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P0304 is the last cylinder in a 4-cylinder engine. On many transverse-mounted 4-cylinders (Civic, Corolla, Altima), cylinder 4 sits closest to the firewall and can be harder to access. A coil-on-plug swap is still the fastest free diagnostic first step. See top-rated scanners on Amazon ↗
🗺️ Where Is the Problem?
Blueprint view - P0304 fault localized to cylinder #4 (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 P0304 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.
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🎯 Top Causes & Probability
50%
#1 - Most Likely
Bad Spark Plug (Cylinder 4)
Worn or fouled spark plug is the leading cause of P0304. On Honda iVTEC engines, oil fouling of the rear plug (cylinder 4) is common when valve stem seals are worn - look for blue smoke on startup as a clue. Replace all 4 plugs at the same time for balanced performance.
🔩 Part
$5–$30
👨🔧 Labor
$0–$80
⚡ DIY Difficulty
Easy
30%
#2 - Check First
Failed Coil-on-Plug (Cylinder 4)
The ignition coil for cylinder 4 has failed or the rubber boot has cracked, allowing high-voltage spark to arc to ground instead of firing the plug. Swap the coil with cylinder 3 to confirm - if the code moves to P0303, replace the coil. On VW Jetta, coils fail in groups and it's common to replace all four.
🔩 Part
$20–$100
👨🔧 Labor
$30–$80
⚡ DIY Difficulty
Easy
14%
#3 - Less Common
Clogged or Leaking Fuel Injector
An injector delivering too little fuel causes a lean misfire; a stuck-open injector causes a rich, wet misfire. Lean misfires are worst under load; rich misfires are worst at idle. An injector balance test (available at most shops with advanced scan tools) can pinpoint this without replacement.
🔩 Part
$25–$150
👨🔧 Labor
$50–$100
⚡ DIY Difficulty
Medium
6%
#4 -
Compression Loss (Cylinder 4)
A leaking head gasket, burned valve, or worn piston rings on cylinder 4 will cause consistent misfires under all conditions that won't respond to ignition or fuel fixes. A compression and leak-down test are needed to diagnose. If all other ignition and fuel checks pass, test compression before buying more parts.
🔩 Part
$50–$2,000+
👨🔧 Labor
$500–$4,000
⚡ DIY Difficulty
Hard
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Inspect the Spark Plug in Cylinder 4 - Remove and inspect the plug. Black sooty deposits = rich/oil fouling. White/blistered = lean or overheating. Tan/grey = normal wear. Swap plug with cylinder 3 and rescan - if P0303 sets, the plug is the culprit.
- Swap the Coil-on-Plug to Cylinder 3 - If the plug was normal, swap the coil and rescan. A migrating misfire code confirms a failed coil. Replace the coil (and consider replacing all of them on high-mileage VW/Audi engines).
📍 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