P0305
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
A flashing check engine light means the misfire rate is high enough to destroy the catalytic converter within minutes of driving, which is the critical case. A steady lamp with an occasional misfire is less urgent but still needs attention, because misfire causes almost always worsen and every misfire event sends unburned fuel into the exhaust.
- Safe to drive
- If the light is flashing, stop and arrange a tow. Steady light with a mild stumble: drive gently, straight to diagnosis.
Cylinder 5 Misfire Detected
Cylinder 5 is not burning its fuel charge reliably. The computer measured the crankshaft momentarily failing to accelerate when that cylinder should have delivered power. The code names the cylinder — it does not name the cause.
The module reads the crankshaft position sensor at high resolution and measures the small acceleration each combustion event produces. When one cylinder's contribution falls short too often within a sampled window, that cylinder is flagged. Because the fault is isolated rather than affecting all cylinders, shared inputs — fuel pump pressure, the mass airflow sensor, a large central vacuum leak — are unlikely; the cause is normally something dedicated to that cylinder's ignition, fueling, or compression. This code only exists on engines with five or more cylinders, and which physical cylinder is number 5 varies by manufacturer.
How the car detected itCrankshaft velocity variation, evaluated across two rolling windows: a short one set at a misfire rate high enough to damage the catalytic converter, which flashes the malfunction indicator lamp, and a longer one at the emissions-threshold rate, which turns the lamp on steady. Most strategies suppress detection over rough roads, at very low fuel level, and during hard shifts to avoid false sets. Window lengths and misfire percentages are vehicle-specific.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- If the light is flashing, stop and arrange a tow. Steady light with a mild stumble: drive gently, straight to diagnosis.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- Unburned fuel raises catalyst temperature until the substrate melts, turning a coil-and-plug repair into a converter replacement. Raw fuel washes the cylinder wall and dilutes the oil. If the cause is mechanical — a burnt valve, a collapsed lifter, a failing head gasket — continued running enlarges the repair every mile.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- 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
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Ignition coil for cylinder 5 failed | Very common | — |
| 2 | Spark plug worn or fouled | Very common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether the check engine light is flashing or steady
This single observation decides between tow it and drive it, and it costs nothing. A flashing lamp means catalyst-damaging misfire rate right now.
Freeze frame — engine speed, load, and coolant temperature when it set
Misfire only when cold points at fuel delivery or a cracked coil; only at idle points at vacuum leaks or low-speed ignition weakness; only under load points at a coil breaking down under cylinder pressure or a weak compression cylinder.
Live misfire counters for all cylinders, not just cylinder 5
If a neighbor is also counting, look for a shared cause — a coil pack driving a pair, an injector harness section, or an intake gasket spanning two runners — rather than one dedicated part.
Fuel trims for that bank
Heavily lean trims alongside one misfiring cylinder steer you toward an intake leak at that runner or an injector problem. Trims near zero steer you toward ignition or compression.
Spark plug age and any recent tune-up work
Worn or out-of-spec plugs are a common and cheap-to-rule-out cause, especially at or beyond the service interval. A recent tune-up shifts suspicion the other way — toward a plug that was dropped, gapped wrong, cross-threaded, or a coil boot not fully seated.
Confirm which physical cylinder is number 5 on this engine
Tools Service information for the specific engine
Look it up before removing anything. On inline-fives, V6s, V8s and V10s the numbering conventions differ, and on some engines cylinder 5 is not where intuition puts it.
ExpectedA confirmed cylinder location and firing order. Do not proceed on assumption.
Swap the coil and spark plug with an adjacent cylinder, clear codes, and re-run
Tools Basic hand tools, spark plug socket, torque wrench, scan tool
Move one part at a time if you can. This is the fastest, cheapest test in misfire diagnosis and it produces a definite answer rather than a guess.
ExpectedIf the misfire code follows the swapped part to the other cylinder, that part is the fault. If it stays on cylinder 5, the cause is in that cylinder itself, its injector, or its wiring.
Read the removed spark plug carefully before fitting anything new
Tools Inspection light, feeler or wire gauge
Look at the electrode wear, the insulator, and any deposits. Compare it against a plug from a healthy cylinder on the same engine.
ExpectedEven light-brown deposits, an intact insulator with no cracks, and a gap within specification for the vehicle. Oil on the plug points at valve stem seals or rings. Coolant staining or a steam-cleaned electrode points at a head gasket or a cracked head.
