P0305 – Cylinder 5 Misfire Detected

Last updated: April 4, 2026

P0305 means the engine computer has detected a misfire on cylinder 5. You may feel a shake at idle, a stumble on acceleration, or flashing check engine light, and fuel economy often drops. The code does not prove which part failed. It only points to a problem affecting cylinder 5 combustion. According to manufacturer OBD-II diagnostic information, this code indicates “Cylinder 5 Misfire Detected.” Misfires can come from ignition, fuel delivery, air leaks, or mechanical issues on that cylinder, so testing must confirm the root cause before replacing parts.

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P0305 Quick Answer

P0305 points to a cylinder 5 misfire. Start by checking cylinder 5 ignition performance (spark plug, coil/boot, wiring) and then confirm fuel injector operation and engine mechanical health on that cylinder.

What Does P0305 Mean?

P0305 means “Cylinder 5 Misfire Detected.” In plain terms, the PCM/ECM sees cylinder 5 not contributing power like it. That shows up as rough idle, hesitation, or a flashing MIL under load. The code does not tell you whether the cause is spark, fuel, air, or compression. It only tells you which cylinder the module flagged.

Technically, the PCM infers misfire by tracking crankshaft speed changes and comparing firing events between cylinders. When cylinder 5 fires weakly or not at all, the crank slows at that point. The PCM counts those events and sets P0305 when the pattern matches a cylinder-specific misfire. This matters because the misfire “detection method” cannot identify the failed component. You must verify ignition, injector control, air leaks, and mechanical condition with tests.

Theory of Operation

Under normal operation, each cylinder burns a metered air-fuel charge and the spark ignites it at the correct time. The crankshaft accelerates slightly after each normal combustion event. The PCM uses crankshaft position sensor data to monitor these tiny speed changes. It expects each cylinder’s contribution to stay consistent for the load and RPM.

P0305 sets when cylinder 5 does not accelerate the crank as expected. Ignition problems weaken or eliminate spark. Fuel problems lean out or flood that cylinder. Air leaks near that runner alter mixture at that cylinder. Mechanical faults reduce compression, airflow, or sealing. The PCM sees the result as a cylinder 5 contribution problem, not a specific failed part.

Symptoms

P0305 symptoms usually show up first as a drivability complaint and a MIL.

• Flashing MIL during acceleration or under load, indicating an active misfire that can damage the catalytic converter
• Rough idle with a noticeable shake, often strongest in gear at a stop
• Hesitation or stumble on tip-in or moderate throttle as cylinder 5 drops out
• Reduced power especially on hills or when merging, sometimes with a “pulsing” feel
• Fuel smell from the exhaust if the cylinder misfires and sends unburned fuel downstream
• Poor fuel economy from misfire corrections and extra throttle to maintain speed
• Related codes such as fuel trim, catalyst efficiency, or additional misfire codes if the condition spreads

Common Causes

• Worn or contaminated spark plug on cylinder 5: A weak spark kernel fails to light the mixture under load, so the ECM counts repeated crankshaft speed drops on that cylinder.
• Ignition coil or coil-on-plug boot fault affecting cylinder 5: Internal coil breakdown or carbon tracking leaks spark energy to ground, which creates an intermittent or consistent misfire event.
• Fuel injector on cylinder 5 restricted, sticking, or electrically faulty: Reduced fuel delivery or poor injector response leans that cylinder and causes unstable combustion the misfire monitor flags.
• Injector/coil circuit wiring issue near cylinder 5: High resistance, an open, or a short in the harness changes current flow, which disrupts coil firing or injector pulse on that cylinder.
• Vacuum leak or intake runner leak feeding cylinder 5: Extra unmetered air near that runner leans only that cylinder, so it misfires even when overall fuel trims look acceptable.
• Low compression or mechanical fault on cylinder 5: A burned valve, worn rings, or a valve train issue reduces effective cylinder pressure and prevents consistent combustion.
• EGR or PCV distribution issue skewing mixture to one cylinder: Improper EGR/PCV flow distribution can dilute cylinder 5 more than others and trigger misfire counts at idle or cruise.
• Fuel quality or fuel pressure/volume problem that shows up first on cylinder 5: Marginal fuel delivery can present as a single-cylinder misfire when that cylinder has the tightest mixture margin.

Diagnosis Steps

Tools you need: a scan tool that shows Mode $06 misfire data, freeze frame, and fuel trims; a digital multimeter; a spark tester; and basic hand tools. For deeper testing, use a lab scope, a noid light, and a fuel pressure gauge if the vehicle provides a test port. Plan for a compression test or relative compression test.

