P0304 on 2015-2020 Ram 2500: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2015-2020 Ram 2500

For the 2015-2020 Ram 2500, the cause of a P0304 code is entirely dependent on the engine. For owners of the 5.7L or 6.4L HEMI V8, the issue is typically a simple ignition component like a coil or spark plug. However, a persistent P0304, especially with a ticking noise, can be an early warning of the infamous 'Hemi tick' caused by valvetrain (lifter/camshaft) issues. For 6.7L Cummins diesel owners, the diagnosis almost immediately points to a more complex and expensive fuel injector failure. These high-pressure common rail injectors are a known failure point due to operating under extreme pressures and sensitivity to fuel contamination.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Generation note: This range covers the end of the fourth-generation Ram (2010-2018) and the beginning of the fifth-generation (2019-present). While engine options (6.4L HEMI, 6.7L Cummins) are similar, some parts and procedures may vary slightly. The fundamental causes and diagnostic steps for P0304 remain consistent across these years.

Symptoms You May Notice

• Rough or shaking idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing Check Engine Light (indicates a severe misfire)
• Smell of unburned fuel from the exhaust
• Reduced fuel economy
• Occasional heavy vehicle shake on cold start

Most Likely Causes

1. Faulty Ignition Coil (HEMI Engines) 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems are a common failure point on modern engines due to constant exposure to high heat and vibration. Forum posts confirm coils can fail prematurely, even at low mileage on these trucks.
   How to confirm: Swap the ignition coil from cylinder 4 with a cylinder on the opposite bank (e.g., cylinder 1 or 3). Clear the codes and drive the truck. If the code changes to P0301 or P0303, the ignition coil is bad. On HEMI V8s, cylinder 4 is the second cylinder from the front on the passenger (right) side.
   Typical fix: Replace the faulty ignition coil. This is a simple repair held by two 10mm bolts and an electrical connector.
   Est. part cost: $40-$90
2. Worn or Fouled Spark Plugs (HEMI Engines) 🔴 High Probability → Shop Spark Plug HEMI engines use two spark plugs per cylinder (16 total), increasing the chance of a single plug failing or becoming fouled over its service life. They are a routine maintenance item often overlooked.
   How to confirm: After ruling out the ignition coil, remove the two spark plugs from cylinder 4. Inspect them for wear on the electrode, cracks in the porcelain, or heavy carbon/oil deposits. Compare them to a new plug.
   Typical fix: Replace the spark plugs in cylinder 4. It is highly recommended to replace all 16 spark plugs at the same time for even performance. Recommended brands include NGK and Champion.
   Est. part cost: $80-$240 for a full set of 16
3. Faulty Fuel Injector (6.7L Cummins Diesel) 🔴 High Probability → Shop Fuel Injector The high-pressure common rail fuel system operates under extreme pressure (over 20,000 psi), making the Bosch injectors a known wear item. They are highly sensitive to fuel contamination (water, debris) which can cause premature failure.
   How to confirm: Requires a capable scan tool to perform a cylinder contribution test or to 'kill' the injector for cylinder 4 to see if there is a change in engine performance. A professional may also perform an injector return flow test.
   Typical fix: Replace the faulty fuel injector and, critically, the high-pressure connector tube to prevent leaks. The new injector's IQA code must be programmed into the ECM. 🎬 Watch: Step-by-step guide to replacing fuel injectors on a 6.7 Cummins. This is often a job for a professional. Bosch is the OEM supplier.
   Est. part cost: $350-$900 per injector
4. Failed Camshaft/Lifters ('Hemi Tick') 🟡 Medium Probability → Shop Engine Camshaft The 5.7L and 6.4L HEMI engines are susceptible to a failure of the roller lifters, particularly on cylinders with MDS (Multi-Displacement System). The roller can seize, grinding a lobe off the camshaft, which often presents as a single-cylinder misfire accompanied by a rhythmic ticking sound.
   How to confirm: Listen for a distinct ticking sound that increases with engine RPM. If plugs and coils are good, this is the next logical suspect. Final confirmation requires removing the valve cover and intake manifold for visual inspection of the camshaft lobes and lifters.
   Typical fix: Requires extensive engine repair, including replacing the camshaft, all 16 lifters (a common Mopar part number is 5038333AB), head gaskets, and related components. This is a job for a professional.
   Est. part cost: $1500-$2500+

Rare But Worth Checking

• Faulty Fuel Injector (HEMI Engines): → Shop Fuel Injector While less common than coils or plugs on the HEMI, a fuel injector can become clogged or fail electrically, causing a P0304.
• Wiring Harness Issue: The wiring to the ignition coil or fuel injector can become damaged, chafed, or have a loose connection, causing an intermittent misfire. A thorough visual inspection of the harness leading to cylinder 4 is warranted.
• Low Cylinder Compression: Internal mechanical problems like a bad valve, worn piston rings, or a leaking head gasket can cause a loss of compression and a persistent misfire that won't be fixed by ignition or fuel parts. A compression test is needed for diagnosis.
• PCM Software Issue: TSB #18-107-22 REV A indicates that for 2015 5.7L HEMI engines, the PCM may require a software update to address conditions that can trigger misfire codes. Similar TSBs exist for the Cummins engine, making a dealer software check a valid step if no mechanical fault is found.

