P0300: Random/Multiple Cylinder Misfire Detected
The ECU has detected misfires that are not concentrated on a single cylinder. The misfire monitor measures crankshaft acceleration after each combustion event; missing acceleration on multiple cylinders, or randomly across cylinders, triggers this code.
What P0300 means
P0300 is a generic OBD-II powertrain code in the misfire & ignition group. In plain English: the engine computer counted combustion events that didn’t fire properly.
- Check engine light
- Usually on
- Emissions-related
- Yes — can fail inspection
- Limp mode possible
- Yes
- Drive cycle to clear
- Required
Get it fixed
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Symptoms
- Check engine light on or flashing
- Rough idle and uneven running
- Power loss, hesitation under acceleration
- Increased fuel consumption
- Possible smell of unburned fuel from exhaust
Common causes
- 01Worn spark plugsMost likely
- 02Vacuum leak affecting all cylindersMost likely
- 03Fuel pressure lowPossible
- 04Timing chain stretched (high mileage)Less likely
Can I drive with P0300?
Only gently, and not if the light is flashing.
A flashing check engine light means an active misfire that is dumping unburned fuel into the exhaust. Stop driving and have it towed or repaired; continuing can melt the catalytic converter.
Why we rate it urgent
- Misfires can overheat and ruin the catalytic converter
- Can put the vehicle in limp mode (reduced power)
- Turns on the check engine light
- Emissions-related: will usually fail an OBD inspection
- Needs a drive cycle to confirm a repair
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose P0300
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just P0300. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Fix other faults first
If other codes are stored, start with power-supply, sensor-circuit and communication codes. They can cause this code as a side effect.
- 3
Find the misfiring cylinder
Use the scan tool's misfire counters to see which cylinders misfire. Swap the ignition coil (or plug) from a misfiring cylinder to another one: if the misfire follows the part, you found it. Check for vacuum leaks and fuel trim if all cylinders misfire.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Worn spark plugs; vacuum leak affecting all cylinders; fuel pressure low; timing chain stretched (high mileage). Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and complete a full drive cycle (cold start, idle, steady highway cruise, deceleration). The code is fixed when the related readiness monitor completes without it returning.
P0300 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Moderate
- Good DIY job?
- Yes, with a scan tool
Needs a scan tool and some experience with testing.
Parts and systems involved
- Ignition coil
- Spark plug
- Fuel injector
- Intake manifold
- Timing chain
P0300 FAQ
What does P0300 mean?
P0300 means “Random/Multiple Cylinder Misfire Detected”. The ECU has detected misfires that are not concentrated on a single cylinder. The misfire monitor measures crankshaft acceleration after each combustion event; missing acceleration on multiple cylinders, or randomly across cylinders, triggers this code.
Is P0300 serious? Can I keep driving?
We rate P0300 as urgent severity. Only gently, and not if the light is flashing. A flashing check engine light means an active misfire that is dumping unburned fuel into the exhaust. Stop driving and have it towed or repaired; continuing can melt the catalytic converter.
What causes P0300?
The most common causes, from most to least likely: Worn spark plugs; Vacuum leak affecting all cylinders; Fuel pressure low; Timing chain stretched (high mileage). Test before replacing parts, because several causes can set the same code.
How much does it cost to fix P0300?
Typical labor is about $65–$750 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix P0300 myself?
DIY difficulty is moderate. Many owners can handle it with a scan tool, a multimeter and basic hand tools. Always confirm the cause before buying parts.
Will P0300 clear itself?
Once the fault is fixed, the check engine light usually goes off by itself after the system passes its self-test over several drive cycles. Clearing the code with a scan tool resets readiness monitors, so an inspection station may turn you away until a full drive cycle is completed.