Diagnostic code
P0303 Code: Cylinder 3 Misfire Detected
P0303 means cylinder 3 has a repeat misfire pattern. A technician will usually confirm the cylinder location, inspect ignition parts, check injector behavior, and rule out compression or valve problems before replacing parts.
Educational reference only, not professional repair advice. A fault code is a starting point, not a diagnosis. Vehicle symptoms, live data, service history, and manufacturer-specific procedures change the repair path. If the check engine light is flashing, the vehicle is overheating, brakes or steering feel unsafe, fuel odor or smoke appears, or the engine is running poorly, stop driving when safe and get professional help.
Sources
Safety limits for this page
Use this page as a reference only. Do not perform hazardous repairs from a web guide. Stop when safe and contact a qualified mechanic if any warning condition below applies.
- Opening fuel lines, fuel tanks, or pressurized fuel components
- Flashing check engine light
- Overheating, smoke, or strong fuel smell
- Loss of power, severe shaking, unsafe braking, or unsafe steering
- Fuel odor, visible leak, smoke, or hard starting after refueling
Can I drive?
If the check engine light is flashing or the engine shakes, stop driving when safe. With a steady light and smooth engine, short local driving to a shop may be possible, but avoid highway speeds and hard acceleration until diagnosed.
Repair range
$100-$1,400
Scanner note
Basic OBD-II scanner with live data
Plain-English Meaning
P0303 means cylinder 3 has a repeat misfire pattern. A technician will usually confirm the cylinder location, inspect ignition parts, check injector behavior, and rule out compression or valve problems before replacing parts.
Common Causes
Common possibilities (most common first):
- Cylinder 3 spark plug wear
- Weak coil or damaged plug wire
- Fuel injector restriction
- Mechanical compression issue
- Localized air leak near cylinder 3
Order varies by make, model, mileage, and operating conditions. Do not replace parts based only on this list — verify with a scan tool and qualified mechanic.
Symptoms
- Shaky idle
- Hesitation
- Poor fuel economy
- Check engine light after acceleration
Diagnostic Steps
- 1Find cylinder 3 on the specific engine
- 2Inspect the plug and coil connection
- 3Swap coil or injector if safe to test
- 4Check fuel trim and misfire counters
- 5Have a qualified shop perform compression or leak-down testing when needed.
Confusable And Related Codes
P0300
Compare →P0300 means the engine is misfiring randomly or across multiple cylinders instead of one clearly identified cylinder. Ignition, fuel, air leaks, compression problems, or timing issues can all interrupt combustion and trigger this code.
P0301
Compare →P0301 means cylinder 1 is not burning its air-fuel mixture consistently. The problem may be isolated to that cylinder, such as a spark plug, coil, injector, compression issue, or local air leak near the intake runner.
P0302
Compare →P0302 means cylinder 2 is misfiring enough for the engine computer to identify it. The cause may be electrical, fuel-related, mechanical, or an air leak that affects that cylinder more than the others.
P0304
Compare →P0304 means cylinder 4 is not firing reliably. The code narrows the search to one cylinder, but the final cause still has to be separated between spark, fuel, air, compression, and wiring faults.
FAQ
Is P0303 serious?
Yes when symptoms are strong or the light flashes, because converter damage can happen quickly.
Can P0303 be an injector?
Yes. An injector that is clogged, leaking, or not commanded correctly can misfire one cylinder.
Why does P0303 happen under load?
Ignition weakness often shows up under load because the spark has to work harder.
This page is educational and is not a substitute for hands-on vehicle diagnosis.