P0306 on 2015-2020 Ram 3500: Cylinder 6 Misfire Causes & Fixes (Hemi & Cummins)

What's Unique About the 2015-2020 Ram 3500

The 2015-2020 Ram 3500 is unique because P0306 can occur on two vastly different engines: the 6.4L Hemi V8 (gasoline) and the 6.7L Cummins I6 (diesel). The diagnostic path is completely different for each. For the Hemi, the cause is typically a simple ignition part, but it can also be an early warning for the platform's widely documented camshaft and lifter failure, often related to the Multi-Displacement System (MDS) lifters. For the Cummins, the problem is almost always fuel-related, pointing directly to the high-pressure fuel injector for cylinder 6, a known wear item sensitive to fuel quality.

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). The core engine designs are similar, but the 2019-2020 6.7L Cummins uses a Bosch CP4.2 high-pressure fuel pump which is subject to a major safety recall (NHTSA ID: 21V-880). This pump can fail internally, sending metal debris throughout the entire fuel system, destroying the injectors and causing misfires like P0306. The recall involves replacing the CP4.2 pump with the more robust CP3 pump.

Symptoms You May Notice

• Check engine light is on or flashing
• Engine running rough or vibrating, especially at idle
• Noticeable loss of power during acceleration
• Hesitation or stumbling from the engine
• Poor fuel economy
• Possible raw fuel smell from the exhaust
• Loud, rhythmic ticking sound from the engine bay (classic 'Hemi tick' associated with lifter failure)
• White smoke from exhaust at idle (common with a failed Cummins injector)

Most Likely Causes

1. Faulty Ignition Coil (6.4L Hemi Only) 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems are exposed to constant, high heat cycles and vibration in the engine bay, leading to eventual failure of the internal windings or epoxy insulation. It is the most common and easiest-to-fix cause for a single-cylinder misfire on this engine.
   How to confirm: Swap the ignition coil from cylinder 6 with another cylinder (e.g., cylinder 4). Clear the codes and drive the truck. If the code changes to P0304, the ignition coil is faulty. Cylinder 6 on the Hemi is the third cylinder back on the passenger side.
   Typical fix: Replace the faulty ignition coil. It is a single component held on by one bolt.
   Est. part cost: $40 - $90
2. Faulty Fuel Injector (6.7L Cummins Only) 🔴 High Probability → Shop Fuel Injector The high-pressure common rail fuel system operates at extreme pressures (over 20,000 psi). Injectors are a known wear item and are sensitive to fuel quality and contamination. Failure of the CP4.2 fuel pump on 2019-2020 models is a known cause of injector destruction.
   How to confirm: A professional diagnosis using a scan tool (like wiTECH) to perform an injector cut-out test is the best method. Other symptoms include a rough idle, hard starting, and white smoke from the exhaust.
   Typical fix: Replace the faulty fuel injector for cylinder 6. It is critical to also replace the high-pressure connector tube at the same time to prevent leaks. 🎬 See how to properly replace fuel injectors on a Cummins. The new injector's trim code (IQA code) must be programmed into the PCM/ECM.
   Est. part cost: $400 - $800 for a single Bosch OEM injector.
3. Worn or Fouled Spark Plugs (6.4L Hemi Only) 🔴 High Probability → Shop Spark Plug The 6.4L Hemi uses two spark plugs per cylinder (16 total), increasing the statistical chance of a single plug failing or wearing out. The service interval is around 100,000 miles, but failure can occur sooner.
   How to confirm: After confirming the coil is good, inspect the two spark plugs from cylinder 6. Look for a worn electrode, carbon tracking, oil fouling, or an incorrect gap. The 6.4L uses one standard and one side-electrode plug per cylinder.
   Typical fix: Replace the spark plugs. It is highly recommended to replace all 16 plugs if they are near or past their service interval.
   Est. part cost: $50 - $150 for a full set of 16 OEM-equivalent plugs.
4. Failed Lifter / Worn Camshaft (6.4L Hemi Only) 🟡 Medium Probability → Shop Engine Camshaft A widely documented issue on Hemi engines is lifter failure, where the roller on a hydraulic lifter (often an MDS lifter) seizes and grinds down the corresponding camshaft lobe. This causes reduced valve lift and a persistent misfire that will not be resolved by ignition part swaps.
   How to confirm: This is often accompanied by a loud, rhythmic ticking sound from the top of the engine, often called the 'Hemi tick'. A compression test on cylinder 6 will show low results. Visual inspection requires removing the valve cover and intake manifold to observe valve movement and inspect the cam lobe. A borescope can also be used to inspect the cam lobe through the lifter bore.
   Typical fix: This is a major engine repair that involves replacing the camshaft, all lifters (often with non-MDS replacements), head gaskets, and related components.
   Est. part cost: $1,500 - $3,000 in parts alone.

