What this code means
P0304 means the PCM may be detecting an uneven contribution from cylinder 4, often by watching crankshaft speed changes.
What the vehicle may do
- The vehicle may idle rough or shake.
- The MIL may illuminate or flash depending on how severe the misfire is.
- The engine may hesitate, stumble, or feel weak under load.
- Stop/start may be disabled while the code is active if the vehicle is equipped with that system.
Possible fault areas
- Possible ignition issues such as a coil or spark plug concern.
- Possible fuel delivery issues such as an injector or fuel quality concern.
- Possible air or vacuum leak concerns.
- Possible engine mechanical, compression, valvetrain, oiling, VVT, or VVL concerns.
- Possible wiring, connector, or PCM-related concerns.
Diagnostic path
Open and frame the code
P0304 on this 2019 to 2024 Ram 1500DT with the 5.7 HEMI is a cylinder 4 misfire code. In plain terms, the PCM is seeing crankshaft speed changes that point to cylinder 4 not contributing evenly. The truck may run rough, shake at idle, hesitate under load, or flash the MIL if the misfire is severe. Broadly, this can come from possible fuel delivery, ignition, air leak, valvetrain, oiling, VVL, mechanical compression, wiring, or PCM-side issues. Start with the basic system checks, then follow a structured diagnostic approach instead of jumping straight to parts.
Confirm the misfire is active
First, turn the ignition on and use the scan tool to read and record the codes and captured conditions. Reproduce the captured conditions on the road test. While the misfire is happening, note when it shows up, watch the downstream oxygen sensor behavior, and use the misfire monitor to confirm cylinder 4 is the cylinder with excessive misfire counts right now. If it is not active during the test, handle it as an intermittent condition rather than forcing the rest of the chart.
Use the monitor gates correctly
Do not over-read the monitor if the enable conditions are not there. The misfire monitor runs only when its operating gates are met, including 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 after the engine reaches 21°C (69.8°F) when required. The engine speed gate is between approximately 400 rpm and 6200 rpm, and the monitor is disabled during a Decel Fuel Shutoff Event. Treat those as monitor gates, not as repairs.
Check for other active paths before chasing cylinder 4
Next, look at the codes you recorded. If other active or pending codes point to the fuel system, ignition system, air handling, VVT, or VVL, check what those mean first and run the applicable diagnostic path. If none of those are active or pending, continue with the cylinder 4 misfire checks.
Do the broad inspection before component testing
Now do a good mechanical and visual inspection. Look for belt or accessory drive issues that could affect crankshaft speed, sensor mounting problems at the CKP, CMP, or MAP sensors, poor terminal fit or connector issues at the sensors, throttle body, injector, ignition coil, or PCM power and ground circuits. Also check for vacuum leaks, air induction or exhaust restrictions, internal engine problems, moisture on ignition parts, insufficient or poor-quality fuel, and operating conditions like bogging or overload. A misfire code can set from anything that disturbs crankshaft speed, and the misfire may not always be caused by a failed component. If you find a problem here, repair it and verify the repair and confirm the code stays gone. If not, keep going.
Inspect MDS connections and intake leaks
With the ignition off, inspect the MDS solenoid harness connectors, jumper connectors, and terminals. You are looking for poor connector seating, damaged locks, corrosion, water intrusion, seal damage if equipped, bent terminals, heat damage from poor contact, pushed-back terminals, spread terminals, and weak terminal tension. Repair anything you find, then verify the repair and confirm the code stays gone. If the MDS connections are clean, check the engine and intake manifold for vacuum leaks. A vacuum leak can affect one or more cylinders depending on where it is. If a leak is found, repair the intake leak and verify the repair. If not, continue.
Check fuel quality, then compare injector behavior
Next, check fuel quality and fuel condition. Look for water in the fuel, wrong fuel type, old or contaminated fuel, and low fuel. Also check for proper ethanol level in the gasoline. If the fuel is the problem, replace the fuel and, if needed, flush and clean the fuel system, then verify the repair. If fuel quality checks out, move into the injector comparison. Before touching fuel lines or fittings, remember the fuel system stays under constant pressure even with the engine off, so release pressure before testing or servicing. Install Fuel Pressure Decay Tester 8978A at the fuel rail, key on, let pressure reach maximum and stabilize, then record that as the starting pressure. Actuate the fuel injector for cylinder 4 while watching the gauge and record the ending pressure. Repeat the same process on several known-good cylinders, always starting at the same starting pressure. Compare the ending pressures. If cylinder 4 looks about the same as the good cylinders, move on. If cylinder 4 stands out, a pressure drop that is too small points you toward debris in the fuel rail or injector. A pressure drop that is too large points toward a faulty or fouled injector as the issue. After each injector actuation, start the engine to clear fuel from the cylinder, because skipping that can damage the engine.
Check spark at the cylinder 4 coil
After the injector test, turn the ignition off, remove the fuel pressure decay tester, and reconnect the fuel lines before continuing. Remove the ignition coil from the misfiring cylinder, reconnect the coil connector, disconnect that cylinder’s fuel injector connector, and install a spark tester on the coil. Crank the engine and look for a crisp blue spark. If the spark is crisp and blue, continue into plug, compression, and mechanical checks. If spark is not present, skip ahead to the ignition coil feed and control circuit tests.
