P0303 Code Meaning and Next Steps

First checks take 10 to 20 minutes for the first checks. No special tools are usually needed for the first checks.

Can you keep driving?

Stop driving if any of these apply

!The check-engine light is flashing or the engine is shaking badly.
!The vehicle is stalling, struggling to accelerate, or obviously running rough.

If the light is steady and the vehicle still drives normally: Maybe, but only for a very short distance if the engine still runs smoothly.

What to check first

Step-by-step checks

1
Safety first
Read the code with the engine off and avoid touching hot or moving components while you inspect the basics
2
Free - no tools
If the check-engine light is flashing or the engine is shaking badly, stop driving before doing anything else
3
Basic tool needed
Check for related cylinder-specific misfire, lean, or fuel-trim codes stored alongside P0303
4
Basic tool needed
Look for loose ignition-coil connectors, obvious intake leaks, or anything recently left disconnected in the engine bay
5
Basic tool needed
Notice whether the fault happens at idle, under load, when cold, or all the time
6
Basic tool needed
If spark plugs or ignition components are accessible, inspect those basics before ordering more expensive parts

If the code returns

-If the misfire follows the spark plug or coil after a swap, replace that confirmed failed part and retest.
-If cylinder 3 still misfires with known-good ignition parts, injector balance or compression testing becomes much more important.
-If lean or fuel-trim codes are also present, solve the shared mixture issue before replacing more cylinder-3 parts.

Background

What this code means

P0303 is a generic OBD-II code for a misfire detected on cylinder 3.

This is more targeted than a random-misfire code, so swap tests and cylinder-specific checks usually pay off faster than broad parts replacement.

Diagnosis

Common causes

Most common

Ignition weakness

Worn spark plugs or a weak ignition coil are among the most common triggers.

Common

Unmetered air

A vacuum leak or split intake hose can upset the mixture enough to cause misfires.

Common

Fuel-delivery fault

Low fuel pressure or injector problems can create a random or repeat misfire pattern.

Possible

Mechanical issue

Low compression, timing problems, or valve faults can also create a misfire when simpler checks do not explain it.

Avoid these mistakes

What not to do

xDo not keep driving with a flashing check-engine light.
xDo not replace several expensive ignition or fuel parts at once without confirmation.

Parts

Parts that may need replacing

PartTypical costNotes

Spark plugs$20-$80 for a basic set on many vehiclesOften the cheapest confirmed fix when plugs are worn or overdue.

Ignition coil$40-$120 eachBest bought after a swap test or stronger cylinder-specific evidence.

Vacuum hose or intake boot$10-$60Relevant when a split or loose air leak is actually found.

Related OBD codes

P0300

P0300 usually means the engine is detecting random or multiple-cylinder misfires.

P0302

P0302 means cylinder 2 is misfiring.

P0304

P0304 usually means cylinder 4 is misfiring.

Source notes

Generic OBD-II (SAE J1979 / ISO 15031-5). P0303 was seeded from dtcdb and then expanded as a cylinder-specific misfire guide with targeted ignition, injector, and compression checks.

This guide is written as a generic multi-make reference, so bulletin history, sensor locations, and repair order can still change by manufacturer and engine family.

This is generic OBD-II guidance and should not override vehicle-specific service information. Exact diagnosis and repair steps vary by make, engine family, and model year.

Last reviewed: 2026-04-10

Reference: Open reference

P0303 - Cylinder 3 Is Misfiring