P0303 on 2015-2020 Ram 3500: Cylinder 3 Misfire Causes and Fixes (Hemi & Cummins)

What's Unique About the 2015-2020 Ram 3500

The 2015-2020 Ram 3500 spans two generations and was offered with two very different engines: the 6.4L Hemi V8 (gasoline) and the 6.7L Cummins I6 (diesel). The causes for P0303 are completely different between them. For the Hemi, causes are typically ignition-related but can be a symptom of the platform's known camshaft/lifter failure, which often starts as a single-cylinder misfire. For the Cummins, the high-pressure fuel system makes the fuel injector the primary suspect. The 2019-2020 diesel models also used a Bosch CP4.2 high-pressure fuel pump that was subject to a major recall for failing and sending metal debris into the fuel system, destroying injectors.

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 start of the Fifth Generation (2019-2020). The most significant difference is the fuel pump on the 6.7L Cummins: 2019-2020 models used the failure-prone Bosch CP4.2 high-pressure fuel pump, while earlier models used a more robust CP3 pump. This makes 2019-2020 diesel models more susceptible to catastrophic fuel system failure causing injector issues. For the 6.4L Hemi, the major change occurred in 2019 with the introduction of the 8-speed ZF automatic transmission, replacing the previous 6-speed. The Hemi engine itself, known as the 'BGE' (Big Gas Engine), remained largely the same.

Symptoms You May Notice

• Rough or shaking idle, sometimes described as a 'loping' feeling
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing Check Engine Light (indicates a severe, catalyst-damaging misfire)
• Reduced fuel economy
• Excessive white or blue-white smoke from exhaust, especially at idle (common with Cummins injector failure)
• Engine knocking or a distinct, rhythmic ticking sound, often called the 'Hemi tick' if coming from a 6.4L engine.

Most Likely Causes

1. Faulty Ignition Coil (6.4L HEMI Only) 🔴 High Probability Coil-on-plug ignition systems are subject to failure from constant heat cycles and vibration in the engine bay. Moisture intrusion can also cause corrosion on the coil spring and contact points, increasing resistance and leading to failure.
   How to confirm: Swap the ignition coil from cylinder #3 with another cylinder (e.g., cylinder #1). Cylinder #3 is the second cylinder from the front on the driver's side. Clear the codes and drive. If the code changes to P0301, the coil is faulty.
   Typical fix: Replace the failed ignition coil. It's often recommended to replace the spark plugs at the same time, especially if they are near their service interval. The OEM Mopar part number is 68238603AA.
   Est. part cost: $50-$85 for a single OEM Mopar coil.
2. Faulty Fuel Injector (6.7L Cummins Only) 🔴 High Probability The high-pressure common rail fuel system operates at extreme pressures, making injectors a common wear item. They are highly sensitive to fuel contamination. On 2019-2020 models, failure of the Bosch CP4.2 fuel pump can send metal debris through the system, destroying the injectors.
   How to confirm: A professional scan tool is needed to perform a cylinder contribution test or to monitor injector balance rates. A reading significantly different from the others on cylinder #3 confirms the issue. An injector return flow test can also be performed; a flow of more than 40mL in 10 seconds at cranking, or one injector contributing more than 40mL to the total during a running test, indicates a failure.
   Typical fix: Replace the cylinder #3 fuel injector and the associated connector tube. This requires extreme cleanliness. The new injector's IQA (Injector Quantity Adjustment) code must be programmed to the ECM using a capable scan tool like wiTECH. For 2015-2018 models, the Bosch part is 0986435621 (for pickups). For 2019-2020 models, the Bosch part is 0445124047 (Standard Output).
   Est. part cost: $400-$650 for a single new Bosch injector.
3. Worn or Fouled Spark Plugs (6.4L HEMI Only) 🔴 High Probability The 6.4L Hemi uses two spark plugs per cylinder (16 total), increasing the chances of a single plug failing. They are a routine maintenance item (recommended every 30,000-100,000 miles depending on type) and can become worn or fouled with carbon or oil.
   How to confirm: After confirming the coil is good, remove the two spark plugs from cylinder #3. Inspect for a worn electrode, cracked porcelain, oil fouling, or heavy carbon buildup. A wet, fuel-smelling plug is a clear sign it is not firing.
   Typical fix: Replace the spark plugs. It is best practice to replace all 16 at the same time. Popular OEM-quality brands include NGK.
   Est. part cost: $100-$200 for a set of 16 NGK plugs.
4. Failed Lifter / Worn Camshaft (6.4L HEMI Only) 🟡 Medium Probability A widely documented issue on HEMI engines where the needle bearings in a lifter's roller can fail, causing the roller to seize and grind against the camshaft lobe, destroying it. This often starts as a single-cylinder misfire accompanied by a distinct ticking sound before escalating. Extended idling is often cited as a contributing factor.
   How to confirm: After ruling out ignition and fuel issues, listen for a loud, rhythmic ticking noise from the engine ('Hemi tick'). A YouTube video by 'vid service information' demonstrates this exact failure on a 2016 Ram 2500 with P0303. A compression test may show low compression on cylinder 3. Visual inspection by removing the valve cover and cylinder head is the definitive diagnosis.
   Typical fix: This is a major and expensive engine repair requiring replacement of the camshaft, all lifters (often upgrading to a non-MDS version), head gaskets, and other related components. Many owners on forums report dealer repair costs in the thousands.
   Est. part cost: $1000-$1800 for a camshaft and lifter kit.