1. Confirm P0305 as pending or confirmed, then record freeze frame data. Focus on RPM, load, coolant temperature, fuel system status (open/closed loop), STFT/LTFT, and vehicle speed. Use that information to decide if it misfires at idle, cruise, or load.
2. Check for related DTCs before touching parts. Look for other misfire codes (P0300/P0301-P0308), fuel trim codes (P0171/P0172), and crank/cam correlation codes. If P0305 shows as pending only, remember many misfire detections behave like Type B logic and may need two trips to confirm.
3. Inspect the basics and power distribution first. Verify ignition and injector fuses, coil/injector feed relays, and any engine control fuses in the underhood box. A shared feed problem can single out one cylinder if a splice or terminal overheats near that branch.
4. Verify ECM and engine grounds with voltage-drop testing under load. Backprobe grounds while the engine runs and electrical loads stay on. Keep ground drop under 0.1V on the circuit operating; high resistance can cause coil and injector control issues that look like a misfire.
5. Do a focused visual inspection at cylinder 5. Check the coil connector lock, terminal tension, oil intrusion in the plug well, and any rubbed-through harness routing on the valve cover or intake. Look for coil boot carbon tracks and signs of arcing on the porcelain.
6. Use the scan tool to identify cylinder 5 misfire behavior. Review Mode $06 misfire counters and compare cylinder 5 to the others at idle and at a steady 2000–2500 RPM. If the scan tool supports it, run a power balance or cylinder contribution test.
7. Swap-test the ignition components only after the connector and power checks pass. Move the cylinder 5 coil (and plug if practical) to another cylinder and clear only the misfire data if the tool allows it. If the misfire follows the part, you confirmed an ignition-side root cause without guessing.
8. Check cylinder 5 injector operation next. Listen for consistent injector clicking with a stethoscope, then verify injector feed voltage with KOEO or engine running (as applicable to the design). If the injector has proper power, verify the ECM control with a noid light or lab scope to confirm pulse integrity.
9. Differentiate a fuel delivery issue from an air leak near cylinder 5. Watch STFT changes while you introduce a small, controlled enrichment source near the cylinder 5 runner area (or use smoke testing for vacuum leaks). A local leak often changes trims and misfire counts most at idle.
10. Check mechanical integrity if ignition and injector tests do not isolate the fault. Perform a compression test on all cylinders and compare cylinder 5 to its neighbors. If compression looks marginal, follow with a leak-down test to locate intake valve, exhaust valve, or ring sealing issues.
11. If the misfire occurs only under load, capture a scan tool snapshot during a road test. Freeze frame shows the conditions when the DTC set; a snapshot captures intermittent events you reproduce. Log RPM, load, misfire counters, STFT/LTFT, spark advance, and fuel system status.
12. Confirm the repair and verify monitor completion. Clear codes only after the fix, then road test under the freeze frame conditions that set P0305. Recheck for pending misfire results, confirm no misfire accumulation in Mode $06, and ensure the appropriate OBD-II readiness monitors return to Ready before an emissions inspection.

Professional tip: If cylinder 5 misfires only at idle and trims go positive, smoke-test the intake and check the PCV fresh-air and valve routing. A small runner leak can mimic an injector issue. If it misfires only under load and trims stay near zero, focus on coil output, plug gap condition, and boot insulation breakdown.

Possible Fixes

• Repair the cylinder 5 ignition fault found during swap-testing: Replace the confirmed bad spark plug, coil, or boot, and correct oil intrusion or carbon tracking that caused secondary leakage.
• Restore injector function on cylinder 5 after verification: Repair the injector circuit, clean a restricted injector when appropriate, or replace the injector if testing proves poor flow or a control/electrical defect.
• Repair wiring/connector issues at the coil or injector: Fix spread terminals, corrosion, broken conductor strands, or chafed insulation, then secure the harness to prevent repeat damage.
• Fix unmetered air entering near cylinder 5: Replace a leaking intake gasket, cracked vacuum line, or PCV component that creates a cylinder-specific lean condition.
• Correct a mechanical cause on cylinder 5: Address low compression causes such as valve sealing faults or valve train problems after compression and leak-down results confirm the failure mode.
• Resolve system-wide fuel delivery problems if testing proves them: Repair low fuel pressure/volume or a control issue that pushes one cylinder over the misfire threshold first under the same operating conditions.

Can I Still Drive With P0305?

You can sometimes drive short distances with a P0305 code, but you should treat it as a “get it checked now” fault. A cylinder 5 misfire can turn into a no-start, severe shaking, or stalling with little warning. If the engine runs rough, the check engine light flashes, or power drops hard under load, stop driving. Continued misfires can overheat and damage the catalytic converter because raw fuel and oxygen reach the exhaust. Keep RPM low, avoid towing, and avoid highway merging until you diagnose it. If you smell fuel, hear backfiring, or see the temperature climb, park the vehicle and diagnose it before further driving.

How Serious Is This Code?

P0305 ranges from an inconvenience to an expensive repair, depending on misfire intensity and when it happens. A light, intermittent misfire at idle may only cause a rough idle and poor fuel economy. A misfire under load can create hesitation, bucking, and unsafe passing power. The biggest risk comes from catalyst damage and overheating when the misfire persists. Many vehicles flash the MIL during active catalyst-damaging misfires. Take that warning seriously. Ignoring P0305 can also wash oil off the cylinder wall, dilute engine oil with fuel, and accelerate wear. Fixing the cause early usually costs far less than replacing a converter.

Common Misdiagnoses

Technicians often misdiagnose P0305 by replacing the spark plug or ignition coil without verifying the misfire stays on cylinder 5. A quick cylinder contribution check and a coil or plug swap test can prevent wasted parts. Another common miss involves fuel delivery. A restricted injector, poor injector power/ground, or low fuel pressure can mimic ignition failure. Vacuum leaks near the #5 runner can also create a lean misfire that “feels” like ignition. Shops also skip the basics on some platforms. They forget to confirm cylinder numbering, especially on V engines. Finally, people chase the misfire while ignoring data. Freeze-frame load, fuel trims, and misfire counters usually point to ignition, fuel, or air before any parts come off.

Most Likely Fix

The most frequently confirmed repair directions for P0305 involve restoring spark or fuel on cylinder 5 after you prove the fault follows the component. Many vehicles end up needing a worn spark plug or a coil-on-plug unit, but you must verify that by swapping components and rechecking misfire counters. The next common direction involves injector flow or injector control. A balance test, injector resistance check, and power/ground verification often catch the problem. If fuel trims run lean at idle and improve off-idle, smoke testing the intake for a leak near cylinder 5 becomes a high-value next step.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.