OBD code information sheet
https://dvireports.com/p0303
Cylinder 3 misfire detected
Selected definitionGeneral catalogue definition1 definitions availableChange
Choose the make reported by the scan tool or vehicle documentation. The same code can have different manufacturer meanings.
Technical explanation
P0303 identifies uneven combustion contribution from cylinder 3, without identifying the failed part. Investigate ignition on spark-ignition engines; on diesel engines focus on injection and mechanical condition, without assuming an ignition coil.
Symptoms
Rough running, hesitation or reduced power.
Steady/flashing engine lamp; possible hard starting and increased consumption.
Top suspects — what to check first
Suggested checking order, not statistical failure rates or a confirmed diagnosis. Priority can change with the engine and associated codes.
Only these cause categories are documented for this record; we do not invent additional suspects to make a top three.
On gasoline engines: spark plug, coil or high-voltage insulation fault, subject to testing.
Check firstOn gasoline engines: spark plug, coil or high-voltage insulation fault, subject to testing.
Injector, fuel supply or unmetered air; low compression or poor cylinder sealing.
Check nextInjector, fuel supply or unmetered air; low compression or poor cylinder sealing.
All causes in the technical record
Some causes overlap with the shortlist above; this is not a probability ranking.
- On gasoline engines: spark plug, coil or high-voltage insulation fault, subject to testing.
- Injector, fuel supply or unmetered air; low compression or poor cylinder sealing.
How to test — practical steps
Start with simple checks before buying parts. This is not a VIN-specific workshop procedure; keep clear of moving parts, fuel pressure and energized circuits.
Safety: Stop safely if the check-engine light flashes or the engine shakes severely. Do not touch energized ignition circuits or open pressurized fuel lines.
- 1
Identify cylinder 3 and save freeze-frame
Locate cylinder 3 using OEM numbering; record freeze-frame and all cylinder misfire counters.
- 2
Note when misfires occur and related faults
Establish cold/hot/load conditions and address related fuel/synchronisation codes first.
- 3
Petrol: inspect ignition for the exact engine
On gasoline engines inspect the plug/connections; if allowed, swap a compatible coil and check whether the misfire follows.
- 4
Workshop: check the correct injector
Use the approved injector/contribution test; on diesel use specified correction/leak-off tests without opening pressurised lines.
Detailed tests / equipment needed (advanced)
Perform only tests for which you have the correct wiring diagram, equipment and training.
Verify cylinder 3 contribution and compression
You need:Misfire-capable scannerIgnition system diagramWorkshop electrical/compression tests
- Use the approved injector/contribution test; on diesel use specified correction/leak-off tests without opening pressurised lines.
- If the fault remains on cylinder 3, test intake sealing, compression and cylinder leakage; avoid loading an engine with a flashing lamp.
What you should see: Results must match the applicable engine-specific test conditions and limits. One reading does not prove a component has failed.
Electrical, fuel, exhaust or timing measurements require suitable equipment and must not be improvised.
Technical references for verification
- Ford PCED — monitor rateuri P0301–P0310 — Definiția familiei și alternativele aprindere/injecție/compresie. Exemplu de monitor pe motoare Ford cu scânteie; nu prescrie teste diesel.
- HELLA — verificarea bobinei — Exemplul Alfa Romeo 147 1.6 TS privește explicit P0303: examinare, alimentare și comandă a bobinei, bujii, verificare după reparație. Valorile și procedura nu se generalizează.
Full technical diagnosis — additional checks
Step-by-step diagnosis
ChecksWork through the steps before replacing parts. Adapt the checks to the manufacturer’s procedure.
Locate cylinder 3 using OEM numbering; record freeze-frame and all cylinder misfire counters.
Establish cold/hot/load conditions and address related fuel/synchronisation codes first.
On gasoline engines inspect the plug/connections; if allowed, swap a compatible coil and check whether the misfire follows.
Use the approved injector/contribution test; on diesel use specified correction/leak-off tests without opening pressurised lines.
If the fault remains on cylinder 3, test intake sealing, compression and cylinder leakage; avoid loading an engine with a flashing lamp.