Rare But Worth Checking

• Incorrect Valve Lash (6.7L Cummins Only): The 6.7L Cummins requires periodic valve lash adjustments (every 150,000 miles). If the clearance on cylinder #3's valves is too tight or too loose, it can cause a loss of compression and a misfire. This is an often-overlooked maintenance item that can cause misfire codes. The spec is typically 0.010" for intake and 0.026" for exhaust, done on a cold engine.
• Injector Wiring Harness (6.7L Cummins Only): The wiring harness that passes through the valve cover to the injectors can become brittle from heat and vibration, causing an open or short circuit to a single injector. This would typically also set a P0203 code. The pass-through connector on the driver's side valve cover is a known weak point.
• Low Cylinder Compression: This indicates a mechanical engine problem like a worn piston ring, a leaking head gasket, or a burnt valve. A compression test is required for diagnosis. On a Hemi, this can be the result of the camshaft/lifter failure.

Diagnosis Steps

1. Read the code with an OBD-II scanner and confirm P0303 is the primary code. Note any other codes present, like P0300 or P0203.
2. For 6.4L HEMI:
3. a. Locate cylinder #3. It is the second cylinder from the front on the driver's side (left bank).
4. b. Swap the ignition coil from cylinder #3 with the coil from cylinder #1 (front-most on the driver's side).
5. c. Clear the codes, start the engine, and see if the misfire code moves to cylinder #1 (P0301). If it does, the coil (Mopar P/N 68238603AA) is bad.
6. d. If the code remains P0303, swap the two spark plugs from cylinder #3 with cylinder #1. If the code moves to P0301, one of the spark plugs is bad.
7. e. If the misfire remains on cylinder #3, listen carefully for a loud, rhythmic ticking noise ('Hemi tick'). This is a strong indicator of camshaft/lifter failure. A compression test is the next step to check for a mechanical sealing issue. If compression is low, suspect a cam/lifter problem.
8. For 6.7L Cummins:
9. a. Locate cylinder #3. It is the third cylinder from the front of the engine.
10. b. Listen for engine knock and check for excessive white smoke at idle, which indicates unburnt fuel from a bad injector.
11. c. Inspect the wiring harness and connector for the cylinder #3 fuel injector. The front pass-through connector on the driver's side valve cover serves cylinders 1-3 and is a common point of failure.
12. d. Use a capable scan tool to run a cylinder contribution test or monitor injector balance rates. This can confirm if cylinder #3 is underperforming due to a fuel injector issue. An injector return flow test can also be performed to check for excessive leakage.

