P0304 on 2020-2025 Jeep Gladiator: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2020-2025 Jeep GLADIATOR

The 2020-2025 Jeep Gladiator uses the 3.6L Pentastar V6 engine. While misfires are common on all engines, this platform has two distinct characteristics. First, the ignition coils on the passenger's side (including cylinder 4) are easily accessible, while those on the driver's side are buried under the intake manifold, making them much more labor-intensive to replace. Second, while less common for a single-cylinder misfire, the Pentastar engine family has a known history of valvetrain issues. A failed rocker arm, due to seized needle bearings, is a common problem that can start as a single-cylinder misfire (like P0304) and is often accompanied by an audible ticking sound, sometimes called the 'Pentastar tick'. If left unaddressed, this can destroy the camshaft lobe.

Diagnostic Flowchart

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

Symptoms You May Notice

• Rough or shaking idle.
• Hesitation or stumbling during acceleration.
• Flashing or solid Check Engine Light.
• Noticeable loss of engine power.
• Engine running rough.
• Smell of unburned fuel from the exhaust.
• Audible ticking or tapping noise from the top of the engine (a strong indicator of rocker arm/lifter failure).
• Intermittent misfire and rough idle only on cold startups, which may clear after restarting the engine.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil Ignition coils are a common wear item. On the 3.6L Pentastar, cylinder 4 is on the passenger side (Bank 2), making the coil easy to access and a common first point of failure. Heat cycles and vibration contribute to their eventual failure.
   How to confirm: Swap the ignition coil from cylinder 4 with another cylinder (e.g., cylinder 2). Clear the codes and drive. If the code changes to P0302, the coil is faulty. This is the most recommended first diagnostic step in owner forums.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all spark plugs at the same time.
   Est. part cost: $30-$70
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug TSB 09-013-20 REV. A was issued for some 2021 Gladiators due to a manufacturing issue causing internal cracks in the spark plugs, leading to misfires. Even without a defect, spark plugs are a standard maintenance item that wear out over time.
   How to confirm: After ruling out the ignition coil, remove the spark plug from cylinder 4. Inspect it for wear (worn electrode), carbon or oil fouling, or physical damage like a cracked insulator. The recommended spark plug gap is typically around 0.043 inches, but check specifications for your exact model year.
   Typical fix: Replace the spark plug. It is best practice to replace all six spark plugs as a set for even performance. If your vehicle falls under TSB 09-013-20 REV. A, all six must be replaced.
   Est. part cost: $10-$25 per plug
3. Leaking Spark Plug Tube Seal 🟡 Medium Probability → Shop Spark Plug The valve cover gaskets incorporate seals for the spark plug tubes. Over time, these seals can harden and leak oil directly into the spark plug well, soaking the ignition coil boot and shorting out the spark, causing a misfire. This is a common issue on many OHC engines, including the Pentastar.
   How to confirm: When you remove the ignition coil from cylinder 4, look for liquid engine oil on the coil boot or down in the spark plug well. A user on JeepGladiatorForum.com with 155,000 miles found oil in the plug well when changing plugs.
   Typical fix: Replace the passenger side valve cover gasket set, which includes the spark plug tube seals. Thoroughly clean all oil from the ignition coil boot and spark plug well before reinstallation.
   Est. part cost: $40-$100 for the gasket set
4. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: If the spark plug and coil are good, the fuel injector is the next suspect. A user on a Wrangler forum with the same engine chased a P0304 by replacing plugs and coils, only to find the plug was wet with fuel, leading them to replace the injector. Diagnosis can be done by swapping the injector with another cylinder (labor-intensive) or using a scan tool to perform a fuel injector balance test.
   Typical fix: Replace the faulty fuel injector. This is often done after codes like P0204 (Injector Circuit/Open) appear alongside P0304.
   Est. part cost: $50-$120

Rare But Worth Checking

• Engine Mechanical Failure (Rocker Arm/Camshaft): Though less common for a new vehicle, this is a widely documented failure on the 3.6L Pentastar. The needle bearings in a rocker arm can fail and fall out, causing the arm to have excessive play and stop actuating the valve correctly. This leads to a misfire and a distinct ticking noise. If ignored, the failed rocker arm will score and destroy the camshaft lobe, requiring replacement of both. Multiple owners on JeepGladiatorForum.com and Reddit have reported a P0304 code as the first symptom of this failure.

