OBD2 Diagnostic Code
P0304
Cylinder 4 Misfire Detected — Safe to Drive? Real Causes & Repair Cost
No, it is not safe to drive — and on a 4-cylinder engine, Cylinder 4's position at the end of the row makes a bad misfire especially hard on the catalytic converter. The good news: the fix is almost always a plug or a coil, and the swap test confirms which one before you spend anything.
Severity
8 / 10
High. Cylinder 4 feeds the exhaust collector directly — unburned fuel hits the catalytic converter with no buffer.
Safe to Drive?
No
Drive only to reach a shop. A flashing check engine light means pull over now — catalytic converter damage is actively occurring.
Typical DIY Fix Cost
Low – Moderate
Plug and coil cover most cases. Access difficulty on some engines adds time but not much cost.
🔍 What This Code Means
Your engine's crankshaft position sensor tracks the exact rotation of the crank after every firing event. P0304 means the sensor detected a rotational stumble — a fractional slowdown in crank speed — at the point in the firing order where Cylinder 4 should have contributed its power stroke. The ECM counts these stumbles. Once the count on Cylinder 4 crosses the calibrated misfire threshold within a set number of engine revolutions, the code is stored.
There is something specific to understand about Cylinder 4 on inline 4-cylinder engines — the layout found in most compact and mid-size vehicles. Cylinder 4 sits at the firewall end of the engine, furthest from the front accessory drives. Every drop of unburned fuel from a Cylinder 4 misfire travels the full length of the exhaust manifold and lands directly at the catalytic converter inlet with very little cooling distance. This is why a severe Cylinder 4 misfire tends to destroy converters faster than a misfire on Cylinder 1 or 2.
⚠️ On many front-wheel-drive vehicles, Cylinder 4 is also the hardest plug to reach. The firewall sits inches behind it, coil packs may be buried under the intake manifold, and some engines require removing a coolant hose or engine mount bracket for clear access. Budget extra time compared to the other cylinders — but the diagnostic process is identical.
Like all cylinder-specific misfire codes, P0304 gives you a precise starting point. The diagnosis is methodical: confirm the plug, confirm the coil, confirm the injector. Only then consider mechanical causes. Most people find their answer by the end of Step 2.
⚡ Common Symptoms
A Cylinder 4 misfire on a 4-cylinder engine hits harder than it would on a V6 or V8. You are running on three of four cylinders — 75% of your engine's designed output. You will notice it.
Strong vibration at idle
Three cylinders firing against one silent one creates a pronounced imbalance. The engine rocks against its mounts. You feel it clearly in the steering wheel and seat, especially at a stop in Drive.
Hesitation and stumble under throttle
Pulling onto a highway or climbing a hill exposes the power deficit immediately. The engine surges, falls back, and surges again. Automatic transmissions may hunt between gears trying to compensate.
Solid or flashing check engine light
Solid = code stored, misfire ongoing but below converter-damage rate. Flashing = severe misfire, catalytic converter damage happening right now. A flashing light is an immediate stop instruction.
Raw fuel smell from the exhaust
Unburned gasoline exiting through Cylinder 4 has a sharp, acrid smell at the tailpipe — distinctly different from normal exhaust. On cold starts this can be especially strong for the first minute of operation.
White or grey smoke under hard acceleration
If coolant is entering Cylinder 4 through a head gasket breach, you may see white steam from the exhaust under load. This symptom alongside P0304 warrants immediate cooling system inspection before further driving.
Fuel economy drops noticeably
Unburned fuel escaping through Cylinder 4 is wasted on every misfire event. The ECM also adjusts fueling to try to smooth the misfire, compounding the loss. Expect a meaningful MPG drop within a day or two of onset.
🎯 Most Likely Causes
Ranked most common to least. Start at #1 and work down. On a 4-cylinder engine, the first two causes account for nearly seven in ten verified P0304 diagnoses — and neither requires a shop visit to confirm.
1
Worn, fouled, or damaged spark plug — Cylinder 4
Spark plugs are a consumable. A plug that has exceeded its service interval develops a widened electrode gap, a cracked insulator, or carbon and oil deposits that prevent a consistent spark. On engines where Cylinder 4 runs slightly hotter due to its position near the firewall with less airflow, plug degradation sometimes accelerates compared to Cylinders 1 and 2. Inspect the tip for fouling and measure the gap with a feeler gauge — compare to your owner's manual spec, typically 0.028–0.060 inches. A plug that is visibly worn, bridged with deposits, or gapped incorrectly is replaced, full stop.
