P0304 on 2012-2019 Jeep Wrangler: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2012-2019 Jeep WRANGLER

While any vehicle can have a misfire, the 2012-2013 Jeep Wrangler with the 3.6L Pentastar V6 is notorious for a specific defect. The left-side cylinder head (affecting cylinders 2, 4, and 6) was prone to excessive valve seat wear, causing poor sealing and misfires. This problem was so common that Chrysler issued Technical Service Bulletin (TSB) #09-002-14 Rev. B and a warranty extension (X56) to cover the repair, which involves replacing the entire cylinder head. Therefore, on these specific model years, P0304 should be investigated with much greater suspicion than a simple tune-up issue. Additionally, the entire 3.6L Pentastar family is known for rocker arm bearing failure, which can also present as a single-cylinder misfire accompanied by a distinct ticking sound.

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 late Jeep Wrangler JK (2012-2018) and the early JL (2018-2019). The most significant issue, a defective left cylinder head, primarily affects 2012-2013 JK models with the 3.6L Pentastar engine built on or before July 25, 2012. Later JK and JL models are not prone to this specific head failure but can still experience P0304 from other common causes, with rocker arm failure being a prominent issue across all years.

Symptoms You May Notice

• Rough idle. [⭐ Bulletin #09-011-25]
• Flashing or solid Check Engine Light
• Noticeable loss of engine power and acceleration
• Shaking or vibrating from the engine
• Hesitation during acceleration
• Unpleasant fuel smell from the exhaust
• Audible ticking noise from the top of the engine (the "Pentastar 🎬 See this shortcut for fixing the common Pentastar engine tick.tick"), which is a strong indicator of rocker arm failure.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability
   How to confirm: Swap the ignition coil from cylinder 4 with another cylinder (e.g., cylinder 2, which is also on the driver's side). Clear the codes and drive. If the code changes to P0302, the coil is bad.
   Typical fix: Replace the faulty ignition coil.
   Est. part cost: $25-$60
2. Worn or Fouled Spark Plug 🔴 High Probability A leaking valve cover gasket can cause oil to pool in the spark plug tube, fouling the plug and causing a misfire.
   How to confirm: After confirming the coil is good, inspect the spark plug for wear, carbon buildup, or oil contamination. You can also swap the plug to another cylinder to see if the misfire follows.
   Typical fix: Replace the spark plug. If oil is present, replace the valve cover gasket and spark plug tube seals as well.
   Est. part cost: $10-$25
3. Failed Left Cylinder Head (2012-2013 Models) 🔴 High Probability Early 3.6L Pentastar engines used cylinder heads (often identified by part numbers ending in 'AA' or 'AB') with improperly hardened valve guides and seats, leading to premature wear and loss of compression on the left bank. TSB #09-002-14 Rev. B directly links P0304 to this issue.
   How to confirm: If basic ignition/fuel swaps don't work, a cylinder leak-down test is required. If cylinder 4 shows leakage of 25% or more, the head is faulty. Air heard escaping from the exhaust is a strong indicator of a bad exhaust valve. A user on the Jeep Wrangler JK Forum confirmed their P0304 was a 35-40% leakdown due to an exhaust valve leak.
   Typical fix: Replace the left cylinder head assembly with the updated part (part numbers ending in 'AC' or newer, such as RL141353AC or 68141353AC).
   Est. part cost: $800-$1500
4. Failed Rocker Arm / Lifter Medium-high Probability The 3.6L Pentastar is widely known for valvetrain issues where the needle bearings in a rocker arm fail, causing the roller to collapse. This creates excessive lash, preventing the valve from opening fully and leading to a misfire.
   How to confirm: A distinct ticking or tapping sound from the upper engine is the primary symptom. As one forum member noted, the sound can be obvious enough for a non-mechanic to notice from 50 feet away. The definitive diagnosis requires removing the valve cover to visually inspect the rocker arms for play. A collapsed rocker arm will have noticeable slop.
   Typical fix: Replace all rocker arms and lifters on the affected bank. It is highly recommended to replace all 24 rocker arms as a preventative measure. Inspect the camshaft lobes for scoring or wear.
   Est. part cost: $150-$300 for a full set of 24 rocker arms
5. Faulty Fuel Injector 🟡 Medium Probability
   How to confirm: If the misfire remains on cylinder 4 after swapping the coil and plug, the next step is to swap the fuel injector with another cylinder. This is more involved as it requires removing the upper intake manifold and fuel rail.
   Typical fix: Replace the faulty fuel injector. It is often recommended to replace all injectors on that bank at the same time.
   Est. part cost: $40-$100

