OBD-II Car P0303 Error
What it means
P0303 is a generic OBD-II code meaning a misfire has been detected in cylinder number 3. A misfire means combustion in that cylinder is incomplete or not happening at all. The engine computer (PCM) detects the misfire by monitoring tiny variations in crankshaft speed and pinpoints the affected cylinder โ here, cylinder 3.
The bottom line on the P0303 error
A P0303 isolates a misfire to cylinder 3, and the usual culprit is a worn spark plug or a failing ignition coil on that cylinder, not the whole engine. It is fairly DIY-friendly, but never keep driving with a flashing check-engine light, which dumps raw fuel into the exhaust and can ruin the catalytic converter.
The cheapest first move costs nothing: swap the cylinder 3 coil with a neighbor โ if the misfire follows to P0304, the coil is bad. Plugs and coils are $10โ$300; only a compression test pointing to a burned valve makes this a $1,000+ job.
P0303 snapshot
Our own at-a-glance triage of the P0303 error, from the verified data on this page. See error-code rankings โ ยท How we score โ
Common causes of the P0303 error
- Worn or fouled spark plug in cylinder 3
- Faulty ignition coil or coil-on-plug for cylinder 3
- Bad spark plug wire (on engines that use them)
- Clogged, leaking, or failed fuel injector for cylinder 3
- Vacuum leak near that cylinder's intake runner
- Low compression from a leaking valve, worn rings, or head gasket issue
- Carbon-fouled intake valves (common on direct-injection engines)
Which OBD-II cars get the P0303 error
P0303 is universal on OBD-II vehicles (1996+) and pinpoints cylinder #3 specifically. It is most common on coil-on-plug engines past roughly 60,000โ100,000 miles โ frequently seen on Honda/Acura, Toyota/Lexus, Ford, GM, and many European four-, six-, and eight-cylinders, where a single worn plug or a failing coil on the #3 cylinder is the usual culprit. On direct-injection engines, carbon buildup on the intake valves is an added cause worth checking.
How to fix the OBD-II Car P0303 error
- 1Swap and inspect the spark plugPull the spark plug from cylinder 3 and check for fouling, wear, cracks, or oil. Replace it if worn. A quick test: swap this plug with one from a known-good cylinder, clear the code, and see if the misfire follows the plug.
- 2Test the ignition coilOn coil-on-plug engines, swap the cylinder 3 coil with an adjacent cylinder. If the misfire moves to the new cylinder (e.g., P0304), the coil is bad and should be replaced.
- 3Check the fuel injectorListen for the injector clicking, or measure its resistance. A clogged or dead injector starves cylinder 3 of fuel. Cleaning or replacing the injector may be required.
- 4Look for vacuum leaksInspect intake hoses, gaskets, and the intake manifold near cylinder 3 for cracks or loose fittings. A localized vacuum leak leans out that cylinder and causes a misfire.
- 5Test compression if the basics check outIf plugs, coil, and injector are all good, perform a compression test on cylinder 3. Low compression points to a mechanical problem โ a burnt valve, worn rings, or a head gasket leak โ that needs deeper repair.
๐งฐ When to call a professional
Call a professional if the misfire continues after replacing the plug and coil, if the check engine light is flashing, or if a compression test shows a low reading on cylinder 3. Flashing-light misfires and mechanical compression problems need prompt professional attention to avoid catalytic converter or engine damage.
OBD-II P0303 vs OBD-II P0301 vs OBD-II P0300: how they compare
How this code stacks up against related error codes for difficulty, urgency, and cost.
| OBD-II P0303 | OBD-II P0301 | OBD-II P0300 | |
|---|---|---|---|
| DIY difficulty (1โ5) | 3 | 3 | 3 |
| Urgency (1โ5) | 4 | 4 | 4 |
| Typical cost | $$ | $$ | $$ |
How much does it cost to fix the P0303 error?
| DIY parts only | $10โ$300 |
| Professional diagnosis & repair | $100โ$500 |
| If major component (low compression / valve) | $1,000โ$3,000+ |
Repair or replace?
P0303 is usually a cheap-to-moderate fix because it isolates one cylinder, so you replace only that cylinder's plug ($10โ$40) or coil ($50โ$300 installed) rather than a full set. This is one of the most DIY-friendly codes: swap the #3 coil with a neighboring cylinder and clear the code โ if the misfire moves (to P0304), you have found a bad coil for a few dollars of diagnosis. Pay a shop when a compression or leak-down test is needed, which points to a burned valve, worn rings, or a head gasket ($1,000+). Prices vary by model and region, so treat these as general ranges. Never keep driving with a FLASHING check engine light โ a sustained single-cylinder misfire pumps raw fuel into the exhaust and can ruin the catalytic converter, which costs far more than the coil that caused it.
Common replacement parts for the P0303 error
| Spark plug (cylinder #3) | $5โ$40 | Cheapest fix; inspect for fouling, oil, or wrong gap | |
| Ignition coil (cylinder #3) | $50โ$300 | Installed; swap-test to confirm before buying | |
| Fuel injector (cylinder #3) | $150โ$500 | If plug and coil are good; clean or replace |
Prices are typical retail ranges and vary by model and supplier โ always confirm the exact part number for your appliance before buying.
OBD-II P0303 error โ FAQ
๏ผ โ Is it safe to drive with a P0303 code?
Only briefly, and not if the check engine light is flashing. A flashing light means an active misfire is dumping raw fuel into the exhaust, which can overheat and destroy the catalytic converter. With a steady light, drive gently straight to a repair and fix it promptly.
๏ผ โ How much does it cost to fix P0303?
It depends on the cause. Spark plugs are cheap (roughly $5โ$25 each). An ignition coil typically runs $50โ$200 with labor. A fuel injector or compression-related repair can run several hundred dollars or more.
๏ผ โ Why does only cylinder 3 misfire and not the others?
P0303 isolates the fault to cylinder 3's components โ its spark plug, coil, injector, or a local vacuum or compression problem. Because each cylinder has its own ignition and fuel parts, one can fail while the rest run fine.
Sources & manufacturer references
This guide is independently written and not affiliated with OBD-II. Always unplug appliances before servicing and follow your model's manual. Error codes and steps can vary by model โ when in doubt, consult a qualified technician.