What are the typical symptoms of P0304?

Cold-start rough running that improves as the engine warms Sweet exhaust smell or white vapour at the tailpipe Coolant level slowly dropping with no external leak Misfire counts appearing only on cylinder 4 Miss returning after road shock or over rough pavement

What causes code P0304?

Head gasket breach letting coolant into cylinder 4 Broken or chafed coil wire at the end of the harness run Plug on cylinder 4 flooded by oil from a leaking valve cover Injector wiring open at the last connector in the loop Low compression on cylinder 4 from ring or valve wear

What recommended checks apply to P0304?

Read the cylinder-4 plug for steam cleaning or coolant residue Wiggle-test the coil and injector connectors while it idles Block-test the coolant for combustion gases if the plug looks washed Reseal the valve cover and clear oil from the plug well Measure relative compression, then leak-down on cylinder 4

P0304Cars & SUVs

Easy

P0304Cylinder 4 misfire detected

Q: What does code P0304 mean?

P0304 places the misfire on cylinder 4, and on many transverse four-cylinder engines that is the hole nearest the firewall and bellhousing where heat, harness chafe, and coolant intrusion all get worse. Confirm the count in Mode $06, then pull the plug and read it: a steam-cleaned white insulator with a sweet smell suggests coolant entering through a head-gasket breach, while black fuzz points to a soaked plug or an over-rich (excess fuel vs air, ~14.7:1 target) mixture. Wiggle-test the coil and injector connectors at idle, because a broken conductor at the last cylinder in the loom is a classic. If the evidence is mechanical, relative compression followed by a leak-down settles the question before you buy anything. Expect an emissions inspection failure until the counts return to zero.

Content reviewed: August 2026Content reviewed by a heavy-duty technician. About our editorial approach

OK to drive

— A misfire caused by coolant intrusion worsens quickly and can hydro-lock or wash the cylinder walls — stop driving if you see white vapour with a dropping coolant level.

What does this severity mean?

General guidance only — not a substitute for a professional inspection or official roadside / out-of-service criteria (e.g. CVSA OOSC) in your jurisdiction.

Part of system: Electrical sensing & powertrain inputs · Wire gauge & ampacity tool

Places the misfire on cylinder 4, and on many transverse four-cylinder engines that is the hole nearest the firewall and bellhousing where heat, harness chafe, and coolant intrusion all get worse. Confirm the count in Mode $06, then pull the plug and read it: a steam-cleaned white insulator with a sweet smell suggests coolant entering through a head-gasket breach, while black fuzz points to a soaked plug or an over-rich (excess fuel vs air, ~14.7:1 target) mixture. Wiggle-test the coil and injector connectors at idle, because a broken conductor at the last cylinder in the loom is a classic. If the evidence is mechanical, relative compression followed by a leak-down settles the question before you buy anything. Expect an emissions inspection failure until the counts return to zero.

Parts that may be involved

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These are generic search links — not a confirmed fit for your vehicle. Verify compatibility on Amazon (Garage / Confirmed Fit) or with the seller before buying.

Decode a VIN on the home page first to add year, make, and model to these Amazon searches.

Coil and plug for cylinder 4 with dielectric grease
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Valve cover gasket and plug-well seals
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Head gasket set if a block test confirms combustion gases
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Vehicles often affected

Common on transverse four-cylinder engines where cylinder 4 sits nearest bellhousing heat, and on high-mileage engines with a weeping head gasket.

Typical symptoms

Cold-start rough running that improves as the engine warms
Sweet exhaust smell or white vapour at the tailpipe
Coolant level slowly dropping with no external leak
Misfire counts appearing only on cylinder 4
Miss returning after road shock or over rough pavement

Likely causes

Head gasket breach letting coolant into cylinder 4
Broken or chafed coil wire at the end of the harness run
Plug on cylinder 4 flooded by oil from a leaking valve cover
Injector wiring open at the last connector in the loop
Low compression on cylinder 4 from ring or valve wear

Recommended checks

1
Read the cylinder-4 plug for steam cleaning or coolant residue
2
Wiggle-test the coil and injector connectors while it idles
3
Block-test the coolant for combustion gases if the plug looks washed
4
Reseal the valve cover and clear oil from the plug well
5
Measure relative compression, then leak-down on cylinder 4

DIY vs professional

DIY: plug reading, harness wiggle test, valve cover reseal. Pro: block-test interpretation, head gasket or valve repair.

Content reviewed by a heavy-duty technician. About our editorial approach