Diagnosis Steps

1. Read the trouble code with an OBD-II scanner and note any other codes, like P0300 or P0204.
2. Identify your engine (HEMI gas or Cummins diesel) as the diagnostic path differs completely.
3. For HEMI Engines: Locate cylinder 4 (second from front on the passenger/right side). Swap the ignition coil from cylinder 4 with cylinder 1 (front-most on the driver/left side).
4. Clear the codes, run the engine, and re-scan. If the code is now P0301, the ignition coil is faulty and needs replacement.
5. If the code remains P0304, the coil is good. Next, inspect the two spark plugs from cylinder 4 for wear, damage, or fouling. Replace if necessary, but replacing all 16 is recommended.
6. If plugs and coils are confirmed good, inspect the wiring to the coil and injector for damage. Listen for a rhythmic ticking noise that could indicate a lifter/camshaft issue.
7. For Cummins Engines: Use a capable scan tool to perform a 'Cylinder Contribution Test' or 'Injector Kill Test' to confirm cylinder 4 is underperforming.
8. If the scan tool confirms a cylinder 4 issue, the fuel injector is the most likely cause. Before replacement, inspect the wiring harness to the injector for any visible damage.
9. For either engine, if no cause is found, check with a dealer for applicable PCM software updates like TSB 18-107-22 REV A (HEMI) or 18-102-22 REV A (Cummins).
10. A compression test may be needed for either engine to rule out internal mechanical issues if the above steps do not identify the cause.

Parts You'll Likely Need

• Ignition Coil (HEMI Engines) (OEM #68238603AA (supersedes 56029129AF, 68060346AA)) — This is the most common cause of a single-cylinder misfire on the 5.7L and 6.4L HEMI engines and is the easiest part to diagnose via swapping.
• Spark Plugs (HEMI Engines) (OEM #N/A (Varies by year/engine, use VIN to confirm)) — A routine maintenance item that is a frequent and simple cause of a misfire. Recommended brands are NGK and Champion. It's best to replace all 16 at once.
• Fuel Injector (6.7L Cummins) (OEM #Bosch 0445124039 (High Output) or 0445124047 (Standard Output) for 2019+ models. Mopar cross-references: 68444790AA, 68444791AA.) — This is the primary failure point for a P0304 on the Cummins diesel engine. Bosch is the original equipment manufacturer.
• Fuel Injector Connector Tube (6.7L Cummins) (OEM #Mopar 5086874AB / Cummins 3979419) — It is highly recommended to replace this tube whenever an injector is replaced to ensure a proper seal and prevent high-pressure fuel leaks.

Related Codes That Often Appear With This One

• P0300 — If the misfire is severe or starting to affect other cylinders due to a systemic issue like a failing camshaft or major vacuum leak, a P0300 (Random/Multiple Cylinder Misfire) code may also be stored.
• P0204 — This code means 'Injector Circuit/Open - Cylinder 4.' If you see P0304 alongside P0204, it strongly points to a problem with the fuel injector on cylinder 4 or its electrical circuit, rather than a spark or compression issue.

Technical Service Bulletins (TSBs) & Recalls

• Applies to 2015 Ram 2500/3500 with the 5.7L HEMI engine. This TSB involves reprogramming the Powertrain Control Module (PCM) to correct software logic that could falsely trigger a range of misfire codes, including P0304. This should be considered if no physical fault is found.
• Applies to 2020 Ram HD trucks with the 6.7L Cummins. This TSB is a PCM software update to address various performance issues like idle instability and engine stumble. While this revision doesn't list P0304, previous versions did, and it shows that software updates are a known fix for driveability issues on the Cummins engine.

Platform-Specific Known Issues

• HEMI Camshaft/Lifter Failure: A widely documented issue on 5.7L and 6.4L HEMI engines is the failure of the roller lifters. The needle bearings can fail, causing the roller to stop spinning and grind against the camshaft lobe, destroying both. This often starts as a persistent ticking sound ('Hemi tick') and can throw a single-cylinder misfire code like P0304.

Mechanic-Grade Diagnostic Values

• 6.7L Cummins Fuel Injector Resistance — expected: Less than 1.0 Ohm, typically ~0.5 Ohms. Failure: A reading of 0 ohms (short circuit) or OL/infinite resistance (open circuit) indicates a failed injector solenoid.
• 5.7L/6.4L HEMI Ignition Coil Primary Resistance — expected: 0.49 - 0.57 Ohms. Failure: A reading significantly outside this range indicates a fault in the primary winding.
• 5.7L/6.4L HEMI Ignition Coil Secondary Resistance — expected: 9.5 - 11.1 kOhms. Failure: A reading outside this range indicates a fault in the secondary winding.
• 6.7L Cummins Injector Return Flow (Running Test) — expected: Maximum 210 mL total for all injectors combined over a 30-second period.. Failure: Flow greater than 210 mL indicates excessive leakage from one or more injectors. Any single injector contributing more than 40 mL is considered faulty.
• 6.7L Cummins Injector Return Flow (No-Start Cranking Test) — expected: Maximum 40 mL total in 10 seconds of cranking at ~200 RPM.. Failure: Flow greater than 40 mL indicates excessive leakage preventing the rail from building enough pressure to start.

