P0304 on 2015-2020 Ram 3500: Cylinder 4 Misfire Causes & Fixes (Hemi & Cummins)

What's Unique About the 2015-2020 Ram 3500

The diagnostic path for P0304 on a Ram 3500 is entirely dependent on the engine. For the 6.4L Hemi V8 (gasoline), diagnosis follows the traditional path of checking spark plugs and ignition coils. However, a persistent P0304, especially with a ticking sound, can be an early warning of the well-documented 'Hemi Tick' caused by a failing valvetrain lifter grinding down the camshaft. For the 6.7L Cummins I6 (diesel), which has no spark plugs, the code almost always points to a problem in the high-pressure fuel system. The most common cause is a faulty fuel injector, a known wear item on these engines due to extreme operating pressures (over 26,000 PSI) and sensitivity to fuel contamination. Additionally, several NHTSA TSBs confirm that on some Cummins models, this code can appear in a cluster with others, indicating a PCM software logic issue that requires a reflash at the dealership, not a hardware replacement.

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 (2015-2018) and the beginning of the fifth generation (2019-2020). While the P0304 code's meaning is the same, specific parts and PCM software may differ. For the 6.7L Cummins, 2019-2020 models were subject to a recall (Y78 / NHTSA 21V880) for a failure-prone Bosch CP4.2 high-pressure fuel pump, which could cause symptoms leading to this code. Injector part numbers also differ between the 4th and 5th generation trucks.

Symptoms You May Notice

• Rough or shaking idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• Flashing or solid Check Engine Light
• Smell of unburned fuel from the exhaust
• Poor fuel economy
• Engine ticking noise that increases with RPM (Hemi V8)

Most Likely Causes

1. Faulty Ignition Coil (6.4L Hemi Only) 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems are a common failure point on modern gasoline engines due to constant exposure to high heat and vibration. This is the most frequent cause for a single-cylinder misfire on the Hemi.
   How to confirm: Swap the ignition coil from cylinder #4 with another cylinder (e.g., cylinder #2 on the same passenger side bank). Clear the codes and drive. If the code changes to P0302, the coil is bad.
   Typical fix: Replace the faulty ignition coil. It is a simple repair involving one 10mm bolt and an electrical connector.
   Est. part cost: $45-$160
2. Faulty Fuel Injector (6.7L Cummins Only) 🔴 High Probability → Shop Fuel Injector High-pressure common rail diesel injectors operate under extreme stress (over 26,000 PSI) and are a known wear item sensitive to fuel quality. They are the most common cause of a single-cylinder misfire on the Cummins engine.
   How to confirm: A professional scan tool like the dealer's wiTECH can run a cylinder contribution test to isolate the misfire. An 'injector return flow test' is the definitive method; high return flow from the cylinder 4 injector indicates it has failed internally. A DIY method is to swap the #4 injector with another cylinder, but this is a complex job.
   Typical fix: Replace the faulty fuel injector. This is an advanced repair that requires extreme cleanliness, replacing high-pressure lines, and programming the new injector's calibration code (IQA code) into the ECM. It is recommended to replace the associated high-pressure connector tube at the same time.
   Est. part cost: $450-$700 for one OEM Bosch injector
3. Worn or Fouled Spark Plugs (6.4L Hemi Only) 🟡 Medium Probability → Shop Spark Plug Spark plugs are a regular maintenance item. The 6.4L Hemi uses two plugs per cylinder (16 total), increasing the chance of a failure over time.
   How to confirm: After confirming the ignition coil is good, remove the two spark plugs from cylinder 4. Inspect for wear (rounded electrode), carbon fouling (black soot), oil fouling, or damage to the ceramic insulator.
   Typical fix: Replace the spark plugs. It's recommended to replace all 16 plugs if they are near their service interval to ensure consistent performance.
   Est. part cost: $80-$150 for a full set of 16
4. Camshaft/Lifter Failure (6.4L Hemi Only) 🟡 Medium Probability → Shop Engine Camshaft A widely documented issue on Hemi engines is the failure of roller lifters, often called 'Hemi Tick'. The lifter's needle bearings can fail, causing the roller to seize and grind down the camshaft lobe, resulting in a persistent misfire. This can be exacerbated by extended idle times which may lead to poor lubrication of the upper valvetrain.
   How to confirm: A persistent, rhythmic ticking noise that increases with engine RPM is a strong indicator. A compression test on cylinder 4 will show low readings. Visual inspection requires removing the valve cover, and confirming a damaged cam lobe requires removing the intake manifold and lifters.
   Typical fix: This is a major, expensive repair requiring replacement of the camshaft, all 16 lifters, head gaskets, and related components. Some owners opt for aftermarket camshafts and non-MDS lifters to prevent recurrence.
   Est. part cost: $1000-$2500
5. Faulty Fuel Injector (6.4L Hemi Only) ⚪ Low Probability → Shop Fuel Injector
   How to confirm: If the coil and plugs are good, swap the fuel injector from cylinder #4 with another cylinder (e.g., cylinder #2). If the misfire code moves to P0302, the injector is the cause. You can also listen to the injector with a mechanic's stethoscope; it should have a consistent, rapid clicking sound.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $50-$120

