P0300: Random or multiple-cylinder misfire detected
Random/multiple cylinder misfire
Severity: Inspect soon
A steady light with P0300 means the engine is misfiring in more than one cylinder; the car is usually drivable gently, but misfire raises exhaust temperature and can damage the catalytic converter, so get it inspected soon.
It gets more serious if:
- Stop: The check engine light is flashing. That means misfire bad enough to damage the catalytic converter is happening right now; Ford warns the extra exhaust heat can damage the converter and nearby parts and could cause a fire. Pull over when safe and avoid further driving, or at most drive gently and slowly to the nearest shop (EPA's own advice is to minimise driving and avoid speed and heavy loads).
P0300 in short
P0300 means the engine computer counted misfires in more than one cylinder, or could not pin them to one. Common causes are worn spark plugs, a vacuum leak, failing ignition coils or low fuel pressure. With a steady light, drive gently to a shop soon; if the light flashes, stop driving.
The engine computer watches how evenly the crankshaft speeds up after each cylinder fires. When a cylinder fails to burn its charge, the crankshaft slows for a moment and the computer counts a misfire.
OBD II rules require the computer to name the cylinder when it can. P0300 is the code it stores when misfires are spread over two or more cylinders, or when it cannot say which cylinder is at fault. That usually points to something shared by all cylinders, such as fuel pressure, a vacuum leak or worn plugs, rather than one bad part.
If the misfire rate is high enough to overheat the catalytic converter, the light blinks while it is happening. A converter damaged this way can later set P0420.
Most likely causes
Cause 1: Worn or fouled spark plugs
Common general cause- Why it ranks here
- Plugs wear in every cylinder at once, so they are a frequent cause of misfire spread across cylinders. NGK notes fouling deposits let voltage leak away so the plug cannot fire.
- What would change this
- Rises if the plugs are past their service interval or show carbon, oil or wet fouling; falls if they are new and clean.
Cause 2: Vacuum leak (cracked hose, intake gasket or PCV leak)
Common general cause- Why it ranks here
- Unmetered air leans out every cylinder; a large leak is a classic cause of random misfire, often at idle.
- What would change this
- Rises if P0171/P0174 are also stored or long-term fuel trims are high at idle and fall at higher rpm; falls if trims are normal.
Cause 3: One or more weak ignition coils
Common general cause- Why it ranks here
- A coil weakened by heat, oil or water contamination or worn plugs can misfire intermittently and on more than one cylinder.
- What would change this
- Rises if misfire counts follow a coil when it is swapped, or coils show cracks or carbon tracks; falls if counts stay on the same cylinders.
Cause 4: Low fuel pressure (weak pump, clogged filter or faulty regulator)
Common general cause- Why it ranks here
- Low pressure starves all cylinders, so misfire shows up on several cylinders, often under load.
- What would change this
- Rises if the misfire is worse when accelerating and fuel trims are high at all speeds; a fuel pressure test confirms or rules it out.
Cause 5: Dirty or clogged fuel injectors
Common general cause- Why it ranks here
- Restricted injectors lean out the cylinders they feed and can cause lean misfire on more than one cylinder.
- What would change this
- Rises with a lean code and normal fuel pressure; an injector balance or flow test confirms it.
Cause 6: Condensation from the charge air cooler (some GM turbo engines)
Supported by vehicle data- Why it ranks here
- GM documented random misfire and a flashing light on 2018 Equinox and Terrain 1.5L and 2.0L turbo engines when water collected in the charge air cooler is drawn in on acceleration.
- What would change this
- Rises only on those vehicles, after long steady drives in humid or wet weather; not relevant to non-turbo engines.
Cause 7: Mechanical problem: low compression or leaking valves
Possible — needs testing- Why it ranks here
- Loss of compression causes misfire, but it is less common than ignition or fuel faults and needs testing to prove.
- What would change this
- A compression or cylinder leakage test showing low readings raises it; even readings rule it out.
Cause 8: Poor-quality or contaminated fuel, or running very low on fuel
Possible — needs testing- Why it ranks here
- Ford's owner guide lists poor fuel, water in fuel and running out of fuel as temporary causes of misfire and a lit check engine light.
- What would change this
- Rises if the misfire started right after a fill-up or the tank ran nearly empty; falls if it persists across tanks of good fuel.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Read all stored and pending codes, freeze-frame data and per-cylinder misfire counts with a scan tool.
IfP0171 and/or P0174 (lean) are stored as well
It meansThe lean condition is the likely primary fault; fix it before chasing ignition parts.
