# P0300: Random/Multiple Cylinder Misfire Detected **Verdict:** Drive to a shop this week Ignition and misfire - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $100-$2,000 - **Urgency:** This week - **Most likely:** Worn spark plugs or a vacuum leak - **DIY?:** Steps 1-3 ## Quick answer P0300 means the engine computer detected misfires in more than one cylinder, or in a pattern it cannot pin to a single cylinder. Because several cylinders are affected, the cause is often something they share, like worn spark plugs, a vacuum leak or low fuel pressure. ## What does P0300 mean? Each cylinder should fire once every two turns of the crankshaft, giving it a small push. The engine computer watches crankshaft speed through the crankshaft position sensor, and when a cylinder fails to fire, the crank slows for a split second. Counting those slowdowns is how the misfire monitor works, and it runs continuously whenever the engine is running. P0300 means misfires are happening in more than one cylinder, or the pattern moves around too much to blame a single one. If the misfire rate is high enough to overheat the catalytic converter, the check engine light flashes and the code can set right away. Lower rates that would only raise emissions usually have to show up on two drive cycles. Because several cylinders are involved, look first for something they share: spark plugs, fuel pressure, intake air or engine timing. ## What are the symptoms of P0300? - Check engine light on, or flashing during bad misfires - Shaking or rough idle - Hesitation, jerking or loss of power when accelerating - Poor fuel economy - Smell of raw fuel or rotten eggs from the exhaust ## What causes P0300? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Worn or wrong spark plugs** (most common). Spark plugs wear gradually, and as the gap widens the coils need more voltage to fire them. Once plugs pass their service interval they misfire across several cylinders, mostly under load or hard acceleration. The wrong plug type, gap or heat range from a previous job does the same. Check service records first; long-overdue plugs are the top suspect. 2. **Vacuum leak or lean condition** (common). A cracked vacuum hose, leaking intake gasket, PCV valve or brake booster hose lets in air the mass airflow sensor never measured. Every cylinder runs lean, and the misfire is usually worst at idle and fades at higher speed. High positive fuel trims and lean codes P0171 or P0174 alongside P0300 point here. A smoke test finds most leaks. 3. **Low fuel pressure or dirty injectors** (less common). A weak fuel pump, clogged filter or faulty pressure regulator cannot keep up, so all cylinders lean out when demand rises, such as on hills or hard acceleration. Dirty injectors on high-mileage engines cause similar random misfires. Low pressure often comes with lean codes and hard starting. A fuel pressure test under load confirms or rules it out. 4. **Multiple failing ignition coils or plug wires** (less common). Older engines with a distributor, a single coil pack or plug wires share ignition parts among cylinders, so one weak part can misfire several. Plug wires break down with age and may arc to the engine, sometimes visible as sparks in the dark. On coil-on-plug engines, several coils failing at once is unlikely unless their power supply is weak. 5. **Mechanical problems such as timing or low compression** (rare). A stretched timing chain, a belt that has jumped a tooth, worn camshaft lobes or leaking valves can make several cylinders misfire at once. This is more likely on high-mileage engines with a poor oil-change history. Timing codes such as P0016 often appear too. A compression test and timing check confirm it before costly parts are bought. 6. **Bad fuel or water in the fuel** (rare). Fuel contaminated with water, debris or the wrong product can misfire every cylinder, often with hard starting or stalling. The key clue is timing: misfires that start right after a fill-up, especially at an unfamiliar station. Topping off with fresh fuel can help a mild case, but heavily contaminated fuel has to be drained from the tank and lines. ## How do you diagnose P0300? 1. Read all codes; any single-cylinder codes (P0301 to P0306) point you to where to start. 2. Watch misfire counters for each cylinder on a scan tool to see which ones misfire most. 3. Check fuel trims; high positive trims point to a vacuum leak or low fuel pressure. 4. Inspect spark plugs for wear, oil fouling or wrong gap, and check coils and plug wires. 