What this code means
P0304 may mean the PCM has detected a misfire pattern on cylinder 4.
What the vehicle may do
- The engine may run rough, stumble, hesitate, or feel down on power.
- The MIL may illuminate or flash depending on misfire severity.
- Stop/start may be disabled while the code is active if the vehicle is equipped with that feature.
Possible fault areas
- Possible ignition faults can include the coil, spark plug, coil control circuit, or related connections.
- Possible fuel faults can include injector performance, fuel quality, fuel contamination, or fuel supply concerns.
- Possible air and engine faults can include vacuum leaks, carbon buildup, oiling issues, compression concerns, valvetrain issues, or coolant entering the cylinder.
- Possible control or wiring faults can include connector problems, circuit faults, VVL-related concerns if equipped, or PCM-related concerns.
Diagnostic path
Open and frame the fault
P0304 is a cylinder 4 misfire. In plain language, the PCM is seeing crankshaft speed variation that points to cylinder 4 not contributing normally. The truck may run rough, stumble, lose power, flash or turn on the MIL, and stop/start may be disabled while the fault is active. This can come from ignition, fuel delivery, air or vacuum issues, VVL or VVT-related faults if equipped, oiling or mechanical problems, carbon buildup, coolant entering the cylinder, wiring, connectors, or in the end the PCM. Start with the basic system checks before you chase the cylinder. Also remember: anything that changes crankshaft speed can set a misfire code, so do not assume a failed part just because the counter names cylinder 4. The monitor gates belong on screen: fuel level above 12%, battery voltage above 10.9 volts, ambient temperature above -23°C (-9.4°F), coolant temperature at engine start above -7°C (19.4°F), or enabled when engine temperature reaches 21°C (69.8°F), and engine speed between approximately 400 rpm and 6200 rpm. The fault sets when crankshaft speed variation between cylinders exceeds a calibrated value based on engine speed and load.
Confirm the misfire is active
Turn the ignition on, read and record all codes and the captured operating data, then reproduce the captured conditions on the road. During the drive, pay attention to when the misfire shows up and watch the downstream oxygen sensor behavior while it is happening. Low-side readings during the misfire point you toward a lean condition, like lack of fuel from a plugged injector or another fuel issue. High-side readings point more toward rich operation, lack of spark, or a leaking injector. Then use the misfire monitor to confirm cylinder 4 is the cylinder showing excessive misfires right now. If cylinder 4 is not actively counting excessive misfires, treat it as an intermittent condition instead of forcing the rest of this path. One more note: during a severe misfire, the PCM can disable the affected injector for a calibrated number of engine revolutions. That can make a high-load or high-rpm misfire appear to continue after the truck comes back to idle until the counter resets.
Check other active clues first
Next, look at the codes you recorded. If there are active or pending clues for fuel, ignition, air handling, VVL if equipped, or VVT, handle those applicable diagnostics first. Do not ignore a related system fault and jump straight to parts on cylinder 4. If those areas are not setting active or pending faults, move into the inspection work.
Base inspection before component testing
Do a practical underhood inspection. Look for belt wear, pulley misalignment or binding, sensor mounting problems at the crank, cam, or MAP sensors, poor terminal fit or connector condition at related components, corrosion on PCM power or ground circuits, vacuum leaks, air inlet or exhaust restriction, signs of internal engine trouble, moisture on ignition components, fuel supply problems, poor fuel quality, or operating conditions like bogging or overload. If you find a problem here, correct it and verify the repair. If the inspection is clean, check oil level and oil condition next. You are looking for oiling or mechanical concerns, including low level, dirty or contaminated oil, incorrect oil filter or viscosity, aerated oil, oil pressure concerns, or engine mechanical tolerance problems. If an oiling or mechanical problem is present, correct that first and verify the repair.
Air, fuel quality, and injector comparison
Now check the intake side for vacuum leaks at the engine and intake manifold. If a leak is found, repair the leak and verify the repair. If not, check fuel quality and supply: water in the fuel, wrong fuel type, old or contaminated fuel, and low fuel. Also perform the ethanol-level check called for in this path. If the fuel is the problem, replace the fuel and, if needed, flush and clean the fuel system, then verify the repair. Before opening the fuel system, release fuel pressure. This system can hold constant pressure even with the engine off, and skipping that step can cause serious or fatal injury. For the injector check, install Fuel Pressure Decay Tester 8978A at the rail. Key on, let pressure reach maximum and stabilize, then record the starting pressure. Actuate the cylinder 4 injector with the scan tool while watching the gauge and record the ending pressure. Repeat the same comparison on several known-good cylinders, always starting from the same starting pressure. After each injector actuation, start the engine to clear fuel from the cylinder; failing to do that can damage the engine. If the ending pressures are about the same, keep going. If cylinder 4 stands out, a pressure drop that is too small sends you toward debris in the fuel rail or injector, and a pressure drop that is too large points toward a faulty or fouled injector. Correct what you find and verify the repair.
