OBD-II · P0301
P0301 Code: Cylinder 1 Misfire — Diagnosis & Real Costs
Cylinder 1 misfire detected
The short version
A steady check-engine light allows short careful drives.
- Severity: Serious
- Most common cause: Worn or oil-fouled spark plug on cylinder 1
- 5 diagnosis steps in order
- 4 symptoms tied to this code
Darker bars are the more common causes; bar length is the top expected repair cost. Start with the dark, cheap causes before the expensive ones.
Can I keep driving?
A steady check-engine light allows short careful drives. A flashing light means raw fuel is reaching the exhaust and can ruin the catalytic converter within minutes — stop driving and diagnose it.
Symptoms
- Distinct shake at idle that partly smooths out at speed
- Power loss and hesitation under throttle
- Higher fuel use and a raw fuel smell from the tailpipe
- Check engine light steady or flashing depending on severity
Causes & repair cost
Worn or oil-fouled spark plug on cylinder 1
Most common$25 – $90
Failed ignition coil on cylinder 1
Most common$110 – $320
Clogged or sticking fuel injector
Possible$150 – $500
Intake manifold gasket leak at that cylinder
Possible$150 – $450
Low compression: valve or piston ring wear
Rare$900 – $3,500
More than half of P0301 cases end with a plug or coil at $25–$320. Low compression is the expensive outcome and usually comes with visible smoke or oil consumption.
Estimate the repair for your carDiagnosis steps in order
- Pull the cylinder 1 plug — its tip color reveals oil or fuel fouling
- Swap coils between cylinders 1 and 2 — if the code becomes P0302, the coil is the fault
- Listen to the injector with a long screwdriver or test its resistance
- Spray intake cleaner near the gasket and watch for idle change
- Run a compression test if plugs and coil changes made no difference
FAQ
Which cylinder is number 1?
It varies by engine; on transverse four-cylinders it is usually nearest the accessory belt, and on V engines it depends on the bank. Check your model's firing-order diagram first.
Should I replace all plugs or just one?
Replace the full set; the price gap is small and the remaining old plugs will fail soon and set another code.
Can a misfire damage the catalytic converter?
Yes — unburned fuel ignites inside the converter and melts its substrate. That is why the flashing light is a stop-driving warning.
Could bad fuel be the cause?
Contaminated fuel usually causes a general P0300 rather than a single-cylinder code. A repeating P0301 points to a physical fault in that cylinder.
Related codes
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