Rare But Worth Checking

• Low Compression / Burnt Valve: Even on later models not affected by the head defect, an individual valve can fail due to carbon buildup or other issues. A compression test followed by a leak-down test will confirm this mechanical engine problem.
• Wiring Harness Issue: The wiring to the ignition coil or fuel injector for cylinder 4 could be damaged or have a loose connection. This can be caused by chafing against other components or rodent damage. A visual inspection of the harness is necessary.

Diagnosis Steps

1. Read the codes with an OBD-II scanner. Confirm P0304 is present and check for any other codes like P0300, P0302, P0306, or P0204.
2. Listen to the Engine: Start the engine and listen for a distinct, rhythmic ticking sound from the driver's side valve cover. This 'Pentastar tick' is a strong indicator of a failed rocker arm.
3. Identify Cylinder 4: It is on the driver's side (left bank), the middle cylinder.
🎬 Watch: Step-by-step maintenance for a Jeep JK cylinder 4 misfire.
4. Swap the Ignition Coil: Swap the coil from cylinder 4 with the coil from cylinder 2 (front cylinder, same bank). Clear codes and test drive. If the code changes to P0302, the coil is bad.
5. Swap the Spark Plug: If the misfire remains on cylinder 4, move the spark plug from cylinder 4 to cylinder 2. If the code moves to P0302, the plug is bad.
6. Inspect for Oil Fouling: When the plug is out, check the spark plug tube for engine oil. If oil is present, the valve cover and tube seals are leaking and must be replaced.
7. Check the Fuel Injector: If the misfire still stays on cylinder 4, the next step is to check the fuel injector. This can be done by swapping it with another cylinder, but this is a more advanced job requiring removal of the upper intake.
8. Perform Compression and Leak-Down Tests: If ignition and fuel component swaps do not solve the issue, a mechanical problem is likely. Perform a dry/wet compression test on cylinder 4. Follow up with a cylinder leak-down test. Per TSB #09-002-14, leakage of 25% or more on 2011-2013 models indicates a faulty cylinder head requiring replacement. A leak-down test will also confirm a burnt valve on later models.
9. Inspect Valvetrain: If a ticking noise is present, remove the driver's side valve cover and inspect all rocker arms for excessive play by wiggling them. A failed rocker arm bearing will allow the arm to move noticeably.

Parts You'll Likely Need

• Ignition Coil (OEM #5149168AH) — A very common failure point for misfires on most modern engines.
  Trusted brands: Mopar, NGK, Delphi
  OEM price range: $40-$60
  Aftermarket price range: $25-$45
• Spark Plug (OEM #SP149125AE) — A standard maintenance item that wears out and is a primary cause of misfires.
  Trusted brands: Champion, NGK
  OEM price range: $15-$25
  Aftermarket price range: $8-$15

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 (Random/Multiple Misfire) may be stored alongside P0304. TSB #09-002-14 Rev. B lists these codes together.
• P0302, P0306 — Because the cylinder head defect affects the entire left bank (cylinders 2, 4, 6), it's common to see misfire codes for these other cylinders appear with P0304.
• P0204 — This code indicates a fault in the fuel injector circuit for cylinder 4. It often appears with P0304 if the issue is a bad injector or a problem with its wiring connector.

