P0304: P0304 Diagnostic Guide

What this code means

P0304 generally means the PCM has detected a possible misfire pattern on cylinder 4.

What the vehicle may do

The vehicle may idle rough or shake.
The vehicle may 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 on equipped vehicles.

Possible fault areas

Ignition components may be involved.
Fuel delivery or fuel quality may be involved.
Air leaks or air handling faults may be involved.
Valve timing, valve lift, or valvetrain issues may be involved.
Wiring, connector, PCM power, or ground issues may be involved.
Cylinder compression or internal mechanical issues may be involved.

Diagnostic path

Opening and code context

On this Ram 1500 Classic with the 5.7 HEMI, P0304 means the PCM has detected a misfire pattern on cylinder 4. The driver may notice a rough idle, hesitation, loss of power, a flashing or steady MIL, and if the truck has stop/start, that feature can be disabled while the code is active. Broadly, this kind of fault may come from ignition, fuel delivery, air leaks, valve timing or lift control, wiring, connector issues, or a mechanical cylinder problem. The PCM is watching crankshaft speed variation between cylinders, and when that variation exceeds its calibrated limit for the current engine speed and load, it flags the misfire. Before judging the repair, make sure the monitor gates can actually be met: 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), and if start engine temperature is below threshold, the monitor enables when engine temperature reaches 21°C (69.8°F). Engine speed also has to be between approximately 400 rpm and 6200 rpm. Treat those as enable conditions, not as the diagnosis by themselves. Also remember that on a severe misfire, the PCM can shut off the affected cylinder’s fuel injector in cycles to help protect the catalytic converter.

Confirm the misfire is active

Start by confirming whether this is an active cylinder 4 problem. Turn the ignition on, pull and record all codes and the captured operating data, then operate the vehicle under those captured conditions. During the drive, pay attention to when the misfire shows up, watch the downstream oxygen sensor activity while it is happening, and use the scan tool misfire monitor to confirm which cylinders are being affected. If cylinder 4 is showing excessive misfires right now, continue the diagnostic path. If it is not acting up, treat it as an intermittent condition instead of forcing the rest of the test.

Check for related active or pending faults first

Next, look at the codes you recorded. If there are active or pending faults in the fuel system, ignition system, air handling system, variable valve timing, or variable valve lift, handle those first because they can drive the misfire. If that check is clean but the next branch in your workflow is not clear, pause, recheck the earlier diagnostic path, and do not guess your way into the next test.

Do the broad mechanical and visual inspection

Now make a careful inspection before getting deep into pinpoint testing. Look for accessory drive issues like a worn belt or pulleys that are misaligned, binding, or loading the engine. Check that the crank, cam, and MAP sensors are mounted correctly, and inspect connector and terminal condition at the key engine controls, including the throttle body, injectors, coils, and sensors. Also look for PCM power or ground corrosion, vacuum leaks, air intake or exhaust restriction, internal engine concerns, moisture on ignition parts, fuel supply or fuel quality problems, and operating conditions like lugging or overload. Repair anything obvious before moving on. If nothing is found and the continuation point is unclear, stop and recheck the path rather than skipping ahead blindly.

Check intake leaks and fuel quality

After the broad inspection, check the engine and intake manifold for vacuum leaks. If the intake system is leaking, repair the leak and verify the repair. If the intake checks good, move to fuel quality. Check for water in the fuel, the correct type of fuel, old or contaminated fuel, and low fuel. Also check the ethanol level in the gasoline. If fuel is the problem, replace the fuel and, if needed, flush and clean the fuel system before verification.

Compare injector pressure drop on cylinder 4

For the injector check, relieve fuel system pressure first; this system can hold pressure even with the engine off, and opening it without releasing pressure can cause serious or fatal injury. Install Fuel Pressure Decay Tester 8978A at the fuel rail. Key on, let pressure build to its maximum and stabilize, then record that as the starting pressure. Actuate the cylinder 4 fuel injector with the scan tool while watching the gauge, and record the ending pressure. After each injector actuation, start the engine to clear fuel from the cylinder; skipping that step can damage the engine. Repeat the same process on several known good cylinders, always starting at the same starting pressure, then compare the ending pressures. If cylinder 4 matches the others, continue to ignition. If cylinder 4 stands out, a small pressure drop points you toward debris in the fuel rail or injector, while a large pressure drop points toward a faulty or fouled injector condition. Repair as needed and verify the repair.