Hidden / Shadow Codes Worth Checking

• Mode 6, Test ID $A6 (example): Mode 6 data provides access to non-continuous monitor test results, including misfire counts for each cylinder over the current and last ten driving cycles. This allows a technician to confirm which cylinder is misfiring and how frequently, sometimes before the count is high enough to set a persistent P0304 code. (see via A professional-grade scan tool with Mode 6 data capability is required.)

Scan Tool Commands That Help

• Chrysler wiTECH or capable aftermarket scanner: Cylinder Contribution Test / Injector Kill Test — For Cummins engines, this command deactivates one injector at a time to observe the change in engine RPM and smoothness. If killing cylinder 4 causes no change, it confirms that cylinder was not contributing, pointing strongly to a bad injector.
• Chrysler wiTECH or capable aftermarket scanner: Fuel Pressure Override Test (Cummins) — This is a specific service function used to perform the injector return flow test. The tool commands the engine to 1200 RPM and rail pressure to 26,107 PSI (1800 bar) to stress the system and accurately measure injector leakage under high pressure.
• Chrysler wiTECH or capable aftermarket scanner: Injector Quantity Adjustment (IQA) Programming — This is a required step after replacing a fuel injector on a 6.7L Cummins. Each new injector has a unique flow-rate code that must be programmed into the ECM to ensure correct fuel trimming and smooth engine operation.
• Chrysler wiTECH: PCM Reprogram/Flash — If a Technical Service Bulletin (TSB) exists for misfire issues on a specific year/engine, this function is used by the dealer to update the PCM software, which may resolve the condition without replacing any parts.

Wiring & Ground Locations

• Engine Block to Firewall Ground Strap — Typically runs from the back of a cylinder head to the vehicle's firewall.. A poor or broken ground here can cause a host of electrical issues, including weak spark or erratic sensor readings that could lead to a misfire code.
• Battery Negative to Frame/Engine Ground — The main ground cable from the negative battery terminal connects to the vehicle body/frame and often directly to the engine block near the starter.. This is the primary ground for the entire electrical system. Corrosion or a loose connection can affect the performance of ignition coils and fuel injectors, potentially causing misfires.

Real Owner Repair Stories

• DodgeForum.com user (2003 Dodge Ram 4.7L (similar HEMI architecture)) — P0304 - Cylinder 4 Misfire
  ❌ Tried (didn't work) Replaced spark plugs, Replaced fuel injector, Replaced ignition coil, Verified compression was good (150 psi)
  ✅ What actually fixed it Upon removing the valve cover, a rocker arm had fallen off. The final repair was replacing one lifter and one roller rocker, which resolved the misfire.
• Reddit user r/ram_trucks (Unknown year Ram truck) — P0304 and P0300, Start/Stop system unavailable
  ❌ Tried (didn't work) Replaced all spark plugs, Swapped ignition coils between cylinders
  ✅ What actually fixed it The user was advised to take the vehicle to the dealer under warranty, as the persistent P0304 after basic ignition parts were ruled out strongly suggests a cam/lifter issue, even if no ticking noise is present.
• 5thGenRams Forum user (2020 Ram with 5778 miles) — P0308 (Cylinder 8 Misfire - same diagnostic principle)
  ❌ Tried (didn't work) PCM software update (lasted one day), Replaced spark plug and ignition coil (lasted one day), Replaced coil, plug, and injector (lasted several months but problem returned)
  ✅ What actually fixed it The issue was persistent and recurring. After multiple failed attempts with ignition and fuel parts, the dealer's next step was to investigate internal mechanical cam/valve issues, highlighting how P030x can be a symptom of a deeper problem.

"I Checked Everything" — The Actual Cause

• In cases where ignition parts (plugs, coils) and fuel parts (injectors) are replaced and compression tests come back normal, a persistent P0304 on a HEMI engine is often caused by a mechanical valvetrain failure. A user on DodgeForum confirmed that after replacing all common parts for a P0304, the actual cause was a fallen rocker arm and a bad lifter, which would not be found with a smoke test or basic electrical checks.

OEM Part Supersession History

• 56029129AF, 68060346AA → 68238603AA — Standard part evolution for improved reliability or manufacturing changes.

Model Year Variations Within This Range

• 2019-2020: For the 6.4L HEMI V8, 2019 marked the replacement of the 6-speed 66RFE automatic transmission with the ZF 8-speed 8HP75 automatic. This provides significantly different gearing and towing performance compared to the 2015-2018 models.