What this code means
P0308 means the PCM may be detecting a misfire pattern on cylinder 8.
What the vehicle may do
- The vehicle may idle rough or shake under load.
- The MIL may illuminate or flash depending on misfire severity.
- Power and drivability can often feel reduced during an active misfire.
Possible fault areas
- Possible ignition concerns, such as a coil or spark plug issue.
- Possible fuel delivery concerns, such as injector or fuel quality issues.
- Possible air or vacuum leak concerns.
- Possible engine mechanical, compression, valvetrain, carbon buildup, oiling, or VCT-related concerns.
- Possible wiring, connector, or PCM-related concerns.
Diagnostic path
What P0308 means on this truck
On this 2019 to 2024 Ram 1500 Classic with the 5.7 HEMI, P0308 means the PCM is seeing a misfire pattern on cylinder 8. The customer may describe a rough idle, a shake under load, reduced power, or a flashing MIL when the misfire is severe. Broadly, this can come from ignition, fuel delivery, air leaks, engine mechanical condition, carbon buildup, VCT operation, wiring, or the PCM, so stay disciplined and prove the fault instead of guessing. The PCM is watching crankshaft speed changes to identify the misfiring cylinder, and the monitor only runs when its operating gates are met. Treat those gates as enable conditions on the screen, not as the repair path.
Confirm the misfire is active
Start with the basic system checks. Turn the ignition on, pull and record all codes and captured operating data, then operate the truck under the same captured conditions. During the drive, note when cylinder 8 starts counting misfires. Watch the downstream O2 sensors while the misfire is happening, and use the misfire monitor to confirm cylinder 8 is the cylinder with excessive counts right now. If the misfire is not active, treat it as intermittent. Also remember that low downstream O2 voltage during the event points you toward a lean or low-fuel condition, while high voltage points toward excess fuel, a lack of spark, or a leaking injector. On a severe flashing-MIL misfire, the PCM may shut off the injector for the affected cylinder to protect the catalyst, so a high-load misfire can appear to stay at idle until the counter resets.
Check other active systems before chasing cylinder 8
Next, look at the other active or pending codes you recorded. If there are fuel system, ignition component, air handling, VVT, or VVL codes present, diagnose those first because they can drive the misfire. If the code list does not point you somewhere else, continue with a broad inspection. If a return point in the path is not clear after one of these no-result checks, pause, recheck the earlier diagnostic path, and avoid guessing.
Do the broad mechanical and installation inspection
Now inspect for the things that can change crankshaft speed or disturb combustion without being a failed cylinder-8 part. Look for belt wear, pulley misalignment or binding, incorrect mounting of the CKP, CMP, or MAP sensor, poor connector or terminal contact at engine sensors, the throttle body, injectors, and coils, corroded PCM power or ground circuits, vacuum leaks, air inlet or exhaust restriction, internal engine issues, moisture on ignition parts, insufficient or poor fuel, bogging on manual-transmission operation, and towing overload. If you find a problem, repair what you found and verify the repair. If nothing turns up and the next move is not obvious, stop and recheck the diagnostic path before moving on.
Check intake leaks and fuel quality
From there, check the engine and intake manifold for vacuum leaks. A vacuum leak can affect one cylinder or several cylinders depending on where the leak is. If the intake is leaking, repair the leak and verify the repair. If it is not leaking, check the fuel itself: water, 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.
Compare injector pressure drop
Next is the cylinder-8 injector check. The fuel system is under 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 and let pressure reach maximum and stabilize; cycling the ignition off and on a couple of times may be needed. Record that as starting pressure. Command the cylinder-8 injector with the scan tool while watching the gauge, then record the ending pressure. Repeat the same process on several known-good cylinders, always starting each actuation from the same starting pressure, and compare the ending pressures. After each injector actuation, start the engine to clear fuel from the cylinder; skipping that can damage the engine. If cylinder 8 stands out, a small pressure drop points toward debris in the rail or injector, while a large drop points toward a faulty or fouled injector. Repair as necessary and verify the repair. If the drops are about the same, move on to spark.
