com/codes/p0300) - [P0301 Cylinder 1 Misfire Detected](https://diagxcar.com/codes/p0301) - [P0302 Cylinder 2 Misfire Detected](https://diagxcar.com/codes/p0302) - [P0304 Cylinder 4 Misfire Detected](https://diagxcar.com/codes/p0304) - [P0305 Cylinder 5 Misfire Detected](https://diagxcar.com/codes/p0305) - [P0306 Cylinder 6 Misfire Detected](https://diagxcar.com/codes/p0306) - [P0307 Cylinder 7 Misfire Detected](https://diagxcar.com/codes/p0307) - [P0308 Cylinder 8 Misfire Detected](https://diagxcar.com/codes/p0308) - [P0316 Engine Misfire Detected on Startup (First 1000 Revolutions)](https://diagxcar.com/codes/p0316) - [P0320 Ignition/Distributor Engine Speed Input Circuit](https://diagxcar.com/codes/p0320) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) - [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle) - [Why does my car shake when I accelerate?](https://diagxcar.com/symptoms/car-shakes-when-accelerating) ## Repair costs - [Ignition coil replacement cost](https://diagxcar.com/cost/ignition-coil-replacement): $150-$400 - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 --- Source page: https://diagxcar.com/codes/p0303 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.