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A cylinder-specific misfire code is a big clue. P0301 pinpoints cylinder 1 - start there. Swap the spark plug and coil pack to an adjacent cylinder and rescan. If the code follows the part, you've found your culprit for under $30. See top-rated scanners on Amazon ↗
🗺️ Where Is the Problem?
Blueprint view - P0301 fault localized to cylinder #1 (spark plug, ignition coil, or fuel injector)
These are statistical causes across ALL vehicles - your exact car may rank differently
For example, on a Honda 4-cyl the downstream O2 sensor causes P0301 64% of the time, but on a GM 5.3L V8 the catalytic converter is the cause 71% of the time. Get a probability ranking built specifically for your year, make, model, and mileage.
🔎 Get the ranking for my exact car - $5.99 →
🎯 Top Causes & Probability
55%
#1 - Most Likely
Bad Spark Plug (Cylinder 1)
A worn, fouled, or cracked spark plug in cylinder 1 is the most common cause of a P0301. Carbon deposits, oil fouling, or a cracked ceramic insulator all prevent proper ignition. Plugs should be inspected every 30k miles and replaced every 60–100k miles depending on type.
🔩 Part
$5–$30
👨🔧 Labor
$0–$80
⚡ DIY Difficulty
Easy
30%
#2 - Check First
Failed Ignition Coil (Cylinder 1)
The coil-on-plug or distributor-connected coil for cylinder 1 has failed internally or developed a crack in the boot, causing a weak or absent spark. Coil failures are especially common on high-mileage vehicles and often happen alongside plug replacement intervals.
🔩 Part
$20–$120
👨🔧 Labor
$30–$100
⚡ DIY Difficulty
Easy
10%
#3 - Less Common
Clogged Fuel Injector (Cylinder 1)
A partially clogged or stuck-open injector delivers too little or too much fuel to cylinder 1, causing an incomplete combustion event. Injector problems are more likely if the misfire occurs only at idle or only under load. Fuel injector cleaner may help minor clogs; severe cases require cleaning or replacement.
🔩 Part
$25–$150
👨🔧 Labor
$50–$100
⚡ DIY Difficulty
Medium
5%
#4 -
Low Compression (Cylinder 1)
If cylinders 1 has low compression due to worn piston rings, a burned valve, or a blown head gasket, the air/fuel mixture won't ignite reliably. A compression test quickly confirms or rules this out. Low compression is a serious mechanical issue requiring engine repair.
🔩 Part
$100–$2,000+
👨🔧 Labor
$500–$3,000
⚡ DIY Difficulty
Hard
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Swap Spark Plug Cylinder 1 → Cylinder 2 - Move the cylinder 1 spark plug to cylinder 2. Clear codes and drive. If P0302 sets instead of P0301, the plug is bad. Replace all plugs while you're in there.
- Swap Ignition Coil Cylinder 1 → Cylinder 2 - If the plug swap didn't move the misfire, swap the coil to cylinder 2. If P0302 now sets, the coil is your culprit.
📍 Find a Trusted Shop Near You
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Tips for Choosing a Shop
- Ask if they charge a diagnostic fee and whether it applies toward the repair
- Request a written estimate before approving any work
- Ask specifically about the part brand - OEM vs. aftermarket matters for this code
- Check Google reviews for recent mentions of the specific repair you need