~ 38% of cases
2
Failed ignition coil (coil-on-plug) — Cylinder 4
On modern coil-on-plug ignition systems, each cylinder has a dedicated coil pack mounted directly on the spark plug. A coil that has cracked internally loses its ability to step voltage up sufficiently under compression. Coil failures often present as intermittent misfires that worsen in wet weather or during hard acceleration — the conditions that demand peak spark output. Confirm with the swap test: move the Cylinder 4 coil to Cylinder 3. If P0303 appears after a test drive, the coil is confirmed faulty. If P0304 stays on Cylinder 4, the coil is cleared and you move to the injector.
~ 30% of cases
3
Clogged or failing fuel injector — Cylinder 4
A partially clogged injector delivers an inconsistent fuel spray pattern. A fully stuck-closed injector delivers nothing. Either way, Cylinder 4 does not get the right mixture and misfires. Unlike coil and plug faults, injector problems do not follow a swap test. Listen for each injector's rapid clicking tick at idle using a mechanic's stethoscope or a long screwdriver held to your ear. No click from Cylinder 4 points directly at the injector circuit. Before ordering a replacement, check for voltage at the harness connector — if voltage is absent, the fault is upstream wiring, not the injector itself.
~ 14% of cases
4
Head gasket breach at Cylinder 4 — coolant intrusion
On inline 4-cylinder engines, Cylinder 4 sits at the end of the cooling jacket furthest from the water pump. Coolant returning from the radiator flows through the entire block before reaching Cylinder 4, arriving slightly warmer than at Cylinder 1. On high-mileage engines or those with a history of overheating, this thermal difference concentrates head gasket wear at Cylinder 4. A small breach allows coolant into the combustion chamber, producing a wet misfire. Clues: white smoke from the exhaust, coolant level dropping without a visible external leak, and engine oil with a milky appearance on the dipstick. Confirm with a combustion gas (block) test before any head gasket diagnosis is made.
~ 10% of cases
5
Low mechanical compression — Cylinder 4
Burned valves, worn piston rings, or a damaged piston on Cylinder 4 reduce compression below the threshold needed to ignite the mixture consistently. This fault does not respond to any ignition or fuel repair. If plug, coil, and injector all check out, perform a compression test on all cylinders. Healthy readings fall between 130–180 PSI. A Cylinder 4 reading more than 15% below adjacent cylinders — or below 100 PSI outright — demands a follow-up wet compression test to separate ring wear from valve or gasket damage before any teardown begins.
~ 6% of cases
6
Wiring fault or damaged harness — Cylinder 4 coil or injector circuit
Cylinder 4's position near the firewall sometimes puts its wiring harness in contact with heat shields, sharp body edges, or coolant hoses — all of which can chafe through insulation over time. A shorted or open-circuit wire to the coil or injector produces an electrical misfire that mimics component failure. Inspect the Cylinder 4 wiring loom for visible damage before condemning a coil or injector that tests fine on the bench. This cause is uncommon but disproportionately found on Cylinder 4 compared to the other positions due to its cramped location.
~ 2% of cases
🔧 What to Check First
Three checks, in this order. On some engines Cylinder 4 access takes a few extra minutes — a plastic engine cover, a stubborn coil connector, or working at an angle toward the firewall. Budget the time, follow the sequence, and do not skip ahead to expensive parts.
1
Access and inspect the Cylinder 4 spark plug
On most inline 4-cylinder engines, the Cylinder 4 plug is at the firewall end — the one furthest back. You may need to remove a plastic engine cover and possibly move a coolant hose out of the way for clear access. Use a plug socket with a long extension and go slowly — cross-threading a plug in an aluminum head is an expensive mistake. Once out, examine the electrode tip against the diagnostic guide in the Causes section. Measure the gap with a feeler gauge. If anything looks wrong — fouling, a worn rounded electrode, a cracked insulator, or a gap outside spec — replace it. If the engine has high mileage and the plugs have not been replaced recently, replace all four at once. One bad plug now usually means another one is close behind.
2
Run the coil swap test
With the Cylinder 4 plug confirmed good (or freshly replaced), swap the Cylinder 4 coil pack with the Cylinder 3 coil. Unplug the connector on each, unbolt the coil (usually one or two bolts), and move them to the opposite positions. Reconnect everything firmly — a half-seated coil connector is its own misfire source. Clear the stored code and drive 10–15 miles, including some steady highway speed where misfires are most detectable. Check the result: P0303 now appears — the Cylinder 4 coil you moved is faulty, replace it. P0304 stays on Cylinder 4 — the coil is innocent, the fault is in the cylinder itself. Do not order a coil without completing this test.
