P0308 on 2019-2022 Chevrolet Express: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2019-2022 Chevrolet EXPRESS

The 2019-2022 Chevrolet Express vans with a P0308 code are equipped with one of two V8 engines: the 6.0L Vortec (L96) used through 2020, or the direct-injected 6.6L (L8T) introduced in 2021. For both engines, cylinder 8 is the rear cylinder on the passenger's side, which can be difficult to access. While both engines are robust, the most common causes are simple ignition system failures. However, GM has issued service bulletins for this engine family (including TSB 19-NA-219) that point to less common but possible mechanical issues like a worn camshaft lobe, sticking valve, or faulty lifter, which can also cause persistent misfires. The 6.0L L96 in the Express does not use Active Fuel Management (AFM), so AFM-related lifter failure is not a concern, though standard lifter failure can still occur.

Diagnostic Flowchart

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

Generation note: This range covers a powertrain update within the same van generation. The 2019-2020 models use the 6.0L V8 (L96), while the 2021-2022 models use the newer 6.6L V8 (L8T). The firing order for both engines is 1-8-7-2-6-5-4-3. While the fundamental causes of a P0308 misfire are the same, specific part numbers for ignition coils, spark plugs, and fuel injectors will differ between the two engines.

Symptoms You May Notice

• Rough or shaking idle.
• Hesitation or stumbling during acceleration.
• Noticeable loss of engine power.
• Flashing or solid Check Engine Light.
🎬 Watch: A quick overview of P0308 causes and fixes.
• Decreased fuel economy.
• Smell of raw fuel from the exhaust.
• Hard shifting from a stop when the misfire is active.

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Coil-on-plug ignition systems are common failure points due to constant heat cycles. It's the most likely culprit for a single-cylinder misfire and a very common failure on GM V8s.
   How to confirm: Swap the ignition coil from cylinder 8 with the coil from another cylinder (e.g., cylinder 6, which is the cylinder in front of it on the same bank). Clear the codes and drive the vehicle. If the code changes to P0306, the ignition coil is faulty.
   Typical fix: Replace the faulty ignition coil. It's a simple unbolt-and-unplug replacement, 🎬 See this walkthrough on how to replace the ignition coil. though access to cylinder 8 can be tight.
   Est. part cost: $55-$110
2. Worn or Fouled Spark Plug 🔴 High Probability
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 8. Inspect it for wear, carbon buildup, oil fouling, or damage to the electrode or insulator. A cracked porcelain insulator is a common finding. A heavily worn or dirty plug cannot produce a strong spark.
   Typical fix: Replace the spark plug. It is often recommended to replace all 8 spark plugs at the same time if they are near their service interval (around 100,000 miles for iridium plugs).
   Est. part cost: $8-$25 per plug
3. Defective Spark Plug Wire 🟡 Medium Probability The spark plug wires on the 6.0L and 6.6L V8s can degrade over time from engine heat, leading to cracks or increased internal resistance that weakens the spark. Coolant leaks from heater core lines have also been known to drip onto the #8 plug wire, causing it to fail.
   How to confirm: Visually inspect the wire for any signs of cracking, melting, or corrosion on the contacts. Check for arcing to the block in the dark with the engine running. Use a multimeter to check the wire's resistance; it should be within the manufacturer's specified range (typically a few thousand ohms per foot).
   Typical fix: Replace the faulty spark plug wire. It's good practice to replace the full set if they are original. 🎬 Watch: How to replace spark plugs and wires on Express vans.
   Est. part cost: $50-$100 for a set
4. Clogged or Faulty Fuel Injector ⚪ Low Probability
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should make a consistent clicking sound. You can also perform a fuel injector balance test with a capable scan tool. Another method is to swap the injector with another cylinder, but this is more labor-intensive. A dirty injector can sometimes be cleaned by running a high-quality fuel system cleaner like Techron through the tank.
   Typical fix: Replace the faulty fuel injector. GM TSB 20-NA-098 warns that using the exact correct part number for your VIN is critical to avoid performance issues, as injectors are flow-matched.
   Est. part cost: $70-$150

Rare But Worth Checking

• Internal Engine Mechanical Failure: GM Service Bulletin 19-NA-219 details that persistent misfires (P0300-P0308) not solved by ignition or fuel system repairs can be caused by a worn camshaft lobe, a collapsed or sticking lifter, a sticking valve, or a broken valve spring. A consistent chirp or tick at half of engine speed can indicate a worn cam lobe or lifter roller. Diagnosis requires a compression test and cylinder leak-down test.
• Wiring Harness Issue: The wiring harness leading to the ignition coil or fuel injector for cylinder 8 can become damaged from heat, chafing, or rodents. A visual inspection of the harness is recommended. An open circuit or short in the wiring can prevent the component from functioning.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner and confirm P0308 is present. Note any other codes, like P0300 or P219B, and check freeze frame data.
2. Locate cylinder 8. On GM V8s, the cylinders on the driver's side are 1, 3, 5, 7 (front to back). The passenger side cylinders are 2, 4, 6, 8 (front to back). Cylinder 8 is the rearmost cylinder on the passenger side.
3. Visually inspect the ignition components for cylinder 8. Look for loose connections, or damage to the coil, wire, and plug.
4. Swap the ignition coil from cylinder 8 to a known good cylinder (like cylinder 6). Clear the codes and run the engine. If the code changes to P0306, the coil is bad.
5. If the code remains P0308, inspect the spark plug wire for cylinder 8. Check for cracks, burns, or corrosion. Test its resistance with a multimeter.
6. If the wire is good, remove and inspect the spark plug from cylinder 8. Look for fouling, wear, or a cracked insulator. If in doubt, swap it with another cylinder's plug to see if the misfire follows.
7. Check the fuel injector on cylinder 8. Listen for a steady clicking with a stethoscope. Check the wiring harness for damage.
8. If ignition and fuel delivery seem okay, perform a compression test on cylinder 8 to check for mechanical engine problems like worn rings or valve issues.
9. If compression is low, perform a cylinder leak-down test to determine if the issue is with rings, valves, or the head gasket.

