Cylinder 3 Misfire Detected
In short
P0303 means the engine computer (PCM) detected a misfire in cylinder number 3 — the cylinder isn't burning its air-fuel mixture properly on one or more firing cycles. The most common real-world cause is a worn or fouled spark plug or a failing ignition coil on that specific cylinder. Because the misfire is isolated to one cylinder, the fix is often a relatively inexpensive ignition component rather than a major repair.
Is it safe to drive with P0303?
Light, intermittent misfires are often safe to drive home or to a shop, but a steady misfire — especially with a flashing check engine light — is not, because unburned fuel can overheat and destroy the catalytic converter. If the light is flashing or the engine is shaking badly, stop driving and reduce load. Prolonged driving with an active misfire risks expensive converter damage and can leave you stranded.
Symptoms
- Rough or shaky idle that may smooth out at higher RPM
- Check engine light on, or flashing during active misfire
- Noticeable loss of power and hesitation under acceleration
- Engine stumble or jerking, especially when cold or under load
- Increased fuel consumption and occasional fuel smell from the exhaust
- Vibration felt through the steering wheel or seat at idle
Common causes (most → least likely)
How to diagnose it (before buying parts)
- 1 Scan for all stored codes and freeze-frame data — note whether other misfire codes (P0300, P0301, etc.) or fuel/sensor codes are present, since these change the diagnosis.
- 2 Visually inspect cylinder 3 ignition components: pull the coil and plug, check for oil in the plug well, cracked boots, carbon tracking, and a worn or fouled plug electrode.
- 3 Swap the cylinder 3 ignition coil with a neighboring cylinder, clear codes, then retest — if the misfire follows the coil to the new cylinder, the coil is the fault.
- 4 Test fuel and air on that cylinder: listen for or test the injector (swap injectors if practical) and check for vacuum/intake leaks near cylinder 3 with carb cleaner or smoke.
- 5 If ignition and fuel check out, perform a compression and/or leak-down test on cylinder 3 to rule out a mechanical fault before buying parts.
Repair options & cost
By manufacturer
Coil-on-plug ignition coils are a frequent failure point and commonly cause single-cylinder misfire codes; replacing the failed coil and the worn plug together is the usual fix.
Coil-on-plug units and, on some V engines, plug boots are common misfire culprits; carbon tracking on boots and worn plugs at extended service intervals often trigger single-cylinder codes.
Generally robust ignition, but aged coil-on-plug units and long-interval iridium plugs eventually cause isolated misfires; check for oil intrusion in the plug tubes from leaking valve-cover seals.
Ignition coil packs are a well-known weak point and often fail one at a time, producing single-cylinder misfire codes; many owners replace coils as a full set when one fails.
Frequently asked questions
Can I drive with a P0303 code?
You can usually drive a short distance with a mild, steady-light misfire, but you should fix it promptly. If the check engine light is flashing, the misfire is active and severe — driving risks overheating and ruining the catalytic converter, so minimize driving and get it diagnosed.
How much does it cost to fix a P0303?
Most P0303 repairs are inexpensive: a spark plug runs about 10-60 dollars and an ignition coil about 40-300 dollars in parts. Shop repairs typically land between 100 and 450 dollars, while rarer mechanical causes like low compression can cost much more.
Will a P0303 clear itself?
The misfire detection can stop and the light may go out if the cause was temporary (such as a one-time bad tank of fuel or moisture), but a failing coil, plug, or injector will not fix itself. The code usually returns until the underlying part is repaired.
What's the difference between P0303 and P0300?
P0303 is a misfire isolated to cylinder 3 specifically, which usually points to a single-cylinder component like that plug, coil, or injector. P0300 is a random or multiple-cylinder misfire, which more often points to a shared cause such as fuel pressure, a vacuum leak, or a bad ground.