Check spark, plug condition, and compression
Before the spark test, turn the ignition off, remove the fuel pressure decay tester, and reconnect the fuel lines. Remove the cylinder-8 ignition coil, reconnect the coil harness, disconnect the cylinder-8 injector connector, install a spark tester on the coil, and crank the engine. If you have a crisp blue spark, inspect the spark plug and cylinder. Look for cracks, carbon tracking, oil, coolant, excess fuel fouling, gap out of spec, or a loose or broken electrode. Also look for oil or coolant in the cylinder. If the plug has one of those faults, replace the spark plug; if oil or coolant is present, diagnose and repair that cause, then verify the repair. If the plug and cylinder look clean, run a compression test on all cylinders and compare cylinder 8 to known-good cylinders. If compression is not within specification, use a cylinder leak down test to separate valvetrain from combustion-chamber problems, repair as necessary, and verify the repair.
See if the plug fault follows, then address carbon
If compression is within specification, swap the cylinder-8 spark plug with a known-good cylinder that is not misfiring, or replace the plug if access makes swapping difficult. Start the engine and watch the misfire monitor. If the misfire follows the plug, replace the plug. If the plug was replaced and the misfire is gone, the repair is complete pending verification. If the misfire stays on cylinder 8, de-carbon the engine with 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 again. If the misfire does not return, the valve and combustion chamber cleaning corrected the fault. If it does return, keep going.
Check oiling, VCT control, and valvetrain parts
Before going deeper, check oil level, oil pressure concerns, mechanical tolerance concerns, correct oil filter, dirty or deteriorated oil, contaminated oil, incorrect viscosity, and aerated oil. Missing one of these can lead to a bad diagnosis. If you find an oiling or mechanical condition, repair it and verify the repair. If not, check VCT solenoid operation. Reconnect the PCM C2 connector, key on, and measure the VCT solenoid control circuit at the solenoid connector. This is a 12.0 volt PWM circuit, and the voltage should be present and roughly match the selected percentage. If it does not, repair the VCT solenoid driver circuit for an open or short to ground and verify the repair. If the VCT control checks out, visually inspect the camshaft lobes, rocker arms, and springs for excessive wear or broken parts. A worn lobe, rocker, or spring may not always show up on compression, so do not skip the visual check. Repair any problem found and verify the repair.
If spark was missing, test power and coil control
If the earlier spark test did not show a crisp blue spark, move to the coil power and control checks. Keep clear of the fan, pulleys, and belts while the engine is operating. Key off, disconnect the cylinder-8 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 F343 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-probed at the coil connector. Crank the engine for 5 seconds. If the light 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. If it stays off constantly, check the coil control circuit for an open.
Finish coil control circuit checks
For the short-to-ground check, key off, disconnect the PCM C2 connector, and use the GPEC Diagnostic Adapter 10436 on the engine-harness side. Do not probe the PCM harness connectors; that can damage the terminals and create a poor terminal-to-pin connection. 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, continue to the final connection checks. For the open-circuit path, measure resistance in the coil control circuit between the ignition coil connector and the GPEC adapter. If resistance is not below 3.0 Ohms, repair the open or high resistance and verify the repair. If resistance is below 3.0 Ohms, move to the harness and PCM connection inspection.
Final connection check and repair verification
For the final pass, disconnect the PCM connectors, related in-line connectors if equipped, and related component connectors. Inspect the connector bodies and terminals for proper installation, damaged locks, corrosion, water intrusion, weather seal damage, bent terminals, overheated or discolored terminals, terminals pushed back in the cavities, spread terminals, and weak terminal tension. Repair anything you find. Then reconnect the PCM, in-line connectors, and component connectors, making sure everything is fully seated and locked. Erase the codes, operate the vehicle under the monitored and set conditions, and read codes again. If P0308 does not return, the wiring or poor connection problem has been repaired. If it does return after all of these checks and repairs, replace and program the PCM, then verify the repair and confirm the code stays gone. Takeaway: prove the active cylinder-8 misfire first, then work through fuel, spark, compression, oiling, VCT, circuit integrity, and connections in order. For more diagnostic training, visit stepdiagnostics.com.
Final check
A cylinder-specific misfire diagnosis works best when the active condition is confirmed first, then fuel, spark, mechanical condition, control circuits, and connections are checked in order.
For more guided automotive diagnostics, visit STEP Diagnostics.
