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Engine Misfire: Causes, Symptoms & How to Fix P0300-P0308

An engine misfire occurs when one or more cylinders fail to produce power during the combustion cycle. You'll feel a stumble, hesitation, or jerking — especially under load. Misfires trigger OBD-II codes P0300 (random misfire) through P0308 (cylinder 8 misfire), making diagnosis straightforward with a scan tool.

What Is It?

In a properly running engine, each cylinder fires in precise sequence: intake, compression, combustion, exhaust. A misfire means the combustion event didn't happen (or happened incompletely) in one or more cylinders. This can be caused by lack of spark, lack of fuel, or lack of compression.

Common Causes

Worn or Fouled Spark Plugs

Very Common

The most common misfire cause. Spark plugs degrade over time — the electrode gap widens, ceramic insulation cracks, or carbon deposits accumulate. This prevents clean ignition of the air-fuel mixture.

Failed Ignition Coil

Very Common

Modern engines use individual coil-on-plug (COP) designs. When a coil fails, the corresponding cylinder gets weak or no spark. Symptoms are consistent misfire on one cylinder (P0301-P0308).

Clogged or Leaking Fuel Injectors

Common

Fuel injectors must deliver precisely metered fuel. Clogged injectors starve the cylinder; leaking injectors flood it. Both conditions cause misfires. Injector issues are often the cause when misfires are intermittent.

Vacuum Leak

Common

Unmetered air entering through a cracked hose or leaking gasket causes a lean condition. The affected cylinder(s) run too lean to combust properly, causing misfires especially at idle.

Low Compression

Moderate

Worn piston rings, a burnt valve, or a blown head gasket reduce compression. Without adequate compression, the air-fuel mixture can't ignite properly. Compression testing reveals this.

Bad Fuel Quality

Moderate

Contaminated or stale fuel doesn't combust properly. Water in fuel, wrong octane rating, or fuel that has sat for months can all cause misfires across multiple cylinders.

How to Diagnose

  1. 1

    Read OBD-II codes to identify which cylinder(s) are misfiring. P0300 = random/multiple, P0301 = cylinder 1, etc.

  2. 2

    For single-cylinder misfire: swap the ignition coil with an adjacent cylinder. If the misfire follows the coil, replace it.

  3. 3

    Inspect and replace spark plugs if worn, fouled, or past their service interval.

  4. 4

    Check fuel injector operation: listen for clicking with a mechanic's stethoscope, or swap injectors between cylinders.

  5. 5

    Inspect vacuum hoses and intake manifold gasket for leaks.

  6. 6

    Perform a compression test if ignition and fuel system check out — low compression points to internal engine issues.

  7. 7

    Check fuel pressure to rule out a weak fuel pump.

Estimated Repair Cost

$50 (spark plugs) to $600+ (fuel injectors or coil pack)

When to See a Mechanic

If the check engine light is flashing, stop driving immediately — a constant misfire can destroy the catalytic converter ($1,000+ repair). If swapping coils and replacing plugs doesn't fix it, professional injector testing and compression testing are needed.

Frequently Asked Questions

What does engine misfire feel like?

You'll feel a stumble, jerk, or hesitation — especially during acceleration. The engine may vibrate more than normal, and you may notice a loss of power. In severe cases, the vehicle shakes noticeably.

Can I drive with an engine misfire?

A steady check engine light with occasional misfire is generally safe for short distances. However, a flashing check engine light means severe misfiring — stop driving immediately to prevent catalytic converter damage.

How much does it cost to fix an engine misfire?

Common fixes: spark plugs ($50-$150), ignition coil ($100-$300), fuel injector replacement ($200-$600). Internal engine issues (compression) can cost $1,000+. Diagnosis typically costs $100-$150.

Can bad fuel injectors cause misfire?

Absolutely. Clogged injectors restrict fuel flow, causing lean misfires. Leaking injectors over-fuel the cylinder, causing rich misfires. Electrically failed injectors deliver no fuel at all. Injector cleaning or replacement resolves this.

Related Parts

Shop fuel injectors →Shop ignition coils →Shop spark plugs →

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