Engine codes
P0300: random or multiple cylinder misfire
The engine computer watches the crankshaft speed up with every firing stroke. When some strokes do not deliver their push, it counts a misfire, and P0300 means it cannot pin them on one cylinder.
P0300: can I drive, and what will the labor cost?
Estimated labor, replace spark plugs (inline 4 cylinder)
$55 to $165
| Labor time (estimate) | 0.5 to 1.5 h |
| Rate used | $110/h |
| Technician wage, United States (average) | $27.13/h |
| High: limit driving, fix within days Limit driving. If the check engine light flashes, stop as soon as it is safe: unburned fuel can overheat the catalytic converter. | |
Labor only: parts and the diagnostic fee are not included. Hours are our estimate for a typical car; book times vary by model. Wage: BLS, May 2025.
P0300 is the generic OBD-II code for "random or multiple cylinder misfire detected". Each cylinder that fires speeds up the crankshaft a little; the computer measures those small accelerations through the crankshaft sensor and counts the strokes that fall short. When misfires come from more than one cylinder, or jump around, it stores P0300, often with codes P0301 to P0308 for the cylinders it could identify. Federal rules require the check engine light to blink when the misfire rate is high enough to damage the catalytic converter (40 CFR 86.1806-05), so a flashing light with P0300 means stop as soon as it is safe. A random misfire usually has a cause shared by all cylinders: worn spark plugs or several weak coils at high mileage, a vacuum leak or lean condition, low fuel pressure or bad fuel, and, less often, low compression on several cylinders. We rate it high: limit driving, fix within days.
Coil swap test: does the misfire follow the coil?
Result
Coil
| What it means: The misfire moved with the coil: replace that ignition coil. |
Swap only with the engine off and the coil connectors clipped back. Read the per-cylinder misfire counters, or wait for the new P030x code, before concluding.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
P0300 at a glance
| Code | P0300 |
| Standard description | Random/Multiple Cylinder Misfire Detected |
| System | Ignition and misfire |
| Severity | High: limit driving, fix within days |
| Common symptoms | shaking at idle, stumbling under load, flashing check engine light, fuel smell from the exhaust |
Why random means shared
A single bad coil makes one cylinder misfire and sets that cylinder's code. When the misfire moves around, the cause is something every cylinder depends on: the air-fuel mixture, the fuel supply, the plugs if they are all worn, the timing. That is why lean codes such as P0171 are often found with P0300, and why replacing one coil rarely fixes it.
When it happens is a clue
- Misfire under hard acceleration, uphill or towing: ignition is weak; worn plugs and coils that break down under high cylinder pressure.
- Misfire at idle, smoother at speed: a vacuum leak or a lean mixture.
- Misfire only on cold starts: fuel delivery, injectors or a carbon problem on some direct-injection engines.
- Misfire after filling up: bad fuel or water in the tank.
Testing the cylinders
The coil swap test on this page works for single-cylinder codes; with a random misfire, start by reading the misfire counters for each cylinder on a scanner. Even cylinder counts point to a shared cause; one or two cylinders far ahead point to an individual fault. A compression test, about an hour to two of labor, settles whether the engine itself is healthy before money goes into parts.
Driving with a heavy misfire sends unburned fuel into the converter, which overheats it. A converter destroyed this way will set P0420 later, so it pays to stop early.
Likely causes of P0300, in order
| Cause | How often |
|---|---|
| Worn spark plugs or several weak ignition coils | most frequent |
| Vacuum leak or lean condition | frequent |
| Low fuel pressure or contaminated fuel | frequent |
| Low compression on several cylinders | less frequent |
| Crankshaft sensor or tone ring signal fault | less frequent |
P0300 repairs and labor time
| Repair | Labor (estimate) | At $110/h |
|---|---|---|
| Replace spark plugs (inline 4 cylinder) | 0.5 to 1.5 h | $55 to $165 |
| Replace spark plugs (V6, rear bank under the intake) | 1.5 to 3.5 h | $165 to $385 |
| Replace one ignition coil | 0.2 to 0.8 h | $22 to $88 |
| Compression and leak-down test | 1 to 2 h | $110 to $220 |