Rare But Worth Checking

• PCM Software Issue (6.7L Cummins): NHTSA TSBs #18-008-24 and #18-007-24 show that P0304 can appear with a cluster of other codes like P0299 (Turbo Underboost) and P20E8 (Reductant Pressure). The official fix in these cases is not hardware replacement, but reprogramming the PCM with updated software to correct faulty logic.
• Loss of Compression (Both Engines): Mechanical issues like a burnt valve, bad piston rings, a leaking head gasket, or a damaged pushrod/rocker arm can cause a misfire. A compression test followed by a cylinder leak-down test is needed to diagnose this after ruling out ignition and fuel issues.
• Injector Wiring Harness (6.7L Cummins): The injector wiring on the 6.7L is routed under the valve cover, where it is exposed to high heat and oil. Over time, the wiring or connectors can become brittle and fail, causing an open circuit to a single injector. This would typically be accompanied by code P0204.

Diagnosis Steps

1. First, identify the engine in your truck: 6.4L Hemi V8 or 6.7L Cummins Diesel. The diagnostic paths are completely different.
2. For 6.4L Hemi V8:
3. Locate Cylinder #4: It is the second cylinder from the front on the passenger side (right side of the engine).
4. Swap the ignition coil from cylinder #4 with the coil from cylinder #2 (the front cylinder on the same side).
5. Clear the codes and start the engine. Let it idle and/or drive it briefly. If the code returns as P0302, the ignition coil is faulty and must be replaced.
6. If the code remains P0304, the coil is good. Next, swap the two spark plugs from cylinder #4 with those from cylinder #2. If the code moves to P0302, the spark plugs were the issue.
7. If the code still remains P0304, listen carefully for a rhythmic ticking sound from the passenger side valve cover area. If a 'Hemi tick' is present, suspect a lifter/camshaft issue.
8. If no tick is present, the problem could be the fuel injector. Listen to the injector with a mechanic's stethoscope; it should have a consistent clicking sound. Consider swapping the injector with another cylinder if you have the tools and experience.
9. If all else fails, perform a compression test on cylinder #4 to check for mechanical engine problems. A low reading points towards issues like a bad valve, piston rings, or the aforementioned lifter/cam failure.
10. For 6.7L Cummins I6 Diesel:
11. Locate Cylinder #4: It is the fourth cylinder back from the front of the engine. The firing order is 1-5-3-6-2-4.
12. Check for applicable TSBs, especially if multiple unrelated codes are present with P0304. Contact a dealer with your VIN to see if a PCM software update (like TSB #18-008-24) is available.
13. Professional diagnosis is highly recommended. A shop will use a scan tool to perform a 'cylinder contribution test' or 'power balance test' to confirm which cylinder is weak.
14. The definitive test is an 'injector return flow test' to measure how much fuel each injector is bypassing. An injector with significantly higher return flow than the others is faulty and must be replaced.
15. If injector tests are inconclusive, check the injector wiring harness under the valve cover for damage. Finally, perform a compression test on cylinder #4.