Rare But Worth Checking

• Injector Wiring Harness Issue (6.7L Cummins): The injector wiring harness passes through the valve cover gasket. Over time, heat and vibration can make the wires brittle, causing an intermittent open or short circuit for a single injector. This should be checked via a continuity test before replacing a costly injector.
• Low Engine Compression: For either engine, a mechanical issue like a burnt valve, damaged piston, or failing head gasket can cause a loss of compression in cylinder 6, resulting in a misfire. A compression test can confirm this. On Hemi engines, a dropped valve seat is a rare but possible cause.
• PCM Software Glitch: Multiple Technical Service Bulletins, including #1800824, #1800724, #1806623, and #1810122REVA, exist for Ram trucks where erroneous misfire codes (P0301-P0306) can be set. The official fix is to have a dealer update the Powertrain Control Module (PCM) with the latest software.

Diagnosis Steps

1. Verify the code P0306 with a reliable OBD-II scanner.
2. Identify the engine: Is it the 6.4L Hemi V8 (gas) or the 6.7L Cummins I6 (diesel)? The diagnostic path depends entirely on this.
3. For the 6.4L Hemi:
4. Locate cylinder 6. It is the third cylinder from the front on the passenger side of the engine.
5. Swap the ignition coil from cylinder 6 with the coil from cylinder 4 (second from front, passenger side).
6. Clear the codes, start the engine, and see if the misfire code returns as P0304. If it does, the ignition coil is bad.
7. If the code remains P0306, inspect and swap the two spark plugs from cylinder 6 with those from cylinder 4. If the code moves, the spark plugs are bad.
8. If the misfire persists on cylinder 6, listen for a distinct ticking noise. If present, investigate for potential lifter/camshaft failure, as this is a common failure path after ruling out ignition components.
9. Perform a compression test on cylinder 6 to check for mechanical engine issues. A healthy cylinder should be between 100-150 PSI.
10. For the 6.7L Cummins:
11. Locate cylinder 6. It is the rearmost cylinder, closest to the firewall.
12. Professional diagnosis is highly recommended. A capable scan tool is needed to perform a cylinder cutout test to confirm the misfire is on cylinder 6.
13. Before replacing the injector, inspect the wiring harness connector at the fuel injector for damage and test the wiring continuity from the main harness plug to the injector. The injector control wire for cylinder 6 is typically a Light Green/Brown wire at the ECM connector.
14. If scan tool data and wiring checks point to the injector, prepare for replacement. This is an involved job requiring special tools and extreme cleanliness.
15. If replacing the injector, always replace the high-pressure connector tube (Mopar P/N 68005335AA or equivalent) at the same time to ensure a proper seal.

