P0304 Misfire detected in cylinder 4

Cylinder 4 misfire

P0304 in short

P0304 means the engine computer detected misfires in cylinder 4. The usual causes are that cylinder's ignition coil or spark plug, then its injector or low compression. Swapping the coil with another cylinder is the quickest first test. Drive gently to a shop soon; if the light flashes, stop driving.

The engine computer measures how smoothly the crankshaft speeds up after each cylinder fires. When one cylinder fails to burn its charge, the crankshaft slows for an instant at that point in the firing order and the computer counts a misfire against that cylinder.

OBD II rules require the computer to identify the cylinder when the misfire is in one cylinder, which is why you get P0304 instead of a general misfire code. Because the problem is in one place, the cause is usually something that serves only cylinder 4: its coil, plug, injector or valves.

Which cylinder is number 4 depends on the engine, so check the service information for your engine before removing parts. If the light flashes, misfire is heavy enough to damage the catalytic converter.

Most likely causes

  1. Faulty ignition coil on cylinder 4

    Common general cause
    Why it ranks here
    On coil-on-plug engines each cylinder has its own coil, and a weak coil is one of the most common reasons a single cylinder misfires. GM's misfire procedure starts with swapping it.
    What would change this
    Rises sharply if the misfire moves to another cylinder when the coil is swapped; falls if it stays on cylinder 4.
    How to test the ignition coil

  2. Worn, damaged or fouled spark plug in cylinder 4

    Common general cause
    Why it ranks here
    A worn or fouled plug cannot fire reliably; NGK notes deposits let voltage leak away. A worn plug also overloads the coil.
    What would change this
    Rises if the plug is past its interval, has a worn electrode or shows carbon, oil or fuel fouling, or if the misfire follows the plug when swapped.
    How to test the spark plug

  3. Oil or water in the plug well, or a carbon-tracked coil boot

    Common general cause
    Why it ranks here
    Oil from a leaking valve cover gasket or water in the plug recess breaks down coil and boot insulation so the spark tracks to ground.
    What would change this
    Rises if the plug well is wet or the boot shows a carbon line; the leak must be fixed or the new coil will fail too.
    How to test the ignition coil

  4. Clogged, leaking or disconnected fuel injector on cylinder 4

    Common general cause
    Why it ranks here
    If ignition parts check out, the injector feeding that one cylinder is the next most likely cause.
    What would change this
    A fuel-wet plug suggests a leaking injector; a dry plug with good spark suggests a restricted injector. An injector balance test or GM-style propane enrichment narrows it.
    How to test the fuel injector

  5. Oil-fouled plug from rotated piston rings (some Honda models)

    Supported by vehicle data
    Why it ranks here
    Honda extended warranty coverage on 2008-2013 Accord, Crosstour, Odyssey and Pilot because rotated piston rings let oil foul the plugs and set cylinder misfire codes.
    What would change this
    Rises only on the listed Honda models with an oily plug in the misfiring cylinder.
    How to test the spark plug

  6. Low compression in cylinder 4 (valve, ring or head gasket)

    Possible — needs testing
    Why it ranks here
    Mechanical faults cause single-cylinder misfire but are less common and must be proved by testing.
    What would change this
    A compression or leakage test showing cylinder 4 low raises it; even readings rule it out.

  7. Damaged wiring or loose connector at cylinder 4's coil or injector

    Possible — needs testing
    Why it ranks here
    Weak supply voltage or poor connections can stop one coil or injector working, but this is less common than the parts themselves.
    What would change this
    Rises if the misfire changes when the harness is moved or the connector is corroded or loose.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all codes and freeze-frame data, and find where cylinder 4 is on your engine in the service information.

    Low riskTool: scan tool, service information

    • IfOnly P0304 is stored (or P0304 plus P0300)

      It meansFocus on cylinder 4.

      Go to step 2

    • IfLean codes P0171/P0174 or misfire codes for several cylinders are also stored

      It meansA shared cause may be involved; follow the P0300 tests as well.

  2. With the engine off and cool, remove cylinder 4's coil and look for cracks, carbon tracks, and oil or water in the plug well.

    Use cautionTool: basic hand tools, inspection light

    Warning:Work only with the engine off and cool; plugs and exhaust parts can burn. Never handle coils, boots or plug wires with the engine running; ignition voltage can shock you.

  3. Swap cylinder 4's coil with the coil from a neighbouring cylinder, clear the codes and drive until the misfire monitor runs again.