Verify power and trigger at the coil connector
Tools Digital multimeter or lab scope, backprobes, wiring diagram
Check both with the connector loaded, and compare against a healthy cylinder's coil. A broken terminal or a chafed trigger wire mimics a failed coil exactly.
ExpectedBattery voltage on the feed and a clean trigger signal matching the other coils. Pin assignments are vehicle-specific — use the correct diagram.
Test the injector for that cylinder
Tools Digital multimeter, mechanic's stethoscope
Compare winding resistance against the other injectors on the engine and listen for a clean click with a stethoscope while running. A clogged or weak injector produces a lean misfire on one cylinder.
ExpectedResistance in line with its siblings and an audible, regular click. Correct absolute resistance is engine-specific — the useful comparison is against the other injectors on the same engine.
Perform a relative compression test, then a compression or leakdown test if needed
Tools Lab scope with amp clamp, or compression gauge and leakdown tester
Relative compression with an amp clamp during cranking is quick and non-invasive. If it flags cylinder 5, follow with a conventional compression or cylinder leakdown test to quantify it.
ExpectedCylinders reasonably consistent with one another. Variation limits are engine-specific — many manufacturers allow up to about 25 percent between the highest and lowest cylinder, while a tighter spread is typical of a healthy engine. Use the manufacturer's stated minimum and maximum variation, and treat one cylinder clearly below its neighbors as a reason to run a leakdown test, not as a diagnosis on its own. Absolute compression figures are engine-specific.
If leakdown is poor, locate the leak path by listening
Tools Leakdown tester, mechanic's stethoscope or hose
With the cylinder at top dead center on compression and pressurized, listen at the throttle body, the tailpipe, the oil filler, and the coolant reservoir.
ExpectedAir escaping at the intake means an intake valve; at the tailpipe, an exhaust valve; at the oil filler, rings or a bore problem; bubbles in the coolant, a head gasket or cracked head.
Smoke-test the intake near that runner if fuel trims were lean
Tools Smoke machine
Introduce smoke into the intake and look for it escaping at the manifold gasket, a cracked runner, an injector seal, or a vacuum port serving that cylinder.
ExpectedNo smoke escaping anywhere. Smoke at that runner's gasket or injector seal explains both the lean trim and the single-cylinder misfire.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the spark plug, or the full set if they are at service age | A worn, cracked, fouled, or misgapped plug found, and the misfire ending when a known-good plug is fitted | $80–$350 | DIY |
| Replace the ignition coil for that cylinder | The misfire following the coil when it is swapped to a different cylinder | $120–$400 | DIY |
| Repair the coil harness, connector, or trigger wire | Missing power or a corrupted trigger signal at the coil connector while the coil itself has been proven good on another cylinder | $100–$400 | DIY |
| Clean or replace the fuel injector for that cylinder | Injector resistance or flow clearly out of line with its siblings, or the misfire following the injector when swapped | $200–$650 | DIY |
| Repair an intake manifold or injector seal leak at that runner | A smoke test showing a leak at that runner, together with lean fuel trims that correct once it is sealed | $150–$600 | DIY |
| Valve or cylinder head repair | Compression on that cylinder outside the manufacturer's stated variation limit, with leakdown air audible at the intake or exhaust and no improvement from a wet test | $1,200–$3,500 | Shop |
| Piston, ring, or short block repair | Low compression that does not improve with a wet test, and leakdown air escaping at the oil filler | $2,500–$7,000 | Shop |
US independent shop rates, parts plus labor. Plug and coil pricing swings widely with access — an inline engine with plugs on top is cheap, a transverse V6 rear bank or an engine requiring intake removal is not. If the catalytic converter has already been cooked by sustained misfire, add its replacement to whatever the underlying repair costs.
- Replacing all the coils because one is suspected, when a swap test would have identified the failed one.
- Not checking whether the lamp was flashing, and driving the vehicle in with the catalytic converter cooking.
- Throwing the old plug away without looking at it. Its appearance frequently narrows the cause to fuel, oil, coolant, or ignition.
- Assuming cylinder 5 is fifth from the front of the engine. Numbering varies and the wrong cylinder wastes the whole job.
- Condemning a cylinder on a shop rule-of-thumb compression spread instead of the manufacturer's stated variation limit, and selling head work on an engine that is within factory tolerance.
- Chasing ignition parts while fuel trims sit heavily lean on that bank, when the real cause is an intake leak or a restricted injector.
- Clearing codes before recording freeze frame, and losing the conditions that would have told you where to look.
- Fitting a new coil and plug and declaring victory without a drive cycle. Confirm the misfire counters stay at zero under the conditions that originally set the code.
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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