What this code means
P0308 means the PCM may be detecting a misfire contribution problem on cylinder 8.
What the vehicle may do
- The vehicle may idle rough or shake.
- The engine may stumble under load or during certain driving conditions.
- The MIL may illuminate or flash depending on misfire severity.
- Stop/start may be disabled while the fault is active, if equipped.
Possible fault areas
- Possible ignition faults may be involved.
- Fuel delivery, injector condition, fuel quality, or fuel level concerns can be possible contributors.
- Vacuum leaks or air handling issues may affect the cylinder contribution.
- Engine mechanical, compression, oiling, valvetrain, or VVL concerns may be involved.
- Wiring, connector, coil control, or PCM control issues can be possible fault areas.
Diagnostic path
Open and frame the fault
On this Ram 1500 with the 5.7 HEMI, P0308 means the PCM is seeing a cylinder 8 misfire pattern. In plain terms, crankshaft speed variation is showing that cylinder 8 is not contributing evenly. The truck may idle rough, stumble under load, flash the MIL if the misfire is severe, and stop/start may be disabled while the fault is active. Broadly, this kind of fault can come from ignition, fuel delivery or fuel quality, intake leakage, engine mechanical or valvetrain problems, or the coil control wiring and PCM side of the circuit. Start with the basic system checks, then reproduce the operating conditions instead of guessing from the code alone.
Monitor gates before testing
Use the running conditions as monitor gates. The misfire monitor is intended to run with 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 if the start temperature is below threshold, when engine temperature reaches 21°C (69.8°F). Engine speed is between approximately 400 rpm and 6200 rpm, and the misfire monitor is disabled during a Decel Fuel Shutoff Event. Once those gates make sense, move into the cylinder 8 path.
Confirm the active cylinder and check companion faults
Turn the ignition on, read and record all codes and captured data, then drive the truck under the captured conditions from when P0308 set. During the road test, note exactly when the misfire shows up, watch the downstream O2 sensors while the misfire is happening, and use the misfire monitor to confirm cylinder 8 is the cylinder showing excessive misfires now. If cylinder 8 is not actively showing excessive misfires, treat it as an intermittent condition and do not force the diagnosis. If other active or pending faults are present in the fuel, ignition, air handling, VVT, or VVL areas, handle what those codes are telling you first. If there are no related active or pending faults, continue with the base misfire checks.
Base inspection, intake leaks, and fuel quality
Now do a grounded visual and mechanical inspection. Look for accessory drive problems like a worn belt or pulley misalignment and binding. Check that the crank, cam, and MAP sensors are mounted correctly. Inspect connector and terminal condition at the crank sensor, cam sensor, MAP sensor, throttle body, injector, ignition coil, PCM powers and grounds, and other related connections. Also check for vacuum leaks, air induction or exhaust restriction, moisture on ignition components, poor or insufficient fuel, low quality fuel, internal engine concerns, manual transmission bog, or towing overload. If you find a problem, repair it and verify the repair. If not, check the engine and intake manifold for vacuum leaks. Repair any intake leak and verify. If the intake system is sealed, check the fuel for water, correct type, age or contamination, and low fuel. Also check for proper ethanol level in the gasoline. If the fuel is wrong or contaminated, replace the fuel and, if needed, flush and clean the fuel system, then verify the repair.
Compare cylinder 8 injector fuel delivery
Before opening the fuel system, remember it can hold constant pressure even with the engine off, so release fuel pressure before testing or servicing any hose, fitting, or line. Install Fuel Pressure Decay Tester 8978A at the fuel rail. Key on, let fuel pressure reach maximum and stabilize, and record that as the starting pressure. Actuate the cylinder 8 injector with the scan tool while watching the gauge, then record the ending pressure. After each injector actuation, start the engine to clear fuel from the cylinder; skipping that can damage the engine. Repeat the same test on several known-good cylinders, starting each injector actuation from the same starting pressure. The ending pressures should be about the same across the tested injectors. If cylinder 8 stands out and the pressure drop is too small, check for debris in the fuel rail or injector and repair as needed. If the pressure drop is too large, suspect a faulty or fouled injector as the issue, then verify the repair.
Check spark, plug condition, and cylinder condition
If injector pressure drop compares normally, remove the fuel pressure tester and reconnect the fuel lines before going farther. Remove the cylinder 8 ignition coil, reconnect the coil harness connector, disconnect the cylinder 8 injector connector, and install a spark tester on the coil. While cranking, look for a crisp blue spark. If spark is missing, move to the coil power and control circuit checks. If spark is good, turn the ignition off and inspect the cylinder 8 spark plug. Look for cracks, carbon tracking, oil fouling, coolant fouling, excess fuel fouling, gap out of specification, or a loose or broken electrode. Also look for oil or coolant in the cylinder. If those plug conditions are present, replace the spark plug. If oil or coolant is in the cylinder, diagnose and repair the cause, then verify the repair.