Components to check
This list does not confirm failed components and is not a replacement recommendation. Verify the fault cause and vehicle applicability first.
- Possible after diagnosis: plug/coil on spark-ignition engines only, injector, wiring or proven mechanical components.
Make- and engine-specific notes
Documented configurations, not proven common defects for all makes or engines. Selecting a vehicle narrows the results.
No reviewed engine-specific exception is available for the selected configuration. Use only the general checks and obtain the exact engine procedure.
Notes are diagnostic leads, not proven faults or disassembly instructions. Do not work on brakes, fuel systems or hot components without proper training.
Check the fuses before ordering a part
An OBD code describes a detected condition; it does not tell you which part to replace. Start with the fuse for the affected circuit.
If the fuse is good, carry on like this:
- Check the relevant supplies and connectionsWhere the diagnostic procedure calls for electrical checks, verify supply, ground and connectors under the manufacturer’s activation conditions.
- Run the specified component testUse the manufacturer’s test for this code and component before deciding it has failed.
- Order only after the fault is demonstratedReplace a part only when the prescribed tests identify it as defective.
Check details
- Identify the circuit and any relevant fuse: Confirm the vehicle, engine, code meaning and stored operating conditions first. A fuse link is a location aid, not evidence that the fuse caused this code. Use the specified type and rating; do not repeatedly replace a fuse that opens.
- Check the relevant supplies and connections: Use the specified engine state, circuit conditions and expected values. Have live measurements performed by someone qualified. For non-electrical faults, follow the relevant mechanical, fluid, pressure or leak checks instead.
- Run the specified component test: Do not apply a generic resistance range. Any resistance or continuity test requires a de-energized circuit and an isolated component, with the correct pins and temperature from the procedure. Some components need a different kind of test.
- Order only after the fault is demonstrated: If results are normal, continue the relevant wiring, control, mechanical or intermittent diagnosis. Check for related faults and verify the repair under the required conditions. A code or a completed checklist alone does not justify replacement.
If the procedure involves airbag/SRS, pretensioners or a high-voltage system, stop DIY testing and use qualified service. Do not probe these circuits with a multimeter.
Vehicle experiences and cases
User-reported cases published after moderation. Approval does not verify the diagnosis; the same code can have different causes.
No experiences have been published for this definition yet. You can be the first to share the checks and outcome.
See all vehicle experiencesManuals, components and vehicle guides
Find the relevant procedure in the workshop manual for your vehicle. A fault code alone does not identify a failed part, powertrain or exact schematic.
Manuals open at their original source. Exact diagram availability depends on the build year, engine and manufacturer access.
System and diagnostic diagrams
Conceptual illustrations of the system; verify the actual configuration and tests in vehicle-specific documentation.
Misfire and cylinder combustion
Illustrative petrol-engine ignition example. Original conceptual diagram, not an exact component location or an OEM repair procedure.
View full-size diagram →Can I fix it myself?
Once the cause is confirmed, open the job sheet: difficulty, time, tools, parts and steps. Common jobs for P0303:
How to fix P0303: step-by-step guide
Symptoms, causes, visual inspection, scan-tool or multimeter tests, replacement and clearing the code, with safety warnings.
Irregular combustion and misfire
General system guidance · not a confirmed diagnosis
Misfire may arise from ignition, fuelling or mechanical condition. Confirm cylinder numbering for the exact engine.
The association is based on the imported description. This information explains the system; it does not confirm a procedure for the exact code, model or engine. Verify applicability in manufacturer documentation.
Checks for this type of system
- Save codes and compare affected cylinders.
- Check spark ignition only on engines equipped with it.
- Separate injection, intake and compression concerns through workshop tests.
Useful diagnostic data, where available
- Per-cylinder misfire counts where supported.
- Fuel trims and occurrence conditions.
Normal values and thresholds vary by vehicle and are not assumed here.
Mistakes to avoid
- Not every misfire is a failed coil.
- Do not apply spark-plug tests to diesels.
System-guide sources
References support general principles, not individual verification of every associated code.
Continue your research
Links to related systems, warning lights and vehicle examples.
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Individually documented guide
The definition and diagnostic guidance were verified for this exact code identity.
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