Cylinder 4 Misfire Detected
The P0304 code means the vehicle's onboard diagnostic system has detected: Cylinder 4 Misfire Detected. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- medium
- Estimated cost
- $8-$30 per spark plug; $30-$80 per coil
The #1 fix for misfire codes. 15-piece set with 14mm, 16mm, 18mm magnetic sockets, T-handle extension, and spark plug gapper.
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Read and clear misfire codes. Shows which cylinder is misfiring so you know exactly what to replace.
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Common Symptoms
- Rough idle
- Vibration at idle
- Check Engine Light on or flashing
- Reduced power
Probable Causes (Ranked by Likelihood)
- Faulty spark plug on cylinder 4 Very High
- Failed ignition coil on cylinder 4 High
- Clogged fuel injector on cylinder 4 High
- Low compression on cylinder 4 Moderate
Step-by-Step Diagnostic Procedure
-
Swap spark plug from cylinder 4 with a known-good plug. If the misfire follows, replace all plugs.
-
Swap coil pack from cylinder 4 to another position. If misfire code changes numbers, replace the coil.
-
Test injector on cylinder 4 resistance and balance.
-
Compression test cylinder 4 as a final step.
Common Fixes by Vehicle
What techs usually find when diagnosing P0304 on specific vehicles — tap your vehicle for the fix and the exact part:
2007-2017 Toyota Camry 2.5L easy DIY
Cylinder 4 misfire is typically an ignition coil failure (Denso 673-1309). Swap the coil from cylinder 4 to cylinder 2 -- if the misfire follows, replace the coil. Cylinder 4 runs hottest on the 2AR-FE and coils fail there first. Also replace the spark plug (NGK 91578) and check the valve cover gasket for oil leaking into the plug tube.
Labor: 20 min2014-2020 Chevrolet Silverado/GMC Sierra 5.3L hard DIY
Cylinder 4 is an AFM cylinder on the GM 5.3L V8. A collapsed AFM lifter is the most common cause. GM TSB #18-NA-355 covers lifter failure diagnosis. Check lifter preload with the valve cover removed. If collapsed, replace all 16 lifters and consider an AFM delete kit (DOD delete) to prevent recurrence.
Labor: 6-8 hours2008-2019 Honda Accord/Civic 2.4L easy (plugs), hard (valves) DIY
Perform a coil swap test first. If the coil is good, check valve lash on cylinder 4 -- Honda K24 engines require periodic valve adjustment. Tight exhaust valves cause misfires when hot. Spec is 0.010-0.012 inch exhaust. Also check fuel injector resistance (should be 11-13 ohms) and clean with Seafoam if sticking.
Labor: 30 min (plugs), 2 hours (valve adjust)2010-2019 Subaru Outback/Forester 2.5L easy DIY
Cylinder 4 on the boxer engine is the driver-side rear cylinder. Heat cycling causes ignition coil cracks (Denso 673-7201). Also check for oil fouling from valve stem seal leaks -- common on the FB25. If compression is low on cylinder 4, suspect a leaking head gasket which is a known Subaru issue on earlier models.
Labor: 30 minFrequently Asked Questions
Does P0304 mean my engine is damaged?
Not necessarily. Most P0304 codes are caused by a simple spark plug or coil failure. Extended misfiring, especially with a flashing CEL, can damage the catalytic converter. Diagnose and repair promptly.
Sources
This page is built from documented references. Verify against your own service info before repair work.
P0304 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
See All Codes & Fixes for Your Vehicle
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