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 engine due to heat and vibration.
  Trusted brands: Mopar, NGK, Delphi
  OEM price range: $60 - $100
• Fuel Injector (6.7L Cummins) (OEM #Bosch 0445124047 / Mopar 68444791AA (2019-2020) or Bosch Reman 0986435518 / Mopar R8415374AA (2013-2018)) — Injectors are a primary failure point for misfires on the Cummins, susceptible to wear and contamination, especially on 2019-2020 models with the CP4.2 fuel pump.
  Trusted brands: Bosch, Mopar
  OEM price range: $400 - $800

Related Codes That Often Appear With This One

• P0300 — If you see P0300 (Random/Multiple Cylinder Misfire) with P0306, it suggests a problem affecting the whole engine, like low fuel pressure, a large vacuum leak, or on a 2019-2020 Cummins, a failing CP4.2 fuel pump sending debris to multiple injectors.
• P0206 — P0206 indicates a fault in the cylinder 6 injector circuit. Seeing these two codes together strongly points to a problem with the fuel injector, its wiring, or the injector driver in the PCM.

Technical Service Bulletins (TSBs) & Recalls

• 18-008-24
• 18-007-24
• 18-066-23
• 18-101-22 REV A
• 21V-880 (NHTSA Recall for CP4.2 Pump)

Platform-Specific Known Issues

• Hemi Cam/Lifter Failure: The 6.4L Hemi engine is known for potential lifter failure, where a roller seizes and destroys the camshaft lobe. This often starts as a single-cylinder misfire (like P0306) accompanied by a loud ticking noise and requires major engine repair.
• Cummins Injector Failure: Fuel injectors are a common failure point on the 6.7L Cummins. Symptoms include rough idle, white smoke, and misfire codes. Replacement is costly and requires care.
• Erroneous Misfire TSBs: Ram has issued several TSBs (e.g., #1800824, #1800724) that list P0306 as a code that could be falsely triggered. The fix is a PCM software update at a dealership. [Bulletin #1800824, Bulletin #1800724]

Mechanic-Grade Diagnostic Values

• 6.4L Hemi Ignition Coil Primary Resistance — expected: 0.49 - 0.57 ohms. Failure: A reading outside of this range indicates a faulty coil winding.
• 6.4L Hemi Ignition Coil Secondary Resistance — expected: 9.5 - 11.1 kilohms. Failure: A reading outside of this range indicates a faulty secondary winding or internal breakdown.
• 6.7L Cummins Fuel Injector Solenoid Resistance — expected: 1.5 - 3.5 ohms. Failure: Resistance outside this range suggests a damaged injector solenoid.
• 6.7L Cummins Low-Pressure Fuel Supply (Lift Pump) — expected: 10-15 PSI at idle. Failure: Pressure dropping near zero under load indicates a weak lift pump or restricted filter, which can starve the high-pressure pump.
• 6.7L Cummins High-Pressure Rail (Cranking) — expected: ~4,000 PSI. Failure: Failure to reach this pressure during cranking will result in a no-start condition, often caused by leaking injectors.
• 6.7L Cummins High-Pressure Rail (Idle) — expected: 6,800 - 7,000 PSI. Failure: Significantly lower or erratic pressure at idle points to issues with the fuel control actuator or injectors.
• 6.7L Cummins Injector Return Flow (Combined) — expected: Max 210 mL in 30 seconds (during Fuel Pressure Override Test). Failure: Exceeding this volume indicates one or more injectors are leaking excessively, returning too much fuel to the tank instead of injecting it.
• 6.7L Cummins Injector Return Flow (Isolated) — expected: Should not contribute more than 40 mL to the total leakage.. Failure: If blocking off one injector reduces the total return flow by more than 40 mL, that specific injector is faulty.

Hidden / Shadow Codes Worth Checking

• Live Misfire Counters (Mode 6 Data): This data shows the real-time count of misfire events per cylinder, even if they haven't reached the threshold to set a hard code like P0306. It can confirm if the misfire is exclusively on cylinder 6 or if other cylinders are also misfiring intermittently. (see via Accessible via the live data stream on a professional scan tool (like wiTECH) or some advanced consumer OBD-II scanners.)

