Number P0300: Random/Multiple Cylinder Misfire Detected
The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Fo
Can you drive with P0300?
Severity: Stop soon — misfire. Only as far as a shop, gently. A flashing check-engine light means an active severe misfire: pull over and arrange a tow.
If you ignore it: A misfiring cylinder dumps unburned fuel into the exhaust, which overheats and destroys the catalytic converter — a $150 ignition fix becomes a $1,500 converter.
This is the #7 most common code in our top-150 library, in the engine misfire family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0300 means
The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Ford 0.9–1.5 %) it comes on after two trips. Ford and GM may shut off a misfiring cylinder's injector to protect the catalyst. P0300 sets when the misfire cannot be assigned to one cylinder: several cylinders each account for more than 10 %, the cylinder changes randomly, or – at Ford – the cylinder cannot be identified at high speed. It often appears together with cylinder codes. Causes are usually systemic: false air, worn plugs on several cylinders, fuel pressure or fuel quality, valve timing – and false detection from a damaged tone wheel, rough roads or vibration. Bulletins name coils, cracked plug insulators and wrong plug gaps.
Systems involved: ignition system, fuel system, intake system, crankshaft position sensor.
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | False air (vacuum leak, crankcase ventilation, intake gasket) | most likely |
| 2 | Worn or wrong spark plugs on several cylinders, faulty ignition coils | most likely |
| 3 | Fuel pressure too low or high, contaminated fuel | possible |
| 4 | Valve timing (stretched chain), valves, compression on several cylinders | less common |
| 5 | False detection: tone wheel or pulley, rough road, vibration, software | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine light on, flashing at a catalyst-damaging rate
- Rough idle, jerking, vibration
- Loss of power, hesitation
- Smell of unburnt fuel, higher consumption
Misdiagnosis traps
- Shotgunning coils and plugs at a random misfire without live misfire counters — the counters tell you whether it is truly random or one cylinder wearing a disguise.
- A vacuum leak or low fuel pressure causes random misfires that ignition parts will never fix.
How a good shop confirms it
- Watch live misfire counters per cylinder while the fault occurs — cold only, idle only, or under load each point somewhere different.
- Check fuel trims: positive trims plus random misfires = air/fuel problem; normal trims = ignition or compression.
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| Ignition coils + spark plugs (full set) | $280 – $645 | $180 – $420 (parts only) | 2/5 |
| Spark plugs — full set | $115 – $340 | $40 – $140 (parts only) | 2/5 |
| Intake / vacuum leak repair (gaskets, hoses) | $338 – $838 | $150 – $400 (parts only) | 3/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor). Published cross-check — Ignition coils + spark plugs (full set): CarMD 2026 national average $480; Spark plugs — full set: CarMD 2026 national average $299.
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.5–6 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
Why does the flashing light matter so much?
A flashing check-engine light means the misfire is severe enough to overheat the catalytic converter right now. Ease off the throttle and get the car serviced promptly — continued hard driving can melt the converter.
Can bad gas cause P0300?
Yes — water-contaminated or stale fuel is a classic random-misfire cause, especially right after a fill-up. Note when the symptom started relative to your last tank.
How much does it cost to fix P0300?
The fix most associated with this code — ignition coils + spark plugs (full set) — runs $280 – $645 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0300?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0300 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0300?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0300?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.