    Use cautionTool: scan tool, basic hand tools

    Warning:Engine off and key out when swapping coils. Do not unplug coils on a running engine to find the bad cylinder; it risks a shock and can damage a weak coil.

  4. Remove cylinder 4's spark plug and inspect it; if it looks normal, swap it with another cylinder's plug and retest.

    Use cautionTool: spark plug socket

    Warning:Engine cold only. Thread plugs in by hand first to avoid cross-threading the cylinder head.

  5. Check cylinder 4's injector connector, then have an injector balance test (or the manufacturer's injector test) done.

    Best left to a shopTool: scan tool with injector tests, noid light

    Warning:Injector testing involves a pressurised fuel system; fuel sprays and ignites easily. Leave pressure tests to a shop.

  6. Run a compression test, then a cylinder leakage test if cylinder 4 reads low.

    Use cautionTool: compression gauge, leak-down tester

    Warning:Disable fuel and ignition as the service information describes before cranking, keep clear of belts and the fan, and let a hot engine cool first. If unsure, have a shop do it.

Symptoms

  • Rough idle or a steady shake
  • Stumble or hesitation when accelerating
  • Loss of power
  • 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

  1. Record the codes and freeze-frame data before clearing them.

  2. Drive gently until it is fixed: no hard acceleration, high speed or towing.

  3. If the swap test proved the coil faulty, replace it, clear the codes and confirm the misfire count stays at zero after a drive.

    Warning:Engine off and key out before unplugging the coil.

  4. If the plug is worn or due by the maintenance schedule, replace it (ideally all plugs) with the type and gap listed for your engine.

    Warning:Cold engine only; start plugs by hand to avoid cross-threading.

  5. If oil was in the plug well, have the valve cover or tube seal leak fixed too, or the new coil will fail.

On your vehicle

Kia

  • 2017–2018 · Forte · 2.0L MPI

    Supported by vehicle data

    Kia service action SA321 covers P0304 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

    Kia ENG181 / Service Action SA321: ignition coil and RFI spring replacement, 2017-2018 Forte 2.0L MPI

    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.

Honda

  • 2008–2012 · Accord

    Supported by vehicle data

    Honda extended warranty coverage for P0304 caused by oil-fouled spark plugs from rotated piston rings. If the plug from the misfiring cylinder is oily, ask a Honda dealer whether the VIN is covered before paying for coils or plugs. Engine fitment was not stated in the record we read.

    NHTSA owner complaints mentioning P0304: 23. That shows what owners reported, not how often a cause sets the code.

  • 2010–2012 · Crosstour

    Supported by vehicle data

    Honda extended warranty coverage for P0304 caused by oil-fouled spark plugs from rotated piston rings. If the plug from the misfiring cylinder is oily, ask a Honda dealer whether the VIN is covered before paying for coils or plugs. Engine fitment was not stated in the record we read.

  • 2008–2013 · Odyssey

    Supported by vehicle data

    Honda extended warranty coverage for P0304 caused by oil-fouled spark plugs from rotated piston rings. If the plug from the misfiring cylinder is oily, ask a Honda dealer whether the VIN is covered before paying for coils or plugs. Engine fitment was not stated in the record we read.

    NHTSA owner complaints mentioning P0304: 20. That shows what owners reported, not how often a cause sets the code.

  • 2009–2013 · Pilot

    Supported by vehicle data

    Honda extended warranty coverage for P0304 caused by oil-fouled spark plugs from rotated piston rings. If the plug from the misfiring cylinder is oily, ask a Honda dealer whether the VIN is covered before paying for coils or plugs. Engine fitment was not stated in the record we read.

    NHTSA owner complaints mentioning P0304: 13. That shows what owners reported, not how often a cause sets the code.

  • A13-078Status unverified

    Honda warranty extension A13-078: spark plug fouling with misfire codes P0301-P0304

    2008–2012accord · NHTSA 10099101

    Per NHTSA's summary, the piston rings can rotate and line up, letting oil reach the combustion chamber and foul the spark plugs, which sets cylinder misfire codes and the check engine light. Engine and repair details were not in the summary; the full bulletin was not retrieved.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • A13-079Status unverified

    Honda warranty extension A13-079: spark plug fouling with misfire codes P0301-P0304

    2010–2012crosstour · NHTSA 10099102

    Per NHTSA's summary, the piston rings can rotate and line up, letting oil reach the combustion chamber and foul the spark plugs, which sets cylinder misfire codes and the check engine light. Engine and repair details were not in the summary; the full bulletin was not retrieved.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • A13-080Status unverified