Technical Service Bulletins (TSBs) & Recalls

• 0900214a / 0900214: Addresses MIL illumination for P0300, P0302, P0304, or P0306 on 2012-2013 models. It outlines checking for excessive cylinder leakage and replacing the left cylinder head under the X56 Warranty Extension.
• 09-011-25: A general TSB for 2019 engines that lists P0304 as 'Cylinder 4 Misfire' and notes 'Rough idle' as a potential customer complaint. [⭐ Bulletin #09-011-25]

Platform-Specific Known Issues

• Warranty Extension X56 for Left Cylinder Head: For 2012-2013 models, Chrysler extended the warranty on the left cylinder head to 10 years or 150,000 miles due to this widespread defect. Owners experiencing P0302, P0304, or P0306 should have contacted a dealer to check for coverage, though this period has likely expired for most vehicles.
• Rocker Arm/Lifter Failure: The 3.6L Pentastar engine is also known for valvetrain tick caused by premature failure of rocker arm bearings, which can lead to misfires if the valve is no longer opening correctly. This is a common repair across all model years covered.

Mechanic-Grade Diagnostic Values

• Fuel Injector Resistance — expected: 12 Ohms to 16 Ohms. Failure: A reading outside of this range, particularly an open circuit (infinite resistance) or a value below 12 Ohms, indicates a faulty injector.
• Ignition Coil Primary Resistance — expected: ~0.6 Ohms. Failure: A significant deviation from this value can indicate a failing coil.
• Cylinder Compression — expected: 120-150 PSI, with less than 25% variation between cylinders.. Failure: A reading significantly lower than other cylinders, such as 75 PSI as noted in one owner's report, points to a serious mechanical issue like a bad valve or piston ring failure.

Scan Tool Commands That Help

• wiTECH: OBD II Monitors -> 'Which cylinder is misfiring' — This function is used to view live misfire counts for each individual cylinder. It is especially useful for diagnosing a P0300 (Random Misfire) to pinpoint which cylinder is the primary offender, or to confirm the misfire is isolated to cylinder 4 when a P0304 is present.

Wiring & Ground Locations

• Engine Ground Strap — A common ground strap runs from the passenger side of the engine block (often near the catalytic converter) to the firewall or frame.. A broken or corroded main engine ground strap can cause a wide range of electrical issues, including weak spark, incorrect sensor readings, and misfires. One owner reported multiple O2 sensor heater circuit codes (P0032, P0038, P0052, P0058) and a battery overcharging issue that were resolved by fixing a missing ground. While not directly for cylinder 4, it shows the importance of good grounds for overall engine electronic stability.
• G201 / G203 — Under the kick panel in the passenger footwell.. These are primary interior grounding points. While less likely to cause a specific engine misfire, they are important for the overall electrical system health and could be a factor in chasing down intermittent electrical gremlins.

Real Owner Repair Stories

• Reddit user on r/MechanicAdvice (2012 Jeep Wrangler, 55K miles) — Check engine light with code P0304.
  ❌ Tried (didn't work) Replacing fuel injector #4
  ✅ What actually fixed it The issue was not the injector. A compression test showed only 75 PSI and a leak-down test showed 57% leakage, pointing to the well-known cylinder head defect for that model year. The final fix required replacing the cylinder head.
• JL Wrangler Forums user (2019 Jeep Wrangler JL 3.6L) — P0304, rough idle on cold start, P0300 also appeared later.
  ❌ Tried (didn't work) Replaced all spark plugs and ignition coils under the intake., Replaced fuel injector #4.
  ✅ What actually fixed it The user reported the #4 spark plug was wet with fuel and that holding paper to the exhaust on startup would have it sucked in, indicating a potential sticking valve. While the user did not post the final confirmed repair, the symptoms strongly point towards a valvetrain or internal engine mechanical issue that was not resolved by replacing common ignition and fuel parts.

OEM Part Supersession History

• 5184296AD, 5184296AE, 5184296AF, 5184296AG → 5184296AH — This is the part number for the engine valve rocker arm, a notorious failure point in the Pentastar engine. The multiple supersessions indicate revisions to address the needle bearing failure that leads to the 'Pentastar tick' and misfires.
  Heads up: While all versions may fit, using the latest 'AH' revision is highly recommended when replacing any rocker arms to prevent repeat failures. It is advised to replace all 12 on the affected bank, not just the one that has failed.