5. If ignition and fuel are good, test compression and check timing. ## How do you fix P0300, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace spark plugs | $120-$450 | Four-cylinder engines are at the low end; V6s with hard-to-reach rear plugs cost more. | | Fix a vacuum leak | $100-$600 | From a cracked hose to an intake gasket. | | Replace ignition coils or plug wires | $150-$700 | Price depends on how many coils are needed. | | Fix fuel delivery (filter, pump or injector cleaning) | $150-$1,300 | After a fuel pressure test confirms the problem. | Full price breakdowns: [spark plug replacement cost](/cost/spark-plug-replacement), [ignition coil replacement cost](/cost/ignition-coil-replacement), [catalytic converter replacement cost](/cost/catalytic-converter-replacement). ## Is it safe to drive with P0300? > **Drive to a shop this week.** If the check engine light is flashing, stop driving, because heavy misfires can overheat and destroy the catalytic converter. With a steady light you can drive gently to a shop. ## Will P0300 fail an emissions test? A car with P0300 and the check engine light on fails an OBD-based emissions test. Misfires also send unburned fuel into the exhaust, raising hydrocarbon emissions and risking catalytic converter damage that can set P0420 later. The misfire monitor rechecks itself quickly after a repair, but the other readiness monitors need several days of driving before a retest. ## After the repair: clearing P0300 Clear the code and watch per-cylinder misfire counts on a scan tool at idle, light cruise and acceleration; many tools show them under Mode 6. Counts should stay at or near zero. Drive through the conditions recorded in the freeze-frame data. If the light stays off for a week of mixed driving and fuel economy recovers, the fix worked. ## How to prevent P0300 - Replace spark plugs at the interval in your owner's manual, using the plug type it specifies. - Buy fuel from busy, reputable stations and use the octane grade your engine requires. - Replace the engine air filter on schedule and fix cracked intake or vacuum hoses promptly. - Change oil on time; sludge and a stretched timing chain can upset valve timing. - Deal with a lean code such as P0171 promptly, since lean running often turns into misfires. ## Codes often seen with P0300 - [P0301](/codes/p0301): Cylinder 1 Misfire Detected - [P0302](/codes/p0302): Cylinder 2 Misfire Detected - [P0303](/codes/p0303): Cylinder 3 Misfire Detected - [P0304](/codes/p0304): Cylinder 4 Misfire Detected - [P0171](/codes/p0171): System Too Lean (Bank 1) - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) ## Frequently asked questions ### Can I drive with a P0300 code? Only gently, and only if the check engine light is steady, not flashing. A flashing light means the misfires are severe enough to damage the catalytic converter. Pull over and have the car towed if it flashes. With a steady light, drive straight to a shop and avoid hard acceleration and towing. ### What is the most common cause of P0300? Worn spark plugs are the most common cause of P0300, with vacuum leaks and fuel delivery problems next. Because several cylinders misfire, look for something they share rather than a single coil or injector. Check how long ago the plugs were changed and whether lean codes such as P0171 are also stored. ### Can bad gas cause P0300? Yes, contaminated or low-quality fuel can cause random misfires across several cylinders. If the misfires began right after filling up, that is a strong clue. Topping up with fresh fuel from a busy station may help a mild case, but badly contaminated fuel has to be drained by a shop. ### How much does it cost to fix P0300? Most P0300 repairs cost $100 to $600, because worn spark plugs and vacuum leaks are the usual causes. Ignition coils or plug wires run $150 to $700 depending on how many are needed. Fuel delivery repairs cost $150 to $1,300, and mechanical problems such as timing chain work cost more. ### What is the difference between P0300 and P0301? P0300 means misfires are random or spread across several cylinders, while P0301 through P0306 name one specific cylinder. A single-cylinder code usually points to that cylinder's plug, coil or injector, and P0300 points to something shared. Both together mean one cylinder is worst but others are misfiring too. ### Can a vacuum leak cause P0300? Yes, a vacuum leak is one of the most common causes of P0300. Unmetered air leans out every cylinder, so the misfires are often worst at idle. A hissing sound, a high or surging idle, and lean codes P0171 or P0174 are clues. A shop smoke test finds the leak quickly. ### Can a bad catalytic converter cause P0300? Rarely; more often P0300 damages the converter. Unburned fuel from misfires burns inside the converter and can overheat or melt it. A badly clogged converter can choke the engine and cause