VVL path if equipped
If this vehicle is equipped with VVL, run the VVL system diagnostic with the scan tool and follow the on-screen routine. If the vehicle is not equipped with VVL, skip this branch. Do not erase the code before running the VVL routine. The scan tool result decides the next move: if VVL is not shown as the cause of the misfire, go on to the ignition and mechanical checks. If VVL is implicated, use the test result to decide the repair path. A rocker arm stuck in Low Lift mode can cause a misfire at higher engine speeds, above approximately 4000 rpm. If the path calls for checking actual oil pressure to the VVL solenoid, key off, remove the solenoid identified by the scan tool, and install Kit, VVL Adapters 2027300090 in its place. Start the engine, let it idle, monitor Dual Oil Pump mode, and let it run long enough for the pump to switch to low pump mode. Compare cylinder head oil pressure to main engine oil pressure. If cylinder head oil pressure is within 5.0 psi of main engine oil pressure, inspect the VVL solenoid screen for debris and check the seals for damage and correct installation. If there is no debris and the seals look good, replace the VVL solenoid, then verify the repair. If the pressure is not within 5.0 psi, diagnose and correct the lack of oil pressure to the VVL solenoid and verify the repair.
Spark, plug, compression, and carbon checks
Before continuing past the injector test, key off, remove the fuel pressure decay tester, and reconnect the fuel lines. Remove the ignition coil from cylinder 4, reconnect the coil connector, disconnect the cylinder 4 injector connector, install a spark tester on the coil, and crank the engine while watching spark. If you have a crisp blue spark, move to the plug and cylinder checks. If spark is not crisp and blue, jump to the coil power and control circuit checks. With spark present, key off and remove the cylinder 4 spark plug. Inspect it for cracks, carbon tracking, oil, coolant or excess fuel fouling, gap out of specification, and a loose or broken electrode. Also look for oil or coolant in the cylinder. If any of those plug problems are present, replace the spark plug. If oil or coolant is present, diagnose and correct the cause, then verify the repair. If the plug and cylinder inspection pass, run a compression test on several cylinders and compare cylinder 4 to known-good cylinders. If compression is not within specification, perform a cylinder leak-down test to determine whether the problem is in the valvetrain or combustion chamber, correct it, and verify the repair. If compression is within specification, swap the cylinder 4 spark plug with a known-good cylinder, or replace it if access makes swapping impractical. Start the engine and watch the misfire monitor. If the misfire moves with the plug, replace that plug and verify the repair. If it goes away after replacement, the repair is complete once verification passes. If the misfire stays on cylinder 4, de-carbon the engine using Mopar Combustion cleaner. After driving and clearing the cleaner from the engine, erase the code, reproduce the captured conditions, and watch the misfire monitor again. If the misfire does not return, the valve and combustion chamber cleaning fixed it. If it does return, inspect the camshaft lobes, rocker arms, and springs for excessive wear or broken parts. Correct any valvetrain issue found and verify the repair. If no issue is found there, continue to the final connector and PCM-side checks.
Coil power and control circuit path
If the spark test failed, move into the electrical side. Keep clear of the fan, belts, pulleys, and loose clothing whenever the engine is operating; serious or fatal injury can result if you get in line with moving parts. Key off, disconnect the ignition coil connector, key on, and actuate the ASD relay with the scan tool. Using a 12-volt test light connected to ground, probe the ASD relay output circuit at the ignition coil connector. If the light does not illuminate brightly, stop all scan tool actuations, repair the open or short to ground between the PDC and the ignition coil connector, and verify the repair. If it does illuminate brightly, check the coil control circuit. With a 12-volt test light connected to a 12 volt source, back probe the coil control circuit at the ignition coil connector. Crank the engine for 5 seconds and watch the light. If it blinks brightly, replace the ignition coil and verify the repair. If the light stays on constantly, check the coil control circuit for a short to ground. Key off, disconnect PCM C2, install Adapter, GPEC Diagnostic 10436 on the engine harness side, and check continuity between ground and the coil control circuit. Do not probe PCM harness connectors directly; that can damage terminals and create a poor terminal-to-pin connection. If continuity to ground is present, repair the short to ground and verify the repair. If the light stayed off constantly during the control test, check the coil control circuit for an open or high resistance. Measure resistance between the ignition coil connector and the GPEC adapter. If resistance is below 3.0 Ohms, continue to the final connector checks. If it is not below 3.0 Ohms, repair the open or high resistance and verify the repair.
Final connection checks and repair verification
At the end of the path, disconnect the PCM connectors, any related in-line connectors if equipped, and the related component connectors. Inspect connector installation, locks, corrosion, water intrusion, weather seals if equipped, bent terminals, overheating from poor contact, pushed-back terminals, spread terminals, and terminal tension. Repair any connector or wiring condition found. Then reconnect everything fully, make sure the locks are engaged, erase the code, and operate the vehicle under the conditions that run and set the monitor. If P0304 does not return, the wiring or poor-connection problem has been corrected. If P0304 does return after those checks, replace and program the PCM, then verify the repair. Keep verification separate from testing: after any repair in this path, run the vehicle under the proper conditions and confirm the misfire counter and code stay gone. The takeaway is simple: prove the cylinder 4 misfire is active, rule out system-level clues first, compare fuel and mechanical results against known-good cylinders, and only then commit to electrical or PCM-side repairs. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0304 diagnosis often works best when the misfire is confirmed active first, then fuel, spark, air, mechanical condition, wiring, and control-side possibilities are narrowed down in order.
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