OK briefly if steady; stop if the light flashes. P0307 means the powertrain control module has detected a misfire isolated to cylinder 7 — that one cylinder is repeatedly failing to burn its air-fuel charge while the others fire normally. It is the cylinder-specific version of the general misfire code P0300.
What P0307 means
The module does not watch combustion directly; it infers a misfire from tiny changes in crankshaft speed. Each power stroke gives the crankshaft a small acceleration, and the module tracks that contribution cylinder by cylinder through the crankshaft position sensor. When a cylinder fails to fire, the crankshaft briefly slows more than expected during that cylinder's stroke. Because the module knows the firing order and exact crank position, it can pin the missing pulse to one cylinder — here, cylinder 7. To keep this accurate it runs an adaptive learn that cancels out machining tolerances in the reluctor ring, and misfire detection runs whenever the engine is running and that learn has updated. A code stores when the speed variation for a single cylinder exceeds a calibrated limit that scales with engine rpm and load. Two thresholds exist: a lower emissions threshold that lights a steady lamp after repeated trips, and a higher catalyst-damaging threshold that flashes the lamp in real time. When a misfire is confirmed, many engines shut off the injector for that cylinder to keep raw fuel out of the catalytic converter.
Symptoms
- Rough idle and a noticeable shake or vibration, worst at a stop and under light load
- Flashing check-engine light when the misfire turns severe, especially under acceleration or heavy load
- Hesitation, stumble, or a dead-spot when you press the throttle
- Loss of power and rougher, less responsive running than normal
- Raw-fuel or rotten-egg smell from the exhaust and worse fuel economy as unburned fuel passes through
Common causes
- Failed or weak coil-on-plug on cylinder 7 — the single most common cause on modern V8s
- Worn, fouled, or wrong-gap spark plug in cylinder 7
- Clogged, leaking, or electrically failed fuel injector on cylinder 7
- Mechanical problem in cylinder 7 — low compression from a burned valve, worn rings, or a valvetrain fault
- Vacuum leak, damaged wiring, or a corroded connector affecting cylinder 7's coil or injector circuit
Severity & driving advice
Severity: Moderate — A single-cylinder misfire can dump raw fuel into the catalytic converter; a flashing light means damage is happening now, so stop driving.
Can I drive? OK briefly if steady; stop if the light flashes.
Diagnostic approach
- Confirm the misfire and read per-cylinder data — Retrieve all stored codes and the freeze-frame, then watch the scan tool's per-cylinder misfire counter. Cylinder 7 should show a clearly higher count than its neighbors. Note the rpm, load, and coolant temperature when it set — a cold-only miss points more toward fuel or a fouled plug, while a load-dependent miss points toward ignition or compression.
- Swap the coil and plug to prove the part — Move the cylinder-7 coil-on-plug to an adjacent cylinder and swap the spark plug the same way. Clear the code and re-run. If the misfire follows the moved part to the new cylinder, that coil or plug is the fault; if it stays on cylinder 7, the problem is that cylinder's injector, wiring, or mechanical condition. This swap test is the fastest way to avoid replacing good parts.
- Inspect the cylinder-7 spark plug and coil — Pull the cylinder-7 plug and read it: oil fouling suggests a mechanical issue, fuel fouling suggests a rich injector, and a cracked insulator causes a hard miss. Set the gap to the factory spec (about 0.043 in / 1.1 mm on the 5.7 HEMI, roughly 0.030-0.034 in on the Ford 5.0). Check the coil boot for carbon tracking and the coil body for cracks.
- Test the cylinder-7 injector and its circuit — Confirm the injector is clicking and measure its resistance — a typical port injector reads around 12 ohms, and an open or shorted winding condemns it. Verify the injector has power and a clean drive circuit; remember the module may have already commanded that injector off after detecting the misfire, so back-probe with the connector seated and look for opens, shorts, or a corroded terminal.
- Check compression and mechanical condition — If a good coil, plug, and injector do not cure it, test cylinder 7 mechanically. Run a compression or cylinder leak-down test; cylinder 7 should be within about 15-25 percent of the strongest cylinder. Low, leaking-down compression indicates a burned valve, worn rings, or a bent pushrod. Also check for a localized vacuum leak that leans out that cylinder before condemning internal parts.
Make & model notes
Ford: On the 5.0-liter Coyote V8 (F-150, Mustang GT), cylinder 7 sits on the driver's-side bank. Ford numbers the passenger bank 1-2-3-4 and the driver bank 5-6-7-8 front to rear, so cylinder 7 is the third plug back on the left head, with a firing order of 1-5-4-8-6-3-7-2. A weak coil-on-plug or a fuel-fouled plug on that cylinder is the usual find; confirm with a coil swap before condemning the injector.
Chrysler: On the 5.7-liter HEMI (300C, Charger, Ram, Grand Cherokee) each cylinder actually has two spark plugs, and cylinder 7 is the rearmost cylinder on the passenger-side bank — Chrysler numbers the right bank 1-3-5-7 and the left 2-4-6-8, firing order 1-8-4-3-6-5-7-2. The factory monitor watches crankshaft speed variation and cuts the injector to a misfiring cylinder. Note that cylinder 7 is one of the cylinders switched off by the MDS cylinder-deactivation system, so a lifter or MDS solenoid fault can also show up as a cylinder-7 miss.
FAQ
What is the difference between P0307 and P0300?
P0307 names a single cylinder — number 7 — as the one misfiring, while P0300 means the module detected a random or multi-cylinder misfire it could not isolate to one cylinder. A specific code like P0307 lets you focus on that cylinder's coil, plug, injector, and compression instead of chasing the whole engine.
Where is cylinder 7 located?
It depends on the engine's numbering. On the Ford 5.0 V8 cylinder 7 is the third cylinder back on the driver's-side bank (bank 2 is 5-6-7-8). On the Chrysler 5.7 HEMI cylinder 7 is the rearmost cylinder on the passenger side (the odd bank is 1-3-5-7). Always confirm against the firing order for your specific engine before pulling parts.
Can I drive with a P0307 code?
Only briefly, and only if the check-engine light is steady. A single-cylinder misfire sends unburned fuel into the exhaust, which overheats and can destroy the catalytic converter. If the light is flashing, the misfire is severe enough to damage the converter right now — stop driving and fix it before you add a costly repair to the bill.
How much does it cost to fix a cylinder 7 misfire?
It depends entirely on the cause. A single coil-on-plug or spark plug is an inexpensive, common repair. A clogged injector is a mid-range job, while low compression from a burned valve or worn rings is a major engine repair. Diagnose with a swap test and a compression check first so you only pay to replace the part that is actually failing.