Scan Tool Commands That Help

• wiTECH or equivalent professional scanner: Cylinder Contribution / Cutout Test — For the 6.7L Cummins, this test deactivates one injector at a time to see the effect on engine speed. If cutting out cylinder 6 causes little or no change in engine RPM, it confirms that cylinder was not contributing power, pointing to an injector or mechanical issue.
• wiTECH or equivalent professional scanner: Fuel Pressure Override Test — For the 6.7L Cummins, this function commands the fuel system to its maximum pressure (approx. 26,000 PSI) at a set RPM. It is used in conjunction with a physical return flow measurement to accurately diagnose leaking injectors under stress.
• wiTECH or dealer-level tool: PCM Flash/Update — If diagnosis reveals no mechanical or electrical faults, a PCM update should be performed. TSBs #1800824, #1800724, #1806623, and #1810122REVA specifically list P0306 as a code that can be falsely set by software glitches, especially on cold starts.

Wiring & Ground Locations

• Cylinder 6 Injector Control Wire (6.7L Cummins) — At the Engine Control Module (ECM) connector. Pin A59 is typically the control wire for injector #6.. The wire is often Light Green/Brown. Testing for continuity and shorts on this specific wire between the ECM and the injector harness can rule out a wiring fault before replacing an expensive injector.
• Engine Block to Chassis Ground — There should be a primary ground strap from the engine block (often at the back of the cylinder head) to the chassis/firewall.. A poor engine ground can cause erratic behavior in sensors and actuators, including the ignition coils and fuel injectors, potentially leading to misfire codes. While not a common cause for a single-cylinder misfire, it should be checked for corrosion and tightness if multiple electrical issues are present.

Real Owner Repair Stories

• RamForum.com user (2014 Ram 1500 5.7L Hemi, 77k miles) — P0306 code appeared.
  ❌ Tried (didn't work) Swapping spark plugs to a different cylinder., Swapping the ignition coil to a different cylinder.
  ✅ What actually fixed it The issue was diagnosed as a lifter/camshaft failure on cylinder 6. The owner was advised by a shop that a full motor swap might be necessary if metal shavings had circulated, but the ultimate fix is replacing the camshaft and lifters.
• RamForum.com user (2014 Ram 3500 6.4L Hemi, 140k miles) — Check engine light with a misfire code.
  ❌ Tried (didn't work) Not specified, diagnosis went straight to internal.
  ✅ What actually fixed it The cause was a failed lifter which required replacement of the entire valve train. The user noted the roller bearings on the lifter failed, causing it to stop rolling and damage the cam lobe.
• RamForum.com user (2015 Ram Hemi, 36k miles) — P0306 code appeared after a remote start on a cold day.
  ❌ Tried (didn't work) Clearing the code (it returned on a subsequent cold start).
  ✅ What actually fixed it The issue was intermittent and only occurred on cold starts. Other users in the thread reported similar cold-start-only misfire codes, suggesting it could be a software glitch (as noted in TSBs) or a borderline fuel injector. The final resolution was not posted, but the pattern points away from a hard mechanical failure.

OEM Part Supersession History

• 56029129AF → 68238603AA — Standard part revision and supplier updates for the 6.4L Hemi ignition coil.
• Bosch CP4.2 Fuel Pump (e.g., 0445010854) → Bosch CP3 Fuel Pump (Conversion Kit) — The original CP4.2 pump was prone to catastrophic failure due to inadequate lubrication from US diesel fuel. A safety recall was issued to replace it with the more robust CP3 design.
  Heads up: This is not a direct swap but a recall-mandated conversion. Failure to perform this recall on an affected 2019-2020 6.7L Cummins leaves the entire fuel system at risk of destruction.

Model Year Variations Within This Range

• 2019-2020: These model years with the 6.7L Cummins engine were equipped with the failure-prone Bosch CP4.2 high-pressure fuel pump. A P0306 code on these trucks, especially if accompanied by P0300 or fuel pressure codes like P0087, should immediately raise suspicion of a failing CP4.2 pump.
• 2019-2020: 6.7L Cummins engines from 2019-onward use hydraulic rocker arms, which changes the torque procedure for valvetrain components during jobs like injector replacement compared to earlier models.