Misfire No.3 cylinder

What P0303 means

The ECM monitors crankshaft rotational speed using the CKP sensor. Every time a cylinder fires, the crank accelerates slightly; a misfire causes a measurable deceleration at that power stroke interval. The ECM also uses the camshaft position sensor to identify which cylinder is at top dead centre, so it can assign each deceleration event to the correct cylinder number. P0303 is stored when cylinder 3's misfire rate exceeds the emission deterioration threshold (typically 2–5% over 1,000 crank revolutions) or the catalyst damage threshold (catalyst-damaging misfires in short windows). The MIL will flash rather than steady-illuminate when the misfire rate is severe enough to threaten the catalytic converter with thermal damage from unburnt hydrocarbons.

Symptoms

• Rough or lumpy idle with a rhythmic shudder at the frequency corresponding to every firing event of cylinder 3
• Check engine light on; MIL flashes if misfires are catalyst-damaging in severity
• Hesitation or stumble under acceleration, most noticeable between 1,500–3,000 RPM
• Reduced power output — the engine effectively runs as if one cylinder is removed
• Fuel economy noticeably lower due to unburnt fuel passing through the exhaust
• Possible exhaust smell of unburnt fuel if misfires are continuous at idle

Common causes

• Worn, fouled, or gap-bridged spark plug in cylinder 3 — the most frequent single-cylinder misfire cause
• Failed ignition coil on cylinder 3 on coil-on-plug designs, or a failed distributor cap/rotor on older distributor systems
• Faulty or clogged fuel injector on cylinder 3 delivering too little fuel or spraying in a poor pattern
• Compression loss in cylinder 3 from a burnt exhaust valve, worn piston rings, or head gasket breach at that cylinder
• Vacuum leak at the intake manifold runner or port gasket serving cylinder 3, creating a locally lean mixture

Severity & driving advice

Severity: High — A flashing MIL means active misfires are sending raw fuel into the catalyst — damage can occur within minutes at high misfire rates.

Can I drive? Flashing MIL: stop driving. Steady MIL: avoid highway.

Diagnostic approach

1. Confirm the misfire is assigned to cylinder 3 and read live misfire counts — Use a scan tool to view per-cylinder misfire counters in the data stream. Factory diagnostic data confirms the ECM tracks misfires per cylinder using VVT and CKP sensor data. Confirm that cylinder 3's misfire count is rising while the other cylinders remain near zero before proceeding.
2. Perform an ignition component swap test — On coil-on-plug ignition systems, swap the coil from cylinder 3 with the coil from a known-good cylinder (e.g. cylinder 1 or 2). Clear the code and run the engine. If P0303 changes to the code for the other cylinder (e.g. P0301), the coil is faulty. If P0303 stays on cylinder 3, the coil is fine — move on to the spark plug and injector.
3. Inspect the spark plug and check for wet fouling — Remove the cylinder 3 spark plug and examine the electrode. Black carbon or oily fouling indicates a rich condition or oil ingestion; white or blistered porcelain indicates a lean/overheated combustion event. Excessive gap or cracked porcelain confirms the plug needs replacement. Factory inspection procedure confirms spark plug condition as the first hardware test.
4. Check cylinder 3 compression and injector operation — Perform a dry compression test on cylinder 3; a result significantly lower than the other cylinders (more than 15% below average) or below specification confirms mechanical wear, a burnt valve, or a head gasket leak. If compression is normal, compare fuel injector pulse width and commanded fuel delivery for cylinder 3 versus its neighbours using a scan tool or injector flow test.

Make & model notes

Toyota: The 1GR-FE V6 engine (FJ Cruiser, 4Runner, Tacoma) mounts cylinder 3 on Bank 1. Factory procedure calls out VVT sensor cross-checking to confirm cylinder identification; if both P0303 and P0340 are stored simultaneously, address the cam sensor circuit first, as the cylinder misfire assignment depends on accurate cam position data.

Ford: The 5.4L 3-valve Triton V8 (F-150, Expedition) has well-documented spark plug thread pull-out issues in the aluminium heads. When servicing cylinder 3 or any cylinder on this engine, use a thread insert repair kit and avoid overtorquing the new plug.

General Motors: LS-family V8 engines with Active Fuel Management (AFM) deactivate cylinders 3, 5, 6, and 7. Lifter failure on any deactivated cylinder, including cylinder 3, causes P0303. A lifter tick accompanying the misfire strongly suggests AFM lifter collapse rather than an ignition or fuel fault.

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