Parts You'll Likely Need

• Ignition Coil (6.4L HEMI) — 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-$90
  Aftermarket price range: $40-$70
• Fuel Injector (6.7L Cummins) — The most common cause of a misfire on the Cummins engine. They are a high-wear item operating under extreme pressure.
  Trusted brands: Bosch
  OEM price range: $450-$600
  Aftermarket price range: $300-$450
• Spark Plugs (6.4L HEMI) — A routine maintenance item that is a frequent cause of misfires when worn or fouled. The HEMI uses 16 plugs.
  Trusted brands: NGK, Champion
  OEM price range: $120-$180 (set of 16)
  Aftermarket price range: $80-$150 (set of 16)

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear with P0303. This is common with the Hemi lifter failure as it progresses.
• P0203 — This code means "Injector Circuit/Open - Cylinder 3". If seen with P0303 on a Cummins, it strongly confirms the problem is electrical with the cylinder 3 fuel injector or its wiring, not a mechanical part of the injector itself.
• P0301, P0302, P0304, etc. — The presence of other cylinder-specific misfire codes points to a larger problem. The NHTSA TSBs #1800824 and #1800724 list all cylinder misfire codes together, suggesting a potential common ECU software fault that can be fixed with a PCM flash.

Technical Service Bulletins (TSBs) & Recalls

• TSB #18-008-24 & #18-007-24: These bulletins apply to 2020 and 2019 Cummins models, respectively. They list P0303 among a wide range of other DTCs that can be resolved by a PCM software update. This suggests that in some cases, the misfire detection may be overly sensitive and can be corrected by a flash, not a mechanical repair.

Platform-Specific Known Issues

• 6.4L HEMI Camshaft/Lifter Failure ('Hemi Tick'): The most significant known issue for the 6.4L Hemi is the failure of the roller lifters. The needle bearings inside the lifter's roller can disintegrate, causing the roller to stop spinning and grind a flat spot on the camshaft lobe. This prevents the valve from opening and closing properly, leading to a persistent misfire on that cylinder (like P0303) and a characteristic ticking sound. It is a major mechanical failure requiring extensive and costly repairs.
• 6.7L Cummins CP4.2 High-Pressure Fuel Pump Failure (2019-2020): A major issue specific to the 2019-2020 Cummins engines was the use of the Bosch CP4.2 high-pressure fuel pump, which was prone to catastrophic failure. When it fails, it sends metal shavings throughout the entire high-pressure fuel system, destroying the fuel injectors. A P0303 code could be a symptom of this failure. A recall (NHTSA 21V-880) was issued to replace the CP4.2 pump with a more reliable CP3-style pump.

Mechanic-Grade Diagnostic Values

• 6.7L Cummins Fuel Injector Solenoid Resistance — expected: Less than 1.0 Ohm, greater than 0 Ohms. Failure: An open circuit (OL) or a reading of 0 Ohms (short).
• 6.7L Cummins Injector Return Flow (Running Test) — expected: Max 210 mL in 30 seconds. Failure: Flow greater than 210 mL indicates excessive leakage from one or more injectors. If one injector contributes more than 40mL of this total, it is faulty.
• 6.7L Cummins Injector Return Flow (Cranking, No-Start) — expected: Max 40 mL in 10 seconds. Failure: Flow greater than 40 mL indicates excessive leakage preventing the rail from building enough pressure to start.
• 6.7L Cummins Fuel Rail Pressure Sensor (KOEO) — expected: ~0.5 Volts. Failure: A reading significantly different from 0.5V may indicate a faulty sensor or wiring issue.
• 6.4L Hemi Ignition Coil Primary Resistance (Generic Spec) — expected: 0.49 - 0.57 Ohms. Failure: A reading outside this range indicates a faulty primary winding.
• 6.4L Hemi Ignition Coil Secondary Resistance (Generic Spec) — expected: 9.5 - 11.1 kOhms. Failure: A reading outside this range indicates a faulty secondary winding.

Scan Tool Commands That Help

• wiTECH: Fuel Pressure Override Test — For the 6.7L Cummins, this function commands the engine to 1200 RPM and rail pressure to 26,107 PSI, creating the specific conditions required to perform an accurate injector return flow test.
• wiTECH: Reprogram PCM — As noted in NHTSA TSBs, a PCM software update may be required to fix erroneous misfire codes. This function flashes the module with the latest software.
• wiTECH: Cylinder Contribution Test / Injector Balance Rates — For both engines, but especially the Cummins, this live data function shows the performance of each cylinder in real-time, helping to confirm if cylinder 3 is underperforming.