Parts You'll Likely Need

• Ignition Coil (OEM #12699382) — This is the most frequent cause of a single-cylinder misfire and is easy to diagnose by swapping cylinders. This part number is widely used on Gen IV and Gen V GM V8s.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $90-$110
  Aftermarket price range: $55-$80
• Spark Plug (OEM #6.0L (L96): 41-162 (supersedes 41-110) | 6.6L (L8T): 41-168) — Spark plugs are a routine maintenance item and can become worn or fouled, leading to a weak or inconsistent spark. It's critical to use the correct plug for your specific engine.
  Trusted brands: ACDelco, NGK
  OEM price range: $12-$25
  Aftermarket price range: $8-$18
• Fuel Injector — Can become clogged or fail electrically, preventing the correct amount of fuel from being delivered to cylinder 8. Per TSB 20-NA-098, the part number must be verified by VIN as there are multiple flow-matched injectors used.
  Trusted brands: ACDelco
  OEM price range: $100-$150
  Aftermarket price range: $70-$110

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) may be stored alongside P0308.
• P219B — This code indicates an air-fuel ratio imbalance on Bank 2 (passenger side). A misfire on cylinder 8 can cause this code to set, as the unburnt oxygen passing the O2 sensor makes the system appear to have an imbalance. Fixing the P0308 misfire will often resolve the P219B code.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: Communicates information needed to aid in diagnosing misfire DTCs P0300-P0308, suggesting a specific diagnostic template when contacting GM's Technical Assistance Center.
• 19-NA-219: Details potential mechanical causes for misfires (P0300-P0308) on this engine family, such as a worn camshaft lobe, sticking valve, or collapsed lifter, when common ignition/fuel faults are not the cause. A key symptom noted is a chirp or tick noise at half of crankshaft speed.
• 20-NA-098: Warns that installing an incorrect part number fuel injector can cause performance issues and set various DTCs, including misfire codes. It stresses the importance of using the VIN to identify the correct flow-matched injector from the parts catalog.
• 19-NA-044: Notes that on direct-injected engines like the 6.6L L8T, a special dual-pulse injection strategy on cold starts can cause a normal condition of rough idle or minimal misfires for about 60 seconds, which should not be diagnosed as a fault if no codes are set.

Platform-Specific Known Issues

• Difficulty Accessing Cylinder 8: Cylinder 8 is located at the rear of the engine on the passenger side, tucked back near the firewall. Access is tight in the Express van's engine bay, making it the hardest spark plug and coil to service. A mechanic on 2CarPros.com n

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Low-Side Supply to High-Pressure Pump) — expected: 72 psi (500 kPa) fixed, or a variable system with a relief set to 84 psi (580 kPa).. Failure: Significantly lower pressure could indicate a weak in-tank fuel pump, affecting the high-pressure system.
• Spark Plug Gap — expected: 0.040 inches for ACDelco 41-162 (6.0L L96).. Failure: An incorrect gap can lead to a weak or inconsistent spark.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Fuel Injector Balance Test — To identify a clogged or failing fuel injector by measuring pressure drop for each injector. TSB PIP5628G specifically calls for this test when diagnosing persistent misfires.
• GDS2 (GM Global Diagnostic System 2): Crankshaft Variation Learn — After certain repairs, this procedure must be performed to allow the ECM to accurately detect misfires. TSB PIP5628G suggests performing this as part of a comprehensive diagnosis.
• GDS2 (GM Global Diagnostic System 2): Misfire Graphic / Session Log — To capture a detailed log of misfire events under specific conditions (hot, cold, under load) to help diagnose intermittent issues, as recommended by GM TSB PIP5628G.

Wiring & Ground Locations

• T8H — On the right (passenger side) rocker cover, at the rear, corresponding to cylinder 8.. This is the official GM designator for the ignition coil for cylinder 8 on the L8T engine. Visually inspecting this specific component and its connector is a primary diagnostic step.
• Cylinder 8 Direct Fuel Injector Control Wire — Pin 49 of the ECM connector, typically a Green wire.. If the fuel injector is suspected, testing for a proper control signal at this pin on the ECM can determine if the fault lies with the wiring, the connector, or the ECM itself.

Real Owner Repair Stories

• Reddit user in r/ChevyTrucks (2011 Suburban (similar GM V8 architecture)) — P0308 code.
  ❌ Tried (didn't work) Initial diagnosis pointed towards common ignition component failure.
  ✅ What actually fixed it The heater core quick-connect lines were dripping coolant directly onto the #8 spark plug wire, causing it to fail. Fixing the coolant leak and cleaning the wire resolved the P0308 code.

OEM Part Supersession History

• 41-110 → 41-162 — Design change and/or supplier change (from Denso to NGK, or vice-versa). The 41-162 plug has a small 'collar' in its design.
  Heads up: While both part numbers are generally considered interchangeable for the 6.0L L96, some owners have noted cosmetic quality differences in the newer 41-162 plugs, though they function correctly. It is best practice to use the latest recommended part number.

Model Year Variations Within This Range

• 2021-2022: The engine was changed from the port-injected 6.0L V8 (L96) to the direct-injected 6.6L V8 (L8T). This changes all fuel system components (injectors, high-pressure fuel pump) and some ignition parts. The L8T uses a cast-iron block and aluminum heads designed for heavy-duty use and operates on regular 87 octane fuel despite a high 10.8:1 compression ratio.