Parts You'll Likely Need

• Ignition Coil (6.4L Hemi) (OEM #56029129AF (superseded by 68238603AA)) — This is the most common cause of a single-cylinder misfire on the Hemi V8 engine.
  Trusted brands: Mopar, NGK, Delphi, Denso
  OEM price range: $70-$110
  Aftermarket price range: $45-$80
• Fuel Injector (6.7L Cummins) (OEM #R8210105AA (Mopar Reman for 2013-2018 Pickups), 4994925 (Bosch New for 2013-2018 Pickups)) — The most common cause of a single-cylinder misfire on the Cummins diesel engine.
  Trusted brands: Bosch (OEM), Mopar (Remanufactured by Bosch)
  OEM price range: $450-$700
  Aftermarket price range: $300-$500 (Reman)
• Spark Plugs (6.4L Hemi) (OEM #SP143877AA) — A regular maintenance item and a common cause of misfires. The Hemi V8 uses two per cylinder.
  Trusted brands: Mopar, NGK, Champion
  OEM price range: $8-$15 per plug
  Aftermarket price range: $5-$10 per plug
• Camshaft and Lifter Kit (6.4L Hemi) — Required for the 'Hemi Tick' repair, which is a known and moderately common major failure on this engine.
  Trusted brands: Mopar, Texas Speed, Comp Cams
  OEM price range: $1200+
  Aftermarket price range: $800-$2000

Related Codes That Often Appear With This One

• P0300 — This indicates a 'Random/Multiple Cylinder Misfire.' If the problem causing P0304 (like a failing lifter or fuel delivery issue) starts to affect other cylinders, P0300 may also be stored.
• P0204 — This code means 'Injector Circuit/Open - Cylinder 4.' If seen with P0304, it strongly points to a problem with the fuel injector on cylinder 4 or its electrical circuit, not a spark or compression issue.
• P0299 / P20E8 / P0607 — On Cummins models, seeing P0304 with codes like P0299 (Turbocharger Underboost), P20E8 (Reductant Pressure Too Low), or P0607 (ECU Internal Performance) may indicate a broader issue that can be resolved with a PCM software update, as noted in TSBs #18-008-24 and #18-007-24.

Technical Service Bulletins (TSBs) & Recalls

• Bulletin #1800824: Mentions P0304-00 - Cylinder 4 Misfire along with other engine and performance codes for 2020 models.
• Bulletin #1800724: Lists P0304-00 - Cylinder 4 Misfire among other codes for 2019 models, suggesting a potential software-related fix.
• Bulletin #1806623: Cites P0304-00 - Cylinder 4 Misfire for 2019 models in relation to power train issues.
• Bulletin #1810122REVA: Includes P0304-00 - Cylinder 4 Misfire for 2019 models as part of a list of diagnostic codes related to the electrical system.

Platform-Specific Known Issues

• 6.4L Hemi Camshaft/Lifter Failure: → Shop Engine Camshaft A persistent P0304, especially when accompanied by a rhythmic ticking sound ('Hemi Tick'), is a strong indicator of a failed roller lifter and subsequent damage to the camshaft. This is a well-documented and expensive mechanical failure for this engine.
• 6.7L Cummins Software Updates: As per TSBs #1800824, #1800724, #1806623, and #1810122REVA, a P0304 code appearing in conjunction with other codes (like P0299 or P20E8) may be resolved by a dealer-installed PCM software update rather than replacing any parts.

Mechanic-Grade Diagnostic Values

• 6.7L Cummins Injector Return Flow (All Cylinders Combined) — expected: Less than 180-210 mL per minute at idle or specified test conditions (1200 RPM).. Failure: A flow rate greater than 210 mL/min indicates excessive leakage from one or more injectors or connector tubes.
• 6.7L Cummins Injector Return Flow (Isolating a Single Injector) — expected: When capping a single injector at the rail, the total return flow should not drop significantly.. Failure: If capping a single injector (e.g., cylinder #4) causes the total return flow to drop by more than 40 mL, that specific injector is identified as the source of the leak.
• 6.4L Hemi Ignition Coil Primary Resistance — expected: 0.49 - 0.57 Ohms.. Failure: A reading outside this range indicates a faulty primary winding in the coil.
• 6.4L Hemi Ignition Coil Secondary Resistance — expected: 9.5 - 11.1 kOhms.. Failure: A reading outside this range indicates a faulty secondary winding in the coil.
• Mode 6 Misfire Counter Data (Both Engines) — expected: Near-zero misfire counts for Test ID $A6 (Cylinder 4) during the current and previous 10 drive cycles.. Failure: A significantly high number of misfires logged for Test ID $A6 compared to other cylinders indicates an intermittent or developing misfire on cylinder 4, even before the P0304 code is set.