IfMisfire counts are concentrated in one or two cylinders
It meansTreat it like a cylinder-specific misfire and test those cylinders' coil and plug.
IfMisfire counts are spread fairly evenly across cylinders
It meansSomething shared by all cylinders is more likely.
Step 2: With the engine warm, compare long-term fuel trim at idle and at about 2,500 rpm.
IfTrims are strongly positive at idle and move toward normal at higher rpm
It meansExtra air is getting in at idle: a vacuum leak.
Points to: Vacuum leak (cracked hose, intake gasket or PCV leak)
IfTrims are strongly positive at all speeds
It meansThe engine is short of fuel; check fuel pressure.
IfTrims are near normal
It meansThe mixture is probably fine; look at ignition.
Step 3: With the engine off and cool, remove the coils and plugs and inspect them for wear, fouling, cracks, carbon tracks and oil or water in the plug wells.
Warning:Do this only with the engine off and cooled down: exhaust parts and plugs get hot enough to burn. Never touch coils, boots or plug wires with the engine running; ignition voltage can give a dangerous shock.
IfPlugs are worn, past their interval or fouled
It meansPlugs are at least part of the problem; also find out why they fouled.
IfCoils or boots are cracked, carbon-tracked or contaminated
It meansCoil insulation is breaking down.
IfEverything looks good
It meansSwap parts between cylinders to see whether the misfire moves.
Step 4: Swap the coil (and then the plug) from a misfiring cylinder with one from a cylinder that is not misfiring, clear the codes, and drive until the misfire counts update.
Warning:Engine off and cool when swapping parts. Do not pull plug wires or coils off a running engine to test them: it risks a shock and can damage a weak coil or the ignition module.
IfThe misfire moves with the coil
It meansThat coil is faulty.
IfThe misfire moves with the plug
It meansThat plug is faulty.
IfThe misfire stays on the same cylinders
It meansIgnition is unlikely; injector or compression is next.
Step 5: Have fuel pressure measured against the manufacturer's specification at idle and under load.
Warning:The fuel system is under pressure and fuel sprays and ignites easily. Direct-injection systems run at very high pressure. Leave this to a shop.
IfPressure is below specification
It meansFuel delivery is the problem.
Points to: Low fuel pressure (weak pump, clogged filter or faulty regulator)
IfPressure is within specification
It meansSupply is fine; suspect restricted injectors.
Step 6: Run a compression test and, if a cylinder is low, a cylinder leakage test.
Warning:Disable fuel and ignition as the service information describes before cranking, keep hands and clothing away from belts and the fan, and let a hot engine cool first. If you are unsure, have a shop do it.
IfOne or more cylinders read low
It meansA mechanical problem (valves, rings or head gasket) is causing the misfire.
Points to: Mechanical problem: low compression or leaking valves
IfCompression is even and within specification
It meansMechanical causes are unlikely; have the injectors tested.
Symptoms
- Rough idle or shaking, especially at a stop
- Hesitation or stumbling when accelerating
- Loss of power
- Fuel smell from the exhaust
- Check engine light flashing Gets more serious: stop and have it checked.
Compare with: Car shaking with the check engine light on, Rough idle with the check engine light on, Check engine light with loss of power
How to fix
Step 1: Write down every code and the freeze-frame data before clearing anything; you will need it to see whether the repair worked.
Step 2: Until it is fixed, drive gently: avoid hard acceleration, high speed, towing and heavy loads.
Step 3: If the plugs are due by the maintenance schedule, replace them with the type and gap listed for your engine.
Warning:Only work on a cold engine, and do not cross-thread plugs in aluminium heads; use a torque wrench if you have one.
Step 4: If the swap test proved a coil faulty, replace that coil and clear the codes, then confirm the misfire counts stay at zero.
Warning:Engine off and key out before unplugging coils.
Parts and cost context
- Catalytic converter:Replacing a converter damaged by misfire without fixing the misfire leads to the new one failing too. Fix the misfire first; prices and labour vary, so get a quote.
Labor varies by vehicle and shop; get a quote before you approve a repair.
Compare used part prices on PartPriceGuideOn your vehicle
ChevroletYour car
2018 · Equinox · 1.5L turbo (LYX), 2.0L turbo (LTG)
Supported by vehicle dataIf P0300 and a flashing light appear when accelerating after a long steady drive in rain or humid weather, GM attributes it to charge air cooler condensation and advises against replacing parts for it.
