Number P0305: Cylinder 5 Misfire Detected
The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Fo
Can you drive with P0305?
Severity: Stop soon — misfire. Only as far as a shop, gently. A flashing check-engine light means an active severe misfire: pull over and arrange a tow.
If you ignore it: A misfiring cylinder dumps unburned fuel into the exhaust, which overheats and destroys the catalytic converter — a $150 ignition fix becomes a $1,500 converter.
This is the #16 most common code in our top-150 library, in the engine misfire family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0305 means
The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Ford 0.9–1.5 %) it comes on after two trips. Ford and GM may shut off a misfiring cylinder's injector to protect the catalyst. P0305 means more than 90 % of the detected misfires fall on cylinder 5. Usual causes are the ignition coil or spark plug of that cylinder, its injector, or low compression; a quick test is to swap coil or plug with a neighbouring cylinder and see whether the misfire moves. GM found heat-damaged coils on its 3.6 V6 caused by a stuck ignition signal – the damaged coil need not be on the misfiring cylinder.
Systems involved: ignition coil cylinder 5, spark plug cylinder 5, fuel injector cylinder 5.
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Ignition coil of cylinder 5 faulty (coil, connector, ground) | most likely |
| 2 | Spark plug of cylinder 5 worn, sooted, cracked or wrongly gapped | most likely |
| 3 | Injector of cylinder 5 clogged or leaking, or its control | possible |
| 4 | Low compression on cylinder 5 (valve, rings, head gasket) or local false air | less common |
| 5 | Water or oil in the plug well | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine light on, flashing at a catalyst-damaging rate
- Rough idle, jerking, vibration
- Loss of power, hesitation
- Smell of unburnt fuel, higher consumption
Misdiagnosis traps
- Replacing the part named by the code title without testing. The code reports a failed self-test; the cause list above is ranked so the likely, cheap causes are checked before the expensive part is condemned.
- Clearing the code and declaring victory when the light stays off for a day — the monitor that sets this code runs only under specific driving conditions.
(General diagnostic guidance for this code family — no code-specific trap is published for this code.)
How a good shop confirms it
- Confirm the code and record any companion codes — companions narrow the system fast.
- Inspect the affected components and their wiring/connectors for damage, corrosion and looseness.
- Test the top-ranked cause with live data or a multimeter before replacing anything.
(Standard diagnostic path for this family, labeled as general guidance.)
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| Ignition coils + spark plugs (full set) | $280 – $645 | $180 – $420 (parts only) | 2/5 |
| Spark plugs — full set | $115 – $340 | $40 – $140 (parts only) | 2/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor). Published cross-check — Ignition coils + spark plugs (full set): CarMD 2026 national average $480; Spark plugs — full set: CarMD 2026 national average $299.
OBDex repair data for this code rates the job difficulty easy with a typical labor time in the 0.3–4 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
How much does it cost to fix P0305?
The fix most associated with this code — ignition coils + spark plugs (full set) — runs $280 – $645 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0305?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0305 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0305?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0305?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.