Mechanical checks after spark and injector checks pass
If the plug and cylinder inspection do not find the cause, run a compression test on all cylinders and compare the known-good cylinders to cylinder 8. If cylinder 8 compression is not within specification, use a cylinder leak down test to separate valvetrain from combustion chamber causes, repair as needed, and verify. If compression is within specification, swap the cylinder 8 spark plug with a known-good cylinder that was not misfiring, or replace the plug if access makes swapping impractical. Start the engine and watch the misfire monitor. If the misfire moves to the previously good cylinder, replace the spark plug. If the plug was replaced and the misfire is gone, the repair is complete; verify it. If the misfire stays on cylinder 8, de-carbon the valves and combustion chambers using Mopar Combustion cleaner. After driving and clearing the cleaner from the engine, erase codes, test drive under the captured conditions, and watch the misfire monitor. If the misfire does not return, the carbon cleaning corrected the fault. If it returns, continue.
Oil, VVL, and valvetrain checks
Next, check engine oil level and condition along with related mechanical conditions. Look for mechanical tolerances out of specification, oil pressure out of specification, low oil level, an oil filter that does not meet OEM specifications, deteriorated or dirty oil, contaminated oil, incorrect oil viscosity, or aerated oil. Repair any oiling or mechanical condition found and verify. If those checks pass, consider VVL only when the symptom fits. A rocker arm stuck in low lift can cause a misfire at or above approximately 5000 rpm and may not set a VVL system fault. If the misfire occurred when engine speed exceeded 5000 rpm and VVL is suspected, check VVL operation, including solenoid O-rings for missing or damaged seals and leakage with the solenoid off. If a VVL issue is found, make the appropriate VVL repair and verify. If VVL checks pass, visually inspect the camshaft lobes, rocker arms, and springs for excessive wear or broken parts. Repair any fault found and verify. If no issue is found there, move to the connection and control circuit side.
Ignition coil power and control circuit path
If the earlier spark test did not show a crisp blue spark, start at the ignition coil feed. Keep clear of the fan, belts, and pulleys when the engine is operating or cranking, and do not wear loose clothing. 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 coil connector. The light should illuminate brightly. If it does not, stop scan tool actuations and repair the open or short to ground between the PDC and the ignition coil connector, then verify. If the feed is good, check the coil control circuit. 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 the coil control circuit for an open or high resistance.
Coil control circuit isolation
For the short-to-ground check, turn the ignition off, disconnect PCM C2, and connect GPEC Diagnostic 10436 to the engine harness side of the PCM C2 connector. Do not probe the PCM harness connectors directly; use the adapter to avoid terminal damage. Check continuity between ground and the coil control circuit at the ignition coil connector. If continuity is present, repair the coil control circuit short to ground and verify. If continuity is not present, move to the final connection checks. For the open-circuit check, measure resistance of the coil control circuit between the ignition coil connector and the GPEC adapter. Resistance should be below 3.0 Ohms. If it is not below 3.0 Ohms, repair the open or high resistance in the coil control circuit and verify. If it is below 3.0 Ohms, continue to the final connection checks.
Final connection check and repair verification
At the end of the path, inspect the related PCM, inline, and component connections. Disconnect the PCM connectors, any related inline connectors, and related component connectors. Check for connector installation problems, damaged locks, corrosion, water intrusion, damaged seals, bent terminals, overheating from poor contact, terminals pushed back in the cavity, spread terminals, and poor terminal tension. Repair any condition found, then reconnect everything fully seated with the locks engaged. Erase the codes, test drive or operate the vehicle under the monitor and captured conditions, and read codes again. If P0308 does not return, the wiring or poor connection issue has been repaired. If it does return after this path, replace and program the PCM, then verify the repair and confirm the code stays gone.
Takeaway
The key with P0308 is to prove cylinder 8 is misfiring now, then work in order: related faults, inspections, intake and fuel, injector comparison, spark, plug and cylinder condition, compression, carbon and oiling concerns, VVL and valvetrain checks, and only then the coil feed, coil control wiring, connections, and PCM decision. Keep verification separate after every repair so you know what actually fixed it. For more diagnostic training, visit stepdiagnostics.com.
Final check
A structured diagnosis may prevent unnecessary parts replacement by proving whether cylinder 8 is losing spark, fuel control, compression, air sealing, or electrical control.
For more guided automotive diagnostics, visit STEP Diagnostics.
