P0303 Code: Cylinder 3 Misfire Causes, Diagnosis, and Repair Cost

The Short Answer

P0303 means "Cylinder 3 Misfire Detected" — your engine's control module caught cylinder 3 failing to fire correctly, and the most common fix is a new ignition coil and spark plug on that cylinder.

What Exactly Is the P0303 Code? P0303 is an OBD-II diagnostic trouble code that translates to **"Cylinder 3 Misfire Detected."** Worth noting: a few popular sites have published this code with a typo calling it a "Cylinder 2" misfire — that's flat-out wrong. It's cylinder 3, full stop. The way the ECM/PCM catches this is clever. There's no sensor sitting inside the combustion chamber watching the flame. Instead, the computer tracks **crankshaft rotational speed**. Every time a cylinder fires, the crank accelerates slightly. If cylinder 3 fires and that expected acceleration doesn't happen, the system logs a misfire event. Enough events in a set window, and P0303 sets. A steady Check Engine Light means the system has logged the code. A **flashing** Check Engine Light is a different situation entirely — it signals severe, ongoing misfires that can push unburned fuel into the exhaust and rapidly destroy your catalytic converter. If your light is flashing, stop driving and diagnose it today. ---

The 6 Most Common Causes of P0303

1. Failed Ignition Coil (Cylinder 3)

This is the culprit the majority of the time. Coil-on-plug systems are common on modern engines, and a coil that's cracked, heat-soaked, or just worn out will produce a weak or absent spark. The good news: it's one of the cheapest possible fixes. ### 2. Worn or Fouled Spark Plug

A plug with excessive electrode wear, carbon fouling, or oil contamination won't ignite the mixture reliably. If your plugs haven't been changed in 60,000–100,000 miles (depending on plug type), this should be your first replacement. ### 3. Faulty Fuel Injector

If the injector on cylinder 3 is clogged, leaking, or electrically dead, the fuel delivery is off and combustion won't happen properly. You can often hear a healthy injector clicking rhythmically with a mechanic's stethoscope — a dead one will be silent or irregular. ### 4. Low Compression on Cylinder 3

This is where things get expensive. Burned valves, worn piston rings, or a failing head gasket can all rob a cylinder of the compression it needs to fire. A compression test will tell you within minutes. Anything below about 120 psi on a gasoline engine (check your spec) deserves attention. ### 5. Vacuum Leak Near Cylinder 3

A cracked intake manifold gasket or a loose vacuum hose close to cylinder 3 can lean out that cylinder's mixture. This usually shows up with a rough idle that smooths out at higher RPM. ### 6. Wiring or Connector Problem

Before you replace expensive parts, wiggle the wiring harness connector at cylinder 3's coil. Corrosion, a broken wire, or a loose pin can mimic a dead coil perfectly. A cheap fix if that's all it is. ---

How to Diagnose P0303 Step by Step

You don't need a lift or a dealership scan tool to work through this logically. 1. Scan and record all codes. If P0300 (random misfire) appears alongside P0303, you likely have a broader problem — don't just swap the cylinder 3 coil and call it done. 2. Check for a flashing CEL. If it's flashing, treat this as urgent. 3. Swap the cylinder 3 coil to a different cylinder (say, cylinder 1) and clear the code. Drive it. If the misfire moves to P0301, your cylinder 3 coil is confirmed bad. If P0303 stays put, the coil is fine and you move on. 4. Replace the spark plug on cylinder 3. While you're in there, inspect it — heavy carbon, oil fouling, or a cracked porcelain tip tells a story. 5. Check the fuel injector. Use a noid light or a stethoscope to confirm it's firing. 6. Run a compression test. This is the last mechanical check before you start talking to a machine shop. My honest take: I've seen shops skip straight to injector replacement when a simple $45 coil swap would've done it. Start cheap and confirm each step. The swap-the-coil test in step 3 takes ten minutes and saves a lot of guessing. ---

What Does It Cost to Fix a P0303 Code? Costs vary a lot depending on the root cause, your vehicle, and whether you DIY. | Repair | DIY Part Cost | Shop Total (Est.) |

If you're replacing one coil anyway, it's worth looking at coil-and-plug kit sets — doing all cylinders at once makes sense if the car has high mileage or you don't know the service history. ---

Which Parts Should You Buy? For ignition coils and spark plugs, brand quality actually matters here. Two names worth knowing:

DENSO supplies ignition components to major automakers as original equipment. Their coil and spark plug kits — available in sets of 6, 8, 12, 16, and more depending on your engine — give you OEM-equivalent quality for a DIY install. Broad coverage across Japanese, domestic, and European applications. NGK, now sold under the parent brand Niterra Co., Ltd. (formerly NGK Spark Plug Co., Ltd.), makes coil-on-plug ignition coils paired with their Laser Iridium spark plugs. Their iridium-tipped plugs have a strong reputation for longevity and consistent spark, especially on import vehicles. Both are legitimate OEM-level choices. If your vehicle is Japanese-brand, check which one was originally spec'd — you may find you're getting the exact same part. > "Don't cheap out on ignition components. A no-name coil that fails six months later costs you double — once in parts, once in labor or your Saturday."

Can P0303 Damage My Engine If Ignored? Yes, and in more than one way. Unburned fuel that makes it into the exhaust stream soaks into the catalytic converter's substrate and can overheat and melt it. Catalytic converters can run $300–$2,000+ to replace depending on the vehicle. Beyond that, a cylinder running without combustion can wash the cylinder walls with raw fuel, stripping oil film and accelerating wear on rings and bores. Fix it before it compounds. ---

Related OBD-II Codes to Know

• P0300 — Random/Multiple Cylinder Misfire (appears with P0303 when multiple cylinders are affected)
• P0301, P0302, P0304+ — Misfire codes for cylinders 1, 2, 4, and beyond
• P0420 / P0430 — Catalyst efficiency codes that can appear *after* a prolonged misfire damages the converter

If P0420 shows up right after you fix P0303, assume the converter took some heat damage during the misfire period. ---