Wiring & Ground Locations

• Cummins Injector Harness Pass-Through Connector (Front) — On the driver's side of the valve cover, the large circular connector closer to the front of the engine.. This single connector carries the wiring for cylinders 1, 2, and 3. It is a very common failure point for P0303 on the Cummins, often due to oil contamination from the CCV system causing a poor connection.
• PCM Connector C1, Pins 13 & 63 (Cummins) — At the Powertrain Control Module (PCM), typically located on the driver's side of the engine block.. These are the specific termination points for the cylinder 3 injector circuit. Pin 13 is the High Side Driver and Pin 63 is the Low Side Driver. This is essential information for checking wire continuity from the injector to the PCM.
• Engine Block Ground — The main negative battery cable connects to the left front of the engine block.. A poor engine ground can cause a host of erratic electrical issues, including misfires. Ensuring this connection is clean and tight is a fundamental step.

Real Owner Repair Stories

• YouTube channel 'vid service information' (2016 Ram 2500 6.4L Hemi) — Engine running rough, ticking noise, P0300 (Random Misfire) and P0303 (Cylinder 3 Misfire).
  ❌ Tried (didn't work) Not specified, but the diagnosis went straight to mechanical after hearing the tick.
  ✅ What actually fixed it Diagnosis confirmed a failed lifter and damaged camshaft lobe on cylinder #3. Due to high mileage and potential for metal debris, a complete engine replacement was recommended as the ultimate fix.
• RamForum.com user (2014 Ram 6.4L Hemi, 120k miles) — Shudder on acceleration, flashing check engine light, codes P0303 and P0300.
  ❌ Tried (didn't work) Replaced all 16 spark plugs., Swapped multiple known-good ignition coils onto cylinder #3., Replaced the fuel injector on cylinder #3.
  ✅ What actually fixed it The user did not post a final resolution, but the story is a textbook example of the Hemi cam/lifter failure where all standard ignition and fuel-part swaps fail to resolve the misfire, strongly pointing to the underlying mechanical problem.
• Reddit user in r/ram_trucks (2013 Ram 1500 5.7L Hemi) — P0303 misfire that returned specifically under heavy load at highway speeds (70-80mph, 4500-5000 RPM), causing shaking.
  ❌ Tried (didn't work) Replaced the spark plug on cylinder #3., Replaced all spark plugs and all ignition coils.
  ✅ What actually fixed it The final fix was not posted, but the symptoms (misfire only at high RPM/load after replacing ignition components) are classic indicators of a failing valve lifter that is unable to function correctly under high valvetrain speeds, leading to the infamous Hemi cam/lifter failure.

OEM Part Supersession History

• 56029129AA, 56029129AB, 56029129AF, 68060346AA → 68238603AA — Standard part consolidation and potential design updates over the model years.
• Various older non-MDS lifter part numbers → Hellcat lifter design — The Hellcat lifter is a more robust design used in high-horsepower factory applications and has become the standard replacement for non-MDS Hemi engines.
  Heads up: If replacing MDS lifters with these non-MDS lifters (e.g., during a camshaft upgrade), the MDS system must be disabled in the PCM calibration.

Model Year Variations Within This Range

• 2019-2020: For the 6.4L Hemi, the 6-speed 66RFE automatic transmission was replaced by the ZF 8-speed 8HP75-LC8 automatic. This significantly changed shift logic and performance feel, though the engine itself remained the same.
• 2019-2020: For the 6.7L Cummins, these models were equipped with the Bosch CP4.2 high-pressure fuel pump, which was subject to a major recall for catastrophic failure. Earlier 2015-2018 models used the more reliable CP3 pump.
• 2015-2020 (All): The 6.4L Hemi used in Ram HD trucks (BGE or 'Big Gas Engine') is different from the version in cars (Challenger/Charger). The truck version has a different camshaft for more low-RPM torque, a different intake manifold, and is painted black instead of orange.