Diagnosis Steps

1. Read the code with an OBD-II scanner and confirm P0304 is present. Note any other codes, especially P0300, P0204, or other cylinder misfires.
2. Listen for an audible ticking noise from the top of the engine. A distinct 'Pentastar tick' strongly suggests a mechanical valvetrain issue (rocker arm/lifter) which requires a different diagnostic path.
3. Locate cylinder 4. On the 3.6L V6, the cylinders are numbered 1-3-5 on the driver's side (Bank 1) and 2-4-6 on the passenger side (Bank 2), from front to back. Cylinder 4 is the middle cylinder on the passenger side.
4. Swap the ignition coil from cylinder 4 with the coil from cylinder 2 (the front cylinder on the same side). This requires a 10mm socket.
5. Clear the codes and drive the vehicle until the check engine light returns.
6. Re-scan for codes. If the code is now P0302, the ignition coil is bad and needs replacement. This is a confirmed fix in many forum threads.
7. If the code remains P0304, the problem is not the coil. Proceed to inspect the spark plug.
8. Remove the ignition coil from cylinder 4 again and inspect the spark plug well for oil. If oil is present, the valve cover gasket/tube seal is leaking and must be replaced.
9. Remove the spark plug from cylinder 4 (typically requires a 5/8" spark plug socket with an extension). Inspect it for excessive wear, carbon buildup, or oil fouling. Replace if necessary. Check if your vehicle falls under TSB 09-013-20 REV. A.
10. If the coil and plug are good and there is no oil in the well, the issue may be the fuel injector or a mechanical problem. A professional diagnosis, including a fuel injector balance test and a cylinder compression/leak-down test, is recommended to check for bad injectors or internal engine damage like a failed rocker arm.

Parts You'll Likely Need

• Ignition Coil (OEM #5149168AJ (supersedes 5149168AI, 5149168AH)) — This is the most common cause of a single-cylinder misfire. The coil generates the high voltage needed to create a spark.
  Trusted brands: Mopar, NGK, Delphi, Bosch
  OEM price range: $85-$120
  Aftermarket price range: $30-$60
• Spark Plug (OEM #SP149125AE (or SP149125AF)) — Spark plugs are a maintenance item that wear out over time, leading to a weak spark and misfires. TSB 09-013-20 REV. A also identified defective plugs on some 2021 models.
  Trusted brands: Champion (OEM), NGK (ILKR7B8 / 93911), Denso
  OEM price range: $15-$25
  Aftermarket price range: $8-$18
• Valve Rocker Arm (OEM #05184296AH) — A known failure point on the 3.6L Pentastar engine. The needle bearings in the roller fail, causing a ticking noise and eventually a misfire code like P0304. If a ticking noise is present, this is a likely culprit.
  Trusted brands: Mopar, Melling
  OEM price range: $15-$25
  Aftermarket price range: $10-$20

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders, a P0300 (Random/Multiple Misfire) code may appear alongside P0304. This can happen if a systemic issue like a failing camshaft begins to affect adjacent cylinders.
• P0302, P0306 — Cylinders 2, 4, and 6 are on the same bank (passenger side). A problem affecting the entire bank, like a failing catalytic converter, vacuum leak, or a valvetrain issue on that bank's camshaft, could cause multiple misfire codes for that bank.
• P0204 — This code specifically points to a fault in the fuel injector circuit for cylinder 4 (e.g., open or short). If you see P0204 along with P0304, the fuel injector or its wiring is the most likely culprit.

Technical Service Bulletins (TSBs) & Recalls

• 09-013-20 REV. A: Subject: '3.6L Pentastar Upgrade Spark Plug Internal Crack'. This TSB applies to certain 2021 model year Gladiators and Wranglers. It states that due to a known manufacturing issue, a misfire code (P0301-P0306) may be set. The prescribed repair is to replace all six spark plugs.
• 09-011-25: This TSB for 2025 models lists P0304 as one of several misfire codes that may be present if a customer complains of a rough idle.
• 09-012-22: While not directly for misfires, this TSB for 2020-2022 Gladiators/Wranglers addresses a spark knock/pinging noise caused by oil intrusion through the PCV system. The extensive repair involves cleaning piston heads and combustion chambers, which could potentially relate to conditions causing misfires.

Platform-Specific Known Issues

• TSB 09-013-20 REV. A: Affects some 2021 Gladiators (and other models) built between Sept-Nov 2020. It addresses a manufacturing defect causing internal cracks in spark plugs, leading to misfire codes including P0304. The fix is to replace all six spark plugs.
• Forum discussions on JeepGladiatorForum.com show members experiencing P0304, sometimes intermittently on cold startups. In one thread, a user with a pending P0304 code tracked misfire counts on cylinder 4 specifically during cold starts.
• A persistent P0304 that is not resolved by new plugs or coils is frequently an early symptom of a failing passenger-side rocker arm and/or camshaft, a known issue with the 3.6L Pentastar engine. One user on the forums noted a P0304 code appeared once, followed a month later by the characteristic engine tick of valvetrain failure.