3
Test the Cylinder 4 injector for pulse and power
With the engine at idle and fully warmed up, press a long screwdriver handle firmly against each injector body in sequence. Put your ear to the handle — a working injector ticks rapidly and consistently. Silence from Cylinder 4 means it is not firing. Next, with the key in the ON position and engine off, unplug the Cylinder 4 injector connector and probe the harness pins with a test light or multimeter. You should see battery voltage (12–14V). Voltage present but no injector tick means the injector has failed electrically and needs replacement. No voltage means there is a wiring fault between the ECM and the injector that must be traced before any part is ordered. Also check for a white residue or wetness around the injector o-ring seal — a leaking injector floods the cylinder and causes a rich-mixture misfire that mimics ignition failure.
🚨 If white smoke is coming from the exhaust alongside P0304, stop driving immediately. White exhaust smoke combined with a cylinder-specific misfire is a strong indicator of coolant entering Cylinder 4 through a head gasket breach. Continued driving risks a severely overheated engine and catastrophic internal damage.
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⚠️ Common Misdiagnosis
Three money-wasting mistakes happen repeatedly with P0304. All three are avoidable with the right sequence of checks.
⚠ Misdiagnosis #1 — Diagnosing head gasket failure based on P0304 alone
A head gasket breach at Cylinder 4 is a real possibility — but it accounts for roughly 10% of P0304 cases. The other 90% are ignition and fuel faults that cost a fraction of a head gasket repair. Some shops see P0304 on an older vehicle with higher mileage and move quickly toward head gasket as the explanation, particularly if there is any history of overheating in the records. Before a head gasket diagnosis can be made responsibly, you need: a fresh spark plug confirmed not to fix the fault, a coil swap test confirmed clear, a compression test on all cylinders, and a combustion gas test on the coolant. P0304 alone does not justify a head gasket recommendation.
Potential savings: Full head gasket repair cost on a car that needed a spark plug
⚠ Misdiagnosis #2 — Skipping access difficulty and replacing the wrong plug
On engines where Cylinder 4 is buried near the firewall, some DIYers count plugs from the wrong direction and replace the Cylinder 1 plug thinking it is Cylinder 4. This is more common than it sounds, especially on transversely mounted engines where the plug order is not immediately obvious from above. The result is a P0304 that does not clear after a plug replacement — because the wrong plug was replaced. Always confirm your engine's cylinder numbering layout before removing anything. A 30-second web search for your specific make, model, and engine eliminates this mistake entirely.
Potential savings: Cost of a second plug replacement and unnecessary shop visit
⚠ Misdiagnosis #3 — Replacing the injector without checking the o-ring seal first
A leaking injector o-ring seal allows fuel to bypass the injector tip and enter the cylinder as a flood rather than a controlled spray. This produces a rich-mixture misfire on Cylinder 4 that sounds exactly like an injector electrical failure. The difference is the repair cost: a new o-ring seal costs almost nothing and takes fifteen minutes to install. A new injector costs significantly more. Before condemning a Cylinder 4 injector that has confirmed electrical pulse, pull the injector and inspect the o-ring for cracking or compression set. Replace the o-ring and retest before ordering a new injector.
Potential savings: Full injector replacement cost vs. the cost of an inexpensive seal
💰 Estimated Repair Cost
DIY cost = parts only. Shop cost = parts plus labour. On some engines Cylinder 4 access adds 30–60 minutes of labour over Cylinder 1 — worth asking about when getting a shop quote.
Spark plug replacement (Cylinder 4)
DIYVery Low
ShopLow–Moderate
LevelEasy–Moderate
Full spark plug set replacement
DIYLow
ShopModerate
LevelEasy–Moderate
Ignition coil replacement (Cylinder 4)
DIYLow–Moderate
ShopModerate
LevelModerate
Injector o-ring seal replacement
DIYVery Low
ShopLow–Moderate
LevelModerate
Fuel injector replacement (Cylinder 4)
DIYModerate
ShopModerate–High
LevelModerate
Catalytic converter replacement (misfire damage)
DIYHigh
ShopVery High
LevelAdvanced
Head gasket repair (coolant intrusion)
DIYHigh
ShopVery High
LevelProfessional
✅ Note the Cylinder 4 access factor in the shop cost column — on some engines the labour to reach the plug and coil is higher than on Cylinder 1. If a shop quotes significantly more for Cylinder 4 work, ask which specific access steps are involved. A clear answer is a good sign.
