CodeXOBD

P0303 — Cylinder 3 Misfire Detected

P0303 means cylinder 3 is not firing correctly. The diagnosis is identical to other cylinder-specific misfires — ignition, fuel, or compression.

High

The engine runs rough with reduced power. Active misfires risk catalytic converter damage. This code will cause an emissions test failure.

$10–$500Parts, typical range
0.3–3 hLabour
estimateNot a quote · 2026 basis

What P0303 means

P0303 is the cylinder 3 equivalent of P0302. The module detected a crankshaft speed drop corresponding to cylinder 3's firing event. The causes and diagnostic approach are the same across all individual cylinder misfires — swap ignition components to isolate the fault, then check fuel delivery and compression.

Technical summary
SystemPowertrain — ignition and misfire
How a misfire is detectedThe crankshaft sensor gives position and speed, the camshaft sensor gives which stroke, and the module times the coil and injector for each cylinder from both. It detects a misfire by the small deceleration of the crankshaft that follows a cylinder which failed to fire, and attributes it to a cylinder when the pattern is consistent.Crank sensorposition, speedCam sensorwhich strokeModuletimes the sparkCoil & injectorper cylinderA misfire is measured as crankshaft deceleration, not as a missing spark

Symptoms you may notice

What causes P0303

Spark plug in cylinder 3 is fouled or worn

Very likelylow costeasy

The most common and cheapest cause of a single-cylinder misfire.

Ignition coil for cylinder 3 has failed

Likelylow costeasy

Swap with another cylinder to confirm. If the misfire follows the coil, replace it.

Fuel injector for cylinder 3 clogged or failed

Possiblemedium costmoderate

A clogged or electrically dead injector starves cylinder 3 of fuel.

Low compression in cylinder 3

Rarehigh costhard

Mechanical failure — worn rings, burned valve, or head gasket leak. This is the least common but most expensive cause.

How to diagnose it

1

Swap the ignition coil with another cylinder

Move the cylinder 3 coil to another cylinder. If the misfire follows, the coil is bad.

Tools: Basic hand toolsIf it passes: Misfire stays on cylinder 3. Continue to step 2.If it fails: Misfire followed the coil — replace it.
2

Inspect the spark plug

Remove and inspect the cylinder 3 spark plug for wear, fouling, or damage.

Tools: Spark plug socketIf it passes: Plug looks normal. Continue to step 3.If it fails: Replace the plug.
3

Check compression

Test compression in cylinder 3 and compare against the others.

Tools: Compression gaugeExpected: Within 10% of other cylindersIf it passes: Compression is normal. The injector is the suspect.If it fails: Low compression — investigate mechanical causes.

Related codes

P0300P0301P0302P0304

Common questions

Is P0303 more serious than P0300?

P0300 (random misfire) suggests a system-wide problem. P0303 is specific to one cylinder, which makes it easier to diagnose — you know exactly where to look.

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)

This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.