Mechanic-Grade Diagnostic Values

• Fuel Injector Resistance — expected: 12 to 16 Ohms. Failure: A reading outside of this range, such as an open circuit (infinite resistance) or a significantly higher reading, indicates a faulty injector. One technician found a bad injector reading double the normal resistance.
• Fuel Pressure — expected: Approximately 43.5 psi (3 Bar) at the rail.. Failure: Significantly lower fuel pressure can cause lean misfires, though it would typically affect multiple cylinders and set a P0300 code.
• Scan Tool Misfire Counter (Mode $06) — expected: Near zero for all cylinders on a healthy engine.. Failure: A scan tool with live data can display misfire counts for each cylinder in real-time. A rapidly increasing count for cylinder 4 confirms the misfire location before the PCM stores a permanent P0304 code. This is useful for diagnosing intermittent issues.

Scan Tool Commands That Help

• wiTECH (or equivalent professional scanner): Cylinder Power Balance Test — This function helps determine if cylinder 4 is contributing less power than the others, which can confirm a misfire without having to swap parts. The tool momentarily disables fuel to each cylinder one by one and measures the corresponding drop in RPM.
• wiTECH (or equivalent professional scanner): Misfire Counter / Live Data — Use this to watch real-time misfire events on each cylinder. This is the fastest way to confirm the misfire is isolated to cylinder 4 and to see if it occurs under specific conditions (e.g., only at idle, or only under load).

Wiring & Ground Locations

• Cylinder 4 Ignition Coil Connector — On the passenger side of the engine (Bank 2), it is the connector for the middle ignition coil. The connector is a USCAR (EV6) style.. This is the primary connection point to test for voltage and ground at the coil, and to inspect for damage or corrosion that could interrupt the signal and cause a misfire.
• Engine Bay Ground Points — There are several key grounds in the engine bay near the battery and on the frame behind the front passenger wheel. A specific ground 'bridge' is located against the top of the passenger fender.. A poor engine ground can cause a host of electrical issues, including weak spark that can lead to intermittent misfires. While not specific to one cylinder, checking that main grounds are clean and tight is a crucial step in diagnosing any electrical fault.
• Passenger Kick Panel Grounds — Inside the cabin, in the passenger-side kick panel area, there are three ground points, one of which may be unused from the factory.. These grounds serve various body and control module functions. A loose ground here could potentially affect PCM operation, though it is a less likely cause for a single-cylinder misfire.

Real Owner Repair Stories

• Reddit user in r/MechanicAdvice (2014 Jeep Grand Cherokee 3.6L with 103,000 miles) — Engine ticking noise started around 93,000 miles. Later, the vehicle began shaking at idle and the Check Engine Light came on with code P0304.
  ❌ Tried (didn't work) The user was considering replacing ignition components but suspected a deeper issue due to the persistent ticking noise.
  ✅ What actually fixed it The combination of a long-term ticking noise followed by a single-cylinder misfire code (P0304) is a classic symptom pattern for a failed rocker arm that has progressed to damaging the camshaft lobe on the 3.6L Pentastar engine. The confirmed fix requires replacing the affected rocker arms and the damaged camshaft.
• YouTube video by El HD300 (Jeep Wrangler 3.6L with 148,000 miles) — Rough idle with active codes P0304 (Cylinder 4 Misfire) and P0204 (Fuel Injector 4 Circuit Open).
  ❌ Tried (didn't work) Initial inspection of spark plugs revealed one was not torqued correctly, but fixing this did not resolve the codes.
  ✅ What actually fixed it Using a multimeter to test the resistance of the fuel injectors, the technician found that the injector for cylinder 4 was reading open (no resistance), while the others read correctly around 13.7 Ohms. Replacing the single faulty fuel injector for cylinder 4 cleared both the P0204 and P0304 codes.

OEM Part Supersession History

• 5149168AH, 5149168AI → 5149168AJ — Standard part revision and improvement by the manufacturer.
  Heads up: The parts are interchangeable; 5149168AJ is the latest revision for the ignition coil.
• 5184296AD, 5184296AE, 5184296AF, 5184296AG → 05184296AH — This is the updated rocker arm assembly for the 3.6L Pentastar engine, designed to address the common needle bearing failure of earlier versions.
  Heads up: The -AH revision is the recommended replacement for all prior versions to prevent repeat failures.