    Honda warranty extension A13-080: spark plug fouling with misfire codes P0301-P0304

    2008–2010odyssey · NHTSA 10099103

    Per NHTSA's summary, the piston rings can rotate and line up, letting oil reach the combustion chamber and foul the spark plugs, which sets cylinder misfire codes and the check engine light. Engine and repair details were not in the summary; the full bulletin was not retrieved.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • A13-081Status unverified

    Honda warranty extension A13-081: spark plug fouling with misfire codes P0301-P0304

    2011–2013odyssey · NHTSA 10099104

    Per NHTSA's summary, the piston rings can rotate and line up, letting oil reach the combustion chamber and foul the spark plugs, which sets cylinder misfire codes and the check engine light. Engine and repair details were not in the summary; the full bulletin was not retrieved.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • A13-082Status unverified

    Honda warranty extension A13-082: spark plug fouling with misfire codes P0301-P0304

    2009–2013pilot · NHTSA 10099105

    Per NHTSA's summary, the piston rings can rotate and line up, letting oil reach the combustion chamber and foul the spark plugs, which sets cylinder misfire codes and the check engine light. Engine and repair details were not in the summary; the full bulletin was not retrieved.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0300Random or multiple-cylinder misfire detected

    Same system Common general cause

    P0300 is stored when misfire spreads to more than one cylinder. If P0304 appears with P0300, cylinder 4 is usually the worst; if other cylinder codes appear too, look for a shared cause.

  • P0420Catalytic converter efficiency below the computer's limit (Bank 1)

    Primary / secondary Common general cause

    Likely primary: P0304. A cylinder that keeps misfiring sends unburned fuel into the catalytic converter, which can overheat and fail, later setting P0420. Fix the misfire before replacing the converter.

  • P0171Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

    Primary / secondary Common general cause

    Likely primary: P0171. If P0171 is stored too, a lean bank 1 may be the shared cause: a lean mixture can make cylinders misfire. Fix the lean condition first and follow the P0300 tests as well.

  • P0174Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

    Primary / secondary Common general cause

    Likely primary: P0174. If P0174 is stored too, a lean bank 2 may be the shared cause: a lean mixture can make cylinders misfire. Fix the lean condition first and follow the P0300 tests as well.

All Misfire codes

P0304 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.

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.

Which cylinder is number 1, 2, 3 and so on?

It depends on the engine. Manufacturers number cylinders differently, especially on V6 and V8 engines where the banks may be numbered side by side or alternately. Check the service information or a repair manual for your exact engine before you pull a coil or plug.

Sources and review

  1. Cal. Code Regs. tit. 13, § 1968.2 - Malfunction and Diagnostic System Requirements (OBD II) — California Air Resources Board (via Cornell LII)
  2. 2001 Lincoln Continental Owner's Guide - Warning lights (Check engine) — Ford Motor Company
  3. Do you drive a 1996 or newer car or light truck? (On-Board Diagnostics, EPA420-F-02-016) — U.S. Environmental Protection Agency
  4. Causes of ignition coil failure — DENSO Aftermarket Europe ,
  5. How To Diagnose & Repair Engine Misfires — AA1Car.com (Larry Carley, automotive technical writer)
  6. GM PIP5062C: P0300 with a misfiring cylinder making little or no power — General Motors (via NHTSA) ,
  7. Technical Information: Fouling of Spark Plugs — NGK Spark Plugs (Niterra UK)
  8. Honda warranty extensions A13-078 to A13-082: spark plug fouling and misfire codes P0301-P0304 — American Honda Motor Co. (via NHTSA)
  9. Ignition system troubleshooting tips to maximise lifetime — DENSO Aftermarket Europe ,
  10. ASE L1 study guide: Misfire codes — OpenExamPrep (independent study site, not affiliated with ASE)
  11. What You Should Know About Catalytic Converters — AA1Car.com (Larry Carley, automotive technical writer)
  12. Kia TSB ENG181 Rev 1 / Service Action SA321: 2017-2018 Forte 2.0L MPI ignition coil and RFI spring replacement — Kia Motors America (via NHTSA) ,
  13. NHTSA ODI consumer complaints flat files — National Highway Traffic Safety Administration (NHTSA), Office of Defects Investigation
Last reviewed
Written by
Saurabh K.
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