What P0301–P0306 means
These six codes are the same fault pointing at different cylinders: P0301 is cylinder 1 misfiring, P0302 cylinder 2, through P0306 for cylinder 6. The computer has caught one specific cylinder repeatedly failing to fire — which is diagnostic gold, because it names exactly where to look.
What you’ll notice
- Rhythmic shaking — the engine limping on one leg, worst at idle
- Power loss and worse economy
- Check engine light, flashing when the misfire is active and severe
- Possible fuel smell
Likely causes and repair cost
Broad US independent-shop bands, parts and labour.
Can you drive with P0301–P0306?
Flashing light: stop — raw fuel from the dead cylinder is burning inside the catalytic converter. Steady light: gentle, minimal driving to diagnosis. Either way, this is a this-week code, not a someday code.
The misdiagnosis to avoid
Replacing parts without the free swap test. Move the suspect cylinder's coil to a neighbour: if the misfire code follows the coil (P0302 becomes P0304), the coil is convicted for zero parts cost. If it stays, the coil is innocent and the plug or injector is next. Ten minutes that prevents hundreds in guessed parts.
How P0301–P0306 actually gets set
The engine computer does not detect combustion directly. It watches the crankshaft position sensor for the tiny deceleration that happens when a cylinder fails to contribute, counts those events over a sampling window, and sets a code when the rate crosses a threshold. That is why a misfire code can appear long before you feel anything, and why a rough-running engine sometimes sets no code at all if the misfire is even across all cylinders.
What a scan tool shows that the code does not
Misfire counters per cylinder are the data that matters, and they are not in the code. A scan tool showing live misfire counts tells you whether one cylinder is responsible or whether the problem is spread across a bank, which splits an ignition fault from a fuel-delivery or vacuum fault immediately. Freeze-frame data adds the conditions: a misfire that only counts under load points somewhere different from one that counts at idle.
The trap with this family of codes
Replacing all the spark plugs and coils at once. It often works, which is exactly why it persists, but it costs several times what the actual repair needed and it teaches you nothing. On a cylinder-specific code, swapping that cylinder's coil with a neighbouring one and re-reading the code convicts or clears the coil for free in ten minutes.
What happens when you clear it
A misfire code will return within a drive cycle or two if the cause is still present, because the monitor runs continuously rather than waiting for specific conditions. That makes misfire codes a useful test of whether a repair actually worked.
This family contains the one emissions code that is genuinely urgent. A flashing light alongside a misfire code means raw fuel is reaching the catalytic converter, which melts it. The repair is usually a few hundred dollars; the converter it destroys is four figures.
Describe what it’s doing in your own words and the symptom checker gives you a safe-to-drive verdict, ranked causes with costs, and notes to take to the shop — free, no sign-up.
A basic code reader costs around $25 and usually pays for itself the first time a warning light comes on. We compare them in choosing an OBD2 scanner.
Questions people ask
How do I know which cylinder is which?
The numbering is set by the manufacturer and printed in service data — cylinder 1's position varies between makes (front of engine, driver's side, passenger's side). Don't guess from the code alone: look up your specific engine's cylinder layout, or the swap test lands on the wrong cylinder.
The misfire moved cylinders after I swapped the coil. What now?
That's the test working: the fault travelled with the part, so the coil is your answer. Replace it — and given one coil failed, ask how old the plugs are; a coil often fails from working overtime against a worn plug's wider gap.
Can a misfire code mean something serious like engine damage?
It can — if coil, plug, and injector for that cylinder check out, the remaining suspect is compression: a burned valve, a broken ring, a failing head gasket at that cylinder. That's why the sequence is cheap tests first, compression test before despair, and no engine-replacement conversations until a leak-down test says where the loss is.