Hidden / Shadow Codes Worth Checking

• Mode 6, Test IDs $A2-$A9: These are not DTCs but onboard monitor results for misfire counts per cylinder (A2=Cyl 1, A3=Cyl 2, etc.). A technician can view the raw count for cylinder 4 (Test ID $A6) to see if it's misfiring before the threshold to trigger the CEL is met. (see via A professional-grade scan tool with Mode 6 data capability.)

Scan Tool Commands That Help

• wiTECH, Autel, Snap-on: Cylinder Performance Test / Cylinder Contribution Test — For the 6.7L Cummins, this function electronically disables injectors one by one to measure the RPM drop of each cylinder. A cylinder with little to no RPM drop when its injector is disabled is not contributing power, confirming the misfire location.
• wiTECH, Autel, Snap-on: Injector Quantity Adjustment (IQA) — Required after replacing a fuel injector on a 6.7L Cummins. Each new injector has a unique calibration code that must be programmed into the PCM to ensure correct fueling and smooth engine operation.
• wiTECH: Fuel Pressure Override Test — For the 6.7L Cummins, this test commands the fuel system to a specific high pressure (e.g., 26,107 PSI) at a set RPM, allowing for a standardized injector return flow test to be performed.

Wiring & Ground Locations

• Engine Block Grounds (6.7L Cummins) — On the passenger side, a ground cable runs from the battery down to the engine block. On the driver's side, there is another ground cable going to the block, sometimes mounted on an 18mm bolt head underneath the intake boot.. A poor engine ground can cause erratic behavior in sensors and actuators, including fuel injectors. Ensuring these main grounds are clean and tight is a crucial preliminary step in diagnosing any electronic issue.
• Body Grounds (6.7L Cummins) — Both the driver and passenger side have smaller ground wires running from the battery negative terminals to the truck's body/fender sheet metal, near the batteries.. The PCM and other control modules rely on solid body grounds. A loose or corroded body ground can cause a host of intermittent electrical problems that could be misdiagnosed as a component failure.
• Actuator-Electronic Turbo (6.7L Diesel) — Connector is located on the lower right front of the engine.. While not directly related to a single cylinder misfire, TSBs link P0304 to other codes like P0299 (Turbo Underboost). Knowing the location of related component connectors is useful for comprehensive diagnosis.

Real Owner Repair Stories

• RamForum.com user (2014 Ram 3500 6.4L Hemi, 140,000 miles) — Check engine light with a misfire code.
  ❌ Tried (didn't work) Not specified, diagnosis went directly to internal engine.
  ✅ What actually fixed it The misfire was caused by a failed lifter. The repair required replacing the entire valve train (camshaft, lifters, etc.). The same user reported an identical failure on their 2014 Ram 5500 with 90,000 miles.

OEM Part Supersession History

• 53021720AB, 53021720AC, 53021720AD, 53021720AE, 5038686AA → 5038784AD — These are part numbers for the non-MDS 'Hellcat' lifters used in 6.4L Hemi engines and as a common upgrade/fix for the 'Hemi Tick' lifter failure. The number has been revised multiple times.
  Heads up: When replacing standard MDS lifters with these non-MDS lifters, the MDS system must be electronically disabled in the PCM via tuning to prevent fault codes.

Model Year Variations Within This Range

• 2013-2018 vs 2019-2020 (6.7L Cummins): The fuel injectors are different between the fourth generation (ending in 2018) and the fifth generation (starting in 2019). For 2019+ trucks, there are also different injectors for Standard Output (SO) and High Output (HO) engine variants. For example, a 2019+ SO injector is Mopar part #68444791AA, while an HO injector is #68444790AA. A 2013-2018 Cab & Chassis injector is a different part number entirely. Using the wrong injector will cause performance issues.