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P0300 — Random or Multiple Cylinder Misfire

P0300 is set when the engine control module's misfire monitor detects that combustion is failing in more than one cylinder, or that misfires are occurring randomly across cylinders rather than being isolated to a single one. The misfire monitor works by measuring tiny variations in crankshaft rotational speed — a missed combustion event causes the crankshaft to decelerate by a detectable amount. When the ECM cannot attribute the event to a single cylinder, it sets P0300.

P0300 means random or multiple cylinder misfire. Stop driving and diagnose it before continuing — it can signal an unsafe condition. The most common cause is worn or fouled spark plugs (all cylinders due for replacement) (typically $60–$350). Causes and cost vary by make and model; confirm the root cause before replacing parts.

Severity: high powertrain Do not drive

Reviewed by MECH AI Editorial · Last verified

What does P0300 mean?

P0300 is set when the engine control module's misfire monitor detects that combustion is failing in more than one cylinder, or that misfires are occurring randomly across cylinders rather than being isolated to a single one. The misfire monitor works by measuring tiny variations in crankshaft rotational speed — a missed combustion event causes the crankshaft to decelerate by a detectable amount. When the ECM cannot attribute the event to a single cylinder, it sets P0300.

What are the symptoms of P0300?

What causes P0300?

Cause Likelihood Estimated repair (USD)
Worn or fouled spark plugs (all cylinders due for replacement) — Especially common past 80,000 miles on platinum or iridium plugs. Most common $60–$350
Failing ignition coils — coil-on-plug systems Most common $80–$600
Vacuum leak causing lean misfire across all cylinders Common $80–$500
Low fuel pressure (weak pump, clogged filter) Common $80–$900
Clogged or leaking fuel injectors Occasional $150–$1,500
Carbon-fouled intake valves (direct-injection engines) Occasional $400–$900
Low engine compression (worn rings, burnt valve, head gasket) Rare $1,500–$5,000
Failed crankshaft or camshaft position sensor causing timing issues Rare $150–$550

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P0300?

No. P0300 is a high-severity code — avoid driving until it is diagnosed and repaired, as it can indicate an unsafe condition or risk further damage.

How to diagnose P0300

  1. Pull every code, including pending and freeze-frame data

    Read all stored, pending, and permanent codes. Note which cylinder(s) show misfire counts in mode 6 or the manufacturer-specific misfire PID. Freeze-frame data tells you the engine load, RPM, and temperature when the misfire occurred — critical for narrowing down the cause.

    Tools: Scan tool with mode 6 / misfire counters

  2. Swap ignition coils between cylinders

    On a coil-on-plug system, move the coil from a suspect cylinder to a known-good cylinder. Clear codes, drive, and re-read. If the misfire follows the coil, the coil is bad. If it stays in the same cylinder, the coil is not the cause.

    Tools: Socket set, Torque wrench (for plug torque on reassembly)

  3. Inspect spark plugs visually

    Remove all spark plugs. Compare their appearance. A black, sooty plug indicates a rich condition for that cylinder. A wet, fuel-smelling plug means no spark or no compression on that cylinder. White or blistered ceramic indicates an overly lean condition. Replace the full set if any are out of spec or past service interval.

    Tools: Spark plug socket, Extension and ratchet, Anti-seize and torque wrench

  4. Test fuel pressure under load

    Connect a fuel pressure gauge and observe pressure at idle, snap-throttle, and steady cruise. A weak pump may hold pressure at idle but drop under load — the time pattern matters as much as the absolute reading.

    Tools: Fuel pressure gauge

  5. Perform a cylinder compression or relative-compression test

    If misfires persist after ignition and fuel system work, perform a compression test on each cylinder, or use a scan tool's relative compression function. Cylinders more than 15 % below the rest indicate a mechanical problem (rings, valves, or head gasket).

    Tools: Compression tester, Scan tool with relative compression

How do I fix P0300?

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P0300: frequently asked questions

What does diagnostic trouble code P0300 mean?

P0300 is set when the engine control module's misfire monitor detects that combustion is failing in more than one cylinder, or that misfires are occurring randomly across cylinders rather than being isolated to a single one. The misfire monitor works by measuring tiny variations in crankshaft rotational speed — a missed combustion event causes the crankshaft to decelerate by a detectable amount. When the ECM cannot attribute the event to a single cylinder, it sets P0300.

What are the symptoms of P0300?

Check Engine Light flashing (severe misfire) or steady (minor misfire). Rough idle and noticeable engine shake at a stop. Hesitation or stumble under load. Loss of power, especially at higher RPM or on hills. Reduced fuel economy. Smell of raw fuel from the exhaust. Possible catalytic converter damage with prolonged operation

What causes P0300?

Worn or fouled spark plugs (all cylinders due for replacement) (most-common). Failing ignition coils — coil-on-plug systems (most-common). Vacuum leak causing lean misfire across all cylinders (common). Low fuel pressure (weak pump, clogged filter) (common). Clogged or leaking fuel injectors (occasional). Carbon-fouled intake valves (direct-injection engines) (occasional). Low engine compression (worn rings, burnt valve, head gasket) (rare). Failed crankshaft or camshaft position sensor causing timing issues (rare)

Is it safe to drive with P0300?

No. P0300 is a high-severity code — avoid driving until it is diagnosed and repaired, as it can indicate an unsafe condition or risk further damage.