If spark is good, inspect the plug and cylinder
With good spark confirmed, turn the ignition off and remove the cylinder 4 spark plug using the proper removal process. Inspect the plug for cracks, carbon tracking, oil fouling, coolant fouling, excess fuel, gap out of specification, and a loose or broken electrode. Also look for oil or coolant in the cylinder. If any of those are present, replace the spark plug. If oil or coolant is found in the cylinder, diagnose and repair the reason it got there, then verify the repair and confirm the code stays gone. If the plug and cylinder look clean, continue.
Check compression before swapping parts
Now check engine compression on all cylinders and compare the known-good cylinders to cylinder 4. If cylinder 4 compression is within specification, continue. If it is not, perform a cylinder leak down test to determine whether the cause is in the valvetrain or combustion chamber, repair as needed, and verify the repair. A compression or valvetrain issue usually shows up across operating conditions, and the affected cylinder will normally carry the heavier misfire count.
Use the spark plug as a comparison test
If compression is acceptable, swap the cylinder 4 spark plug with a known-good cylinder that is not misfiring, or replace the plug if access makes swapping impractical. Start the engine and watch the misfire monitor. If the misfire follows the plug to the other cylinder, replace the spark plug. If the plug was replaced and the misfire is gone, the repair is complete. Either way, verify the repair and confirm the code stays gone. If the misfire stays on cylinder 4, continue.
Address possible carbon buildup
The next step is to clean carbon from the valvetrain and combustion chambers using Mopar Combustion Cleaner. After driving the vehicle and clearing the cleaner from the engine, erase the code and test drive under the captured conditions. Watch the misfire monitor again. If the misfire does not return, the cleaning corrected the fault and you still verify the repair. If the misfire returns, keep going. Carbon buildup can show up under different operating conditions and is often more noticeable at cruise or under heavier load.
Check oiling, VVL, and valvetrain hardware
Before going deeper, check for mechanical tolerances out of specification, oil pressure out of specification, low oil level, an oil filter that does not meet OEM specifications, dirty or deteriorated oil, contaminated oil, incorrect viscosity, and aerated oil. If any of that is present, repair the oiling or mechanical condition, including issues such as drain back, sludge, oil pump, pickup tube, or seal problems, then verify the repair. If oiling and mechanical basics are OK, look at the VVL path only if the misfire occurred when engine speed exceeded 5000 rpm and VVL is suspected. One VVL failure pattern can show up only at or above approximately 5000 rpm. Check the solenoid O-rings for missing or damaged seals and make sure the solenoid is not leaking when off before checking rocker arm operation. If VVL issues are found, make the appropriate VVL repair and verify the repair. If not, visually inspect the camshaft lobes, rocker arms, and springs for excessive wear or broken parts. If you find wear or damage there, repair it and verify the repair. If none of those issues are found, move to the final harness and PCM connection checks.
If spark was missing, test coil power and control
If the spark test did not show a crisp blue spark, start at the coil feed. Turn the ignition off, disconnect the ignition coil connector, turn the ignition on, and actuate the ASD relay with the scan tool. With a 12-volt test light connected to ground, probe the ASD Relay (F342) 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 ignition coil connector, then verify the repair. If it does illuminate brightly, test the coil control circuit. Use a 12-volt test light connected to a 12 volt source, back probe the coil control circuit at the ignition coil connector, and crank the engine for 5 seconds while watching the light. If it blinks brightly, replace the ignition coil and verify the repair. If it stays on constantly, check the control circuit for a short to ground. If it stays off constantly, check the control circuit for an open.
Check the coil control circuit for short or open
For a control circuit that stays on constantly, turn the ignition off, disconnect the PCM C2 connector, and install the GPEC Diagnostic 10436 adapter on the engine harness side. Do not probe the PCM harness connectors directly, because that can damage the terminals and create poor terminal-to-pin contact. Check continuity between ground and the coil control circuit at the ignition coil connector. If continuity is present, repair the coil control circuit for a short to ground and verify the repair. If continuity is not present, go to the final harness and PCM connection checks. For a control circuit that stays off constantly, measure resistance in the coil control circuit between the ignition coil connector and the GPEC adapter. If resistance is below 3.0 Ohms, go to the final harness and PCM connection checks. If it is not below 3.0 Ohms, repair the open or high resistance in the coil control circuit and verify the repair.
Final connection check and repair verification
The final step is a complete related harness and connector inspection. Disconnect the PCM connectors, any related in-line connectors if equipped, and the related component connectors. Inspect connector seating, locks, corrosion, water intrusion, seals if equipped, bent terminals, overheated terminals, pushed-back terminals, spread terminals, and terminal tension. Repair anything you find. Then reconnect the PCM, in-line, and component connectors and make sure everything is fully seated and locked. Erase the code, operate or test drive the vehicle under the operating conditions, and read codes again. If P0304 returns at this point, replace and program the PCM, then verify the repair and confirm the code stays gone. If it does not return, the wiring or poor connection problem has been repaired. The takeaway is simple: prove cylinder 4 is actively misfiring, separate fuel, spark, air, mechanical, and circuit causes in order, and keep repair verification separate from testing. For more diagnostic training, visit stepdiagnostics.com.
Final check
A cylinder-specific misfire can have several possible causes, so the diagnosis should confirm the active cylinder and then separate fuel, spark, air, mechanical, and circuit faults in order.
For more guided automotive diagnostics, visit STEP Diagnostics.