Check spark, plug condition, compression, and carbon

Once the injector comparison passes, remove the fuel pressure tester and reconnect the fuel lines. Pull the ignition coil from cylinder 4, reconnect the coil connector, disconnect that cylinder’s injector connector, install a spark tester, and crank the engine. If you have a crisp blue spark, continue with the plug and cylinder checks. If you do not, move into the ignition circuit tests. With spark present, remove the spark plug and inspect it for cracks, carbon tracking, oil, coolant, excess fuel fouling, an out-of-spec gap, or a loose or broken electrode. Also look for oil or coolant in the cylinder. If any of that is present, replace the spark plug as needed, and if oil or coolant is in the cylinder, diagnose and repair the reason it got there. If the plug and cylinder look good, run a compression test on all cylinders and compare cylinder 4 against known good cylinders. If compression is not within specification, use a leak down test to separate valvetrain from combustion chamber causes, then repair as needed. If compression is good, 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 follows the plug, or goes away after replacement, the plug was the fault path. If it does not follow the plug, de-carbon the valves and combustion chambers using Mopar Combustion cleaner, drive the cleaner out, clear the code, then reproduce the captured conditions and watch for the misfire to return.

Check oiling, VCT control, and valvetrain hardware

If the misfire returns after de-carboning, check engine oil level and condition along with mechanical and oiling concerns. Look for mechanical tolerances out of specification, oil pressure out of specification, low oil level, the wrong oil filter, dirty or deteriorated oil, contaminated oil, wrong viscosity, or aerated oil. If any of those are present, repair the oiling or mechanical problem, such as drain-back trouble, sludge, oil pump damage, or pickup tube and seal issues, then verify. If the oiling and mechanical checks pass, check VCT solenoid operation. Reconnect the PCM C2 connector, turn the ignition on, and measure the VCT Solenoid K442 control circuit at the solenoid connector. This is a 12.0 volt PWM circuit, so voltage should be present and should match approximately to the percentage selected. If it does not, repair the VCT solenoid driver circuit for an open or short to ground. If it does, visually inspect the camshaft lobes, rocker arms, and springs for excessive wear or broken parts. Repair any issues found, then verify.

If spark is missing, test the coil feed and control circuit

If the earlier spark test did not show a crisp blue spark, work the coil circuit path. Keep clear of the fan, belts, and pulleys when the engine is operating, and avoid loose clothing around the front of the engine. Turn the ignition off, disconnect the ignition coil connector, turn the ignition on, and actuate the ASD relay with the scan tool. Using a 12-volt test light connected to ground, probe the ASD Relay F343 output circuit at the coil connector. A bright light means the feed side is present and you can move to coil control. No bright light means stop scan tool actuations and repair the open or short to ground between the PDC and ignition coil connector, then verify. For the coil control check, use a 12-volt test light connected to a 12 volt source, back probe the coil control circuit at the coil connector, and crank the engine for 5 seconds. If the light blinks brightly, replace the ignition coil and verify. If the light stays on constantly, check the coil control circuit for a short to ground. If the light stays off constantly, check that circuit for an open. When working at the PCM side, do not probe the PCM harness connectors; use the GPEC diagnostic adapter because probing those terminals can damage them. For the short-to-ground check, disconnect PCM C2, connect the adapter to the engine harness side, and check continuity between ground and the coil control circuit. Continuity means repair the short to ground. No continuity moves you to the final harness and PCM connection check. For the open check, measure resistance between the ignition coil connector and the GPEC adapter. If resistance is below 3.0 Ohms, move to the final connector check. If it is not below 3.0 Ohms, repair the open or high resistance in the coil control circuit and verify.

Final connector check and verification

At the end of the path, inspect the related PCM, inline, and component harness connections. Disconnect the PCM connectors, related inline connectors if equipped, and related component connectors. Check connector installation, locks, corrosion, water intrusion, weather seals, bent terminals, heat damage from poor connection, pushed-back terminals, spread terminals, and terminal tension. Repair anything found, then reconnect every connector fully and make sure the locks are engaged. Clear the code with the scan tool, operate or test drive the truck under the conditions that allow the monitor to run, and read codes again. If P0304 returns after that complete connector and circuit path, replace and program the PCM, then verify the repair. If it does not return, the wiring or poor connection issue has been corrected. The takeaway is simple: prove the cylinder 4 misfire is active, rule out related system faults, then work fuel, spark, compression, valvetrain, oiling, circuits, and connectors in order. For more diagnostic training, visit stepdiagnostics.com.