PIP5511Status unverified
GM PIP5511: flashing MIL, P0300, random misfire after steady driving in humid or wet conditions
2018equinox · 1.5L turbo (LYX), 2.0L turbo (LTG) · issued · NHTSA 10137703
Water condensing in the charge air cooler during long steady drives is drawn into the engine on acceleration, causing random misfire, a flashing light and history P0300. GM's preliminary guidance says not to replace components for this concern. Also applies to the 2018 GMC Terrain.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
PIP5062CStatus unverified
GM PIP5062C: P0300 with a misfiring cylinder making little or no power
2012–2014camaro, equinox, impala · issued · rev. C · NHTSA 10112049
GM's diagnostic order for a misfiring cylinder: swap the coil and plug to another cylinder, inspect the plug wet or dry, test the injector, use propane enrichment on a dry cylinder to find a weak injector driver, then a cylinder leakage test. NHTSA's record also lists 2012-2014 Buick LaCrosse, Cadillac SRX, GMC Terrain and 2013-2014 Cadillac ATS and XTS.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
GMCYour car
2018 · Terrain · 1.5L turbo (LYX), 2.0L turbo (LTG)
Supported by vehicle dataSame charge air cooler condensation pattern as the 2018 Equinox: random misfire and a flashing light on acceleration after long steady driving in wet or humid weather.
PIP5511Status unverified
GM PIP5511: flashing MIL, P0300, random misfire after steady driving in humid or wet conditions
2018equinox · 1.5L turbo (LYX), 2.0L turbo (LTG) · issued · NHTSA 10137703
Water condensing in the charge air cooler during long steady drives is drawn into the engine on acceleration, causing random misfire, a flashing light and history P0300. GM's preliminary guidance says not to replace components for this concern. Also applies to the 2018 GMC Terrain.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
KiaYour car
2017–2018 · Forte · 2.0L MPI
Supported by vehicle dataKia service action SA321 covers P0300 on Fortes built May 16, 2016 to Sept 7, 2017: with a misfire code stored, all four ignition coils are replaced, so check whether the VIN is covered before buying parts. Supersession status of the bulletin is not confirmed, so this is labelled vehicle data rather than a confirmed bulletin.
TSB ENG181 (Rev 1)Status unverified
2017–2018forte · 2.0L MPI · issued · rev. Rev 1 · NHTSA 10167299
For Fortes built May 16, 2016 to Sept 7, 2017 (VIN check required): if the check engine light is on with P0300 to P0304, Kia replaces all four ignition coils; if no misfire code is stored, only the coil RFI springs are replaced.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
Related codes
P0300 questions
Can I drive with a flashing check engine light?
Not if you can avoid it. A flashing light means misfire severe enough to overheat the catalytic converter is happening now. Ford's owner guide warns the heat can damage the converter and could start a fire; EPA advises keeping driving to a minimum. Drive gently to the nearest shop at most, or have it towed.
Why P0300 and not a specific cylinder code?
The computer stores a cylinder code when it can pin the misfire to one cylinder. P0300 means misfires are spread across two or more cylinders, or it could not tell which. Look for a cause shared by all cylinders, such as plugs, a vacuum leak or fuel pressure.
Can bad gas cause P0300?
It can. Ford's owner guide lists poor-quality fuel, water in the fuel and running out of fuel as causes of misfire. If the code appeared right after filling up, try a tank of fuel from a busy station before replacing parts, and stop driving if the light flashes.
How do I tell a bad coil from a bad spark plug?
Swap them. Move the coil from the misfiring cylinder to a neighbouring cylinder, clear the code and drive. If the misfire moves with the coil, the coil is bad. If it stays put, swap the spark plug the same way. If it still stays, look at the injector and compression.
Sources and review
- Cal. Code Regs. tit. 13, § 1968.2 - Malfunction and Diagnostic System Requirements (OBD II)
- 2001 Lincoln Continental Owner's Guide - Warning lights (Check engine)
- Do you drive a 1996 or newer car or light truck? (On-Board Diagnostics, EPA420-F-02-016)
- Technical Information: Fouling of Spark Plugs
- How To Diagnose & Repair Engine Misfires
- ASE L1 study guide: Misfire codes
- Causes of ignition coil failure
- Ignition system troubleshooting tips to maximise lifetime
- GM PIP5511: Flashing MIL, P0300, random misfire after extended steady driving in humid or wet conditions
- GM PIP5062C: P0300 with a misfiring cylinder making little or no power
- What You Should Know About Catalytic Converters
- Kia TSB ENG181 Rev 1 / Service Action SA321: 2017-2018 Forte 2.0L MPI ignition coil and RFI spring replacement
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
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