P0308 on 2008-2015 GMC Sierra 1500: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2008-2015 Gmc SIERRA 1500

The 2008-2015 GMC Sierra 1500 V8 engines are equipped with Active Fuel Management (AFM), which deactivates cylinders to save fuel. This system is a known weak point, as the specialized AFM lifters are prone to failure. A collapsed or stuck AFM lifter is a common cause for a P0308 code on this platform, often leading to more extensive and expensive repairs than a simple ignition component failure. The failure can be caused by issues with the Valve Lifter Oil Manifold (VLOM), low oil pressure, or internal lifter pin damage.

Diagnostic Flowchart

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

Generation note: This year range covers two generations: the GMT900 (2008-2013) and the K2XX (2014-2015). While the fundamental causes of a P0308 misfire are similar, the K2XX platform introduced new EcoTec3 engines with direct injection. Both generations feature Active Fuel Management (AFM) and are susceptible to lifter failures that can cause this code. TSBs like PIP4138R and 19-NA-219 cover these issues across model years.

Symptoms You May Notice

• Rough or shaky idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing or solid Check Engine Light
• Decreased fuel economy
• Engine noise described as a tick, chirp, or squeak, especially on a cold start (if related to cam/lifter issues)
• Smell of fuel from the exhaust

Most Likely Causes

1. Faulty Spark Plug or Wire 🔴 High Probability → Shop Spark Plug TSB #PIP5159C specifically calls out loose spark plug wires as a potential cause for P0300-P0308 codes on these vehicles, sometimes due to a missing 'C' clip on the wire end. Cylinder 8's location at the rear of the engine bay can also expose it to more heat. In some cases, a coolant leak from heater core connections can drip onto the #8 plug wire, causing a misfire.
   How to confirm: Inspect the spark plug for wear, damage, or fouling. Check the spark plug wire for a secure connection at both the coil and the plug, and look for any signs of damage or a missing 'C' clip as mentioned in the TSB.
   Typical fix: Replace the spark plug and/or spark plug wire for cylinder 8. 🎬 See this walkthrough on how to change your spark plugs.
   Est. part cost: $15-$50
2. Failed Ignition Coil 🟡 Medium Probability → Shop Ignition Coil
   How to confirm: Swap the ignition coil from cylinder 8 with a coil from another cylinder (e.g., cylinder 6). Clear the codes and drive the vehicle. If the misfire code moves to the other cylinder (e.g., P0306), the coil is bad.
   Typical fix: Replace the faulty ignition coil. An OEM ACDelco coil for a 2014 Sierra is part number 12611424.
   Est. part cost: $40-$115
3. Failed AFM Lifter 🟡 Medium Probability → Shop Engine Valve Lifter Active Fuel Management (AFM) lifters are a well-documented failure point on these GM V8 engines, leading to misfires. TSBs #PIP4138R and #19-NA-219 directly link misfire codes to collapsed AFM lifters. A collapsed lifter will fail to open the valve, causing a complete lack of compression and a persistent misfire.
   How to confirm: This is an advanced diagnosis. It often starts with a compression test. If compression is low, a technician will need to inspect the valvetrain, which may involve removing the valve cover and potentially the cylinder head to access the lifters. A consistent ticking noise from the upper engine is a strong indicator. 🎬 Watch this guide to diagnosing and replacing a collapsed AFM lifter.
   Typical fix: Requires replacing the failed lifters, guides, and often the camshaft, as a failed lifter can damage the cam lobe. Many owners opt to install an AFM-delete kit to prevent future failures. The repair involves significant labor.
   Est. part cost: $300-$1000+
4. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: Listen to the injector with a mechanic's stethoscope to ensure it's clicking. You can also check the resistance with a multimeter or swap the injector with another cylinder to see if the misfire follows