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🚗 Most Reported On
P0304 appears across all makes. These models generate above-average Cylinder 4 misfire reports, often linked to coil boot deterioration, cooling system design, or carbon buildup patterns particular to their engine architecture.
Toyota Camry (2007–2014, 2.5L 2AR-FE — coil and plug)
Honda CR-V (2007–2011, 2.4L K24 — valve cover oil leak)
Ford Fusion (2010–2016, 2.5L Duratec — injector)
Volkswagen Jetta (2011–2018, 1.4T / 2.0T — coil failure)
Chevrolet Cruze (2011–2016, 1.4T Ecotec — coil boot)
Subaru Impreza (2012–2017, 2.0L FB20 — head gasket)
BMW 3 Series (2012–2019, B48 2.0T — carbon and coil)
Mazda3 (2010–2018, 2.0L / 2.5L SkyActiv — plug fouling)
❓ Frequently Asked Questions
Is it safe to drive with P0304?
No. A Cylinder 4 misfire sends raw unburned fuel into the exhaust on every misfiring cycle. That fuel combusts inside the catalytic converter, overheating and destroying the ceramic substrate. On a 4-cylinder engine, Cylinder 4 is the last cylinder before the exhaust collector — raw fuel arrives at the converter with very little cooling distance, making the damage faster than it would be from Cylinder 1. Drive only as far as needed to reach a repair facility. If the check engine light is flashing rather than steady, stop driving immediately and do not restart until the misfire is repaired.
Why is Cylinder 4 harder to access than the others on some engines?
On most front-wheel-drive vehicles the engine sits transversely — sideways in the bay — with the exhaust manifold facing the firewall. Cylinder 4 is at the firewall end of the engine, which means the plug and coil pack are often tucked behind coolant hoses, the brake master cylinder, and sometimes the engine mount. On some Honda, Toyota, and Nissan engines you can reach it with a long extension and patience. On others — particularly some VW and BMW applications — meaningful access requires moving or removing additional components. If you are doing this yourself, look up the procedure for your specific model before starting. Knowing what is in the way prevents broken bolts and dropped sockets.
P0304 came back after I replaced the spark plug. What do I do next?
Run the coil swap test immediately. Move the Cylinder 4 coil to Cylinder 3 and install the Cylinder 3 coil on Cylinder 4. Clear the code and drive 10–15 miles. If P0303 appears, the coil you moved out of Cylinder 4 is faulty — replace it. If P0304 stays on Cylinder 4 with a confirmed-good plug and confirmed-good coil, move to the injector: listen for clicking at idle, confirm voltage at the harness connector, and inspect the o-ring seal. If the injector checks out, a compression test on Cylinder 4 is the next step.
Can a cooling system problem cause P0304 specifically on Cylinder 4?
Yes — this is one of the more overlooked causes of Cylinder 4 misfires on high-mileage engines. On many inline 4-cylinder designs, the coolant jacket at Cylinder 4 runs slightly hotter than at Cylinder 1 because it sits furthest from the thermostat housing. Over time, this thermal difference concentrates head gasket stress at Cylinder 4. A small breach allows coolant into the combustion chamber, producing a wet misfire. Signs that point toward this cause rather than ignition: white smoke from the exhaust, coolant level dropping without a visible external leak, sweet-smelling exhaust gas, and a milky or frothy appearance on the oil dipstick. Confirm with a combustion gas test on the coolant before any head gasket decision is made.
I have P0304 and P0300 together. Which do I fix first?
Fix P0304 first. P0300 is the ECM's way of saying the overall misfire count across the engine has exceeded a threshold — which happens when one cylinder misfires severely enough to push the whole-engine count over the limit. P0300 and P0304 appearing together almost always means Cylinder 4 is the primary offender driving the random count. Address the Cylinder 4 fault and clear both codes. In the vast majority of cases P0300 does not return once the cylinder-specific fault is repaired.
🔗 Related Codes
These codes share the same fault family as P0304 or commonly appear alongside it.
This guide is for informational purposes only. Always consult a qualified mechanic before performing any vehicle repair. OBDPlatform is not responsible for actions taken based on this content.