rough running, but fix the misfire first, then have the converter checked if P0420 appears or power stays low. ### Why is my check engine light flashing with P0300? A flashing check engine light means the misfire rate is high enough to damage the catalytic converter. Ease off the throttle, avoid hard acceleration and get off the road when it is safe. If the light keeps flashing even at a gentle idle, shut the engine off and arrange a tow. ### Will P0300 pass inspection? No, a car with P0300 and the check engine light on fails an OBD-based emissions inspection. The misfire must be fixed and the code cleared. Clearing also resets the readiness monitors, so drive normally for several days and confirm the monitors show ready on a scan tool before retesting. ### Can a bad timing chain cause P0300? Yes, though it is less common than plug or air problems. A stretched timing chain throws off valve timing so several cylinders misfire, and very low or sludged oil can upset variable valve timing. A rattle at cold startup, or timing codes such as P0016 alongside P0300, points toward this cause. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other ignition and misfire codes - [P0301 Cylinder 1 Misfire Detected](https://diagxcar.com/codes/p0301) - [P0302 Cylinder 2 Misfire Detected](https://diagxcar.com/codes/p0302) - [P0303 Cylinder 3 Misfire Detected](https://diagxcar.com/codes/p0303) - [P0304 Cylinder 4 Misfire Detected](https://diagxcar.com/codes/p0304) - [P0305 Cylinder 5 Misfire Detected](https://diagxcar.com/codes/p0305) - [P0306 Cylinder 6 Misfire Detected](https://diagxcar.com/codes/p0306) - [P0307 Cylinder 7 Misfire Detected](https://diagxcar.com/codes/p0307) - [P0308 Cylinder 8 Misfire Detected](https://diagxcar.com/codes/p0308) - [P0316 Engine Misfire Detected on Startup (First 1000 Revolutions)](https://diagxcar.com/codes/p0316) - [P0320 Ignition/Distributor Engine Speed Input Circuit](https://diagxcar.com/codes/p0320) - [P0019 Crankshaft Position - Camshaft Position Correlation (Bank 2 Sensor B)](https://diagxcar.com/codes/p0019) - [P0087 Fuel Rail/System Pressure - Too Low](https://diagxcar.com/codes/p0087) - [P0171 System Too Lean (Bank 1)](https://diagxcar.com/codes/p0171) - [P0172 System Too Rich (Bank 1)](https://diagxcar.com/codes/p0172) ## Symptoms - [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) - [Why does my car stall while driving?](https://diagxcar.com/symptoms/car-stalls-while-driving) - [Why is black smoke coming from my exhaust?](https://diagxcar.com/symptoms/black-smoke-from-exhaust) - [Why is blue smoke coming from my exhaust?](https://diagxcar.com/symptoms/blue-smoke-from-exhaust) - [Why does my car jerk when I accelerate?](https://diagxcar.com/symptoms/car-jerks-when-accelerating) - [Why does my car lose power when accelerating?](https://diagxcar.com/symptoms/car-loses-power-when-accelerating) - [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle) - [Why does my car shake when I accelerate?](https://diagxcar.com/symptoms/car-shakes-when-accelerating) - [Why does my car smell like gas?](https://diagxcar.com/symptoms/car-smells-like-gas) - [Why does my car smell like rotten eggs?](https://diagxcar.com/symptoms/car-smells-like-rotten-eggs) - [Why is my engine knocking?](https://diagxcar.com/symptoms/engine-knocking-noise) - [Why is my car getting bad gas mileage?](https://diagxcar.com/symptoms/poor-gas-mileage) - [Why does my car smell sweet?](https://diagxcar.com/symptoms/sweet-smell-from-car) ## Repair costs - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 - [Ignition coil replacement cost](https://diagxcar.com/cost/ignition-coil-replacement): $150-$400 - [Catalytic converter replacement cost](https://diagxcar.com/cost/catalytic-converter-replacement): $1,000-$3,000 - [Crankshaft position sensor replacement cost](https://diagxcar.com/cost/crankshaft-position-sensor-replacement) - [Engine replacement cost](https://diagxcar.com/cost/engine-replacement) - [Fuel injector replacement cost](https://diagxcar.com/cost/fuel-injector-replacement) - [Head gasket replacement cost](https://diagxcar.com/cost/head-gasket-replacement) - [Valve cover gasket replacement cost](https://diagxcar.com/cost/valve-cover-gasket-replacement) ## Warning lights linked to this - [Check engine light](https://diagxcar.com/warning-lights/check-engine) ## Terms explained - [What is Check engine light?](https://diagxcar.com/glossary/check-engine-light) - [What is DTC?](https://diagxcar.com/glossary/dtc) - [What is Misfire?](https://diagxcar.com/glossary/misfire) - [What is OBD-II?](https://diagxcar.com/glossary/obd-ii) --- Source page: https://diagxcar.com/codes/p0300 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.