What P0304 Means
P0304 rounds out the individual cylinder misfire codes on this list, isolated to cylinder 4. As with P0301 through P0303, the PCM has determined this misfire consistently traces to one specific cylinder, pointing the diagnosis toward that cylinder's spark plug, ignition coil, fuel injector, or a mechanical fault like low compression, rather than a system-wide cause affecting multiple cylinders. The coil/plug swap test remains the standard first diagnostic step, since it quickly distinguishes between a faulty ignition component and something else specific to cylinder 4's position, such as its injector or internal mechanical condition. It's also worth noting that misfire codes are inherently statistical — the PCM counts misfire events over a rolling window of engine revolutions rather than flagging a single missed firing event, so a code like P0304 means a consistent pattern has been detected over time, not just one-off noise. This is part of why the code can sometimes take a drive cycle or two to appear even after a fault has started.
Common Causes
- Worn, fouled, or damaged spark plug in cylinder 4
- Failing ignition coil serving cylinder 4
- Clogged, leaking, or electrically failed fuel injector on cylinder 4
- Low compression in cylinder 4 from worn piston rings or a valve sealing issue
Symptoms You Might Notice
- Check Engine Light on, flashing if the misfire is severe or sustained
- Noticeable rough idle or engine shake
- Hesitation or stumbling under acceleration
- Reduced power and fuel economy
What to Check First
- 1
Inspect and replace the spark plug in cylinder 4 if worn, fouled, or improperly gapped
- 2
Swap the ignition coil between cylinder 4 and a neighboring cylinder, clear the code, and see whether the misfire follows the coil
- 3
Check the cylinder 4 fuel injector using resistance or balance testing if ignition components test fine
- 4
Perform a compression or leak-down test specifically on cylinder 4 if fuel and ignition components both check out
- 5
Check whether the misfire count for cylinder 4 correlates with specific driving conditions (cold start, high load, steady cruise) using scan tool freeze-frame data, since certain fault types are more likely to show up only under specific conditions
Frequently Asked Questions
If I have multiple single-cylinder misfire codes at once, what does that mean?
Multiple individual cylinder codes appearing together — rather than one isolated code or the general P0300 — often points toward a shared but component-level cause, like an entire set of worn spark plugs reaching end of life together, rather than one isolated failure.
Do I need special tools to diagnose P0304 myself?
A basic OBD-II scanner to read live misfire counts per cylinder, a spark plug socket, and a multimeter cover most of the initial diagnostic steps. A compression tester is needed if ignition and fuel components check out fine and a mechanical cause is suspected.
Can a vacuum leak specific to one cylinder cause P0304?
Yes, though it's less common than a spark plug or coil issue — a leaking intake gasket or cracked vacuum line feeding specifically into cylinder 4's intake runner can lean out that cylinder enough to cause a consistent, isolated misfire.
Why did this code take a while to show up after I first noticed rough running?
The PCM tracks misfires as a count over a rolling window of engine revolutions rather than reacting to a single missed firing event, so it can take a short period of consistent misfiring before the threshold for setting the code is actually reached.
Reviewed by Emmanuel Erere, Auto Mechanic. Causes and diagnostic steps checked for workshop accuracy.