P0303
UniversalCylinder 3 Misfire Detected
The engine computer has detected combustion that is irregular or incomplete in cylinder 3. It generally identifies a misfire by monitoring changes in engine operation. A misfire can cause rough running, reduced power, or increased emissions, though the code alone does not identify the cause and the vehicle may show no obvious symptoms.
⚠️ Before You Begin (Safety First)
If the check-engine light is flashing, the engine is running very roughly, or power is significantly reduced, stop driving when safe and arrange diagnosis. Continued driving with an active misfire can overheat and damage the catalytic converter. Keep clear of moving or hot engine parts, and do not handle ignition components while the engine is running.
Common Symptoms
- Rough idle or vibration may occur if cylinder 3 is not contributing consistently.
- Hesitation or reduced power may occur when the engine is under load.
- The check-engine light may turn on or flash; flashing can indicate a misfire that risks catalytic-converter damage.
- Fuel economy may worsen if combustion is incomplete.
- There may be no noticeable symptoms, especially if the misfire is intermittent or mild.
Common Causes
- A spark plug, ignition coil, or related connection may fail to provide reliable ignition.
- An injector, its wiring, or a fuel-delivery issue may prevent cylinder 3 from receiving or using fuel correctly.
- Low compression or another mechanical problem may prevent the cylinder from combusting normally.
- Damaged wiring or a poor electrical connection may disrupt signals or operation associated with cylinder 3.
- An air leak or other engine-management issue may contribute to uneven combustion; testing is needed to determine whether it affects this cylinder.
Diagnostic Steps
- Note whether the check-engine light is steady or flashing and whether the engine is running roughly. Stop driving and seek help if the light flashes or the engine runs severely unevenly.
- With the vehicle parked safely, use a compatible scan tool to record all codes and available freeze-frame information before clearing anything. Other codes or operating conditions may help explain the misfire.
- With the engine off and cool, visually check for obvious damage or loose connections around accessible ignition and engine wiring. Do not pull on wires or touch ignition parts with the engine running.
- Check the vehicle's owner information for the correct procedure and service requirements before inspecting or removing any parts. If access is difficult or testing requires specialized equipment, have a qualified technician continue diagnosis.
- Avoid replacing a coil, plug, or injector based on this code alone; confirm the cause with appropriate testing.
When to Call a Professional
A technician can review stored codes and freeze-frame data, check cylinder-specific misfire data, and test ignition, injector operation, wiring, and compression as appropriate. The code identifies a detected condition, not a confirmed failed part; diagnosis should guide any replacement.
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Commonly Asked Questions
Does P0303 prove that the cylinder 3 ignition coil is bad?
No. The code reports a detected misfire in cylinder 3. Ignition parts are possible causes, but fuel delivery, wiring, compression, or other issues can also contribute.
Can I drive with P0303?
A steady warning light with normal vehicle behavior may allow only cautious travel to a repair location, but prompt diagnosis is advisable. If the light flashes, the engine runs roughly, or power drops substantially, stop driving when safe because continued misfiring can damage the catalytic converter.
Why might there be no noticeable symptoms?
A mild or intermittent misfire may not be obvious to the driver, even though the engine computer has detected irregular combustion.
How is the cause usually distinguished?
Diagnosis may combine scan data with checks of ignition, fuel-injector operation, wiring, and cylinder compression. The applicable tests and procedures depend on the vehicle's model, year, and engine.
Browse Available Code Guides
Previous and next available guides in database record order, across makes. Adjacent entries may share a code number and do not necessarily describe related faults.