2021 Honda P0304: What It Means & Is It Safe to Drive?
P0304 on the 2021 Honda: Cylinder 4 Misfire Detected. Here's what it means, what usually causes it, what repairs typically cost, and whether it's safe to keep driving.
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What this code means
Misfire confirmed on cylinder 4. On 4-cylinder engines, this is the last cylinder—check coil and plug first, then injector, then compression.
Common causes on the 2021 Honda
- Worn spark plug on cylinder 4
- Failed ignition coil on cylinder 4
- Faulty or clogged fuel injector
- Low compression or valve issue
Symptoms
- Check Engine Light
- Rough idle and shaking
- Poor acceleration or power loss
Typical fixes
- Replace spark plug and COP coil on cylinder 4
- Clean or replace fuel injector
- Compression and leak-down test if basic parts check out
Repair cost range
For a 2021 Honda, repairs for P0304 typically fall between $150 and $900, depending on the root cause and labor rates in your area.
Known issues on Honda
Honda K-series and R-series P0304 often comes from a worn spark plug on cylinder 4. This last cylinder can run slightly hotter. Inspect plug condition first—OEM Honda plugs last 60k miles but aftermarket plugs can fail earlier and cause persistent P0304.
Most commonly affected models
Accord 2.4L K24 2003–2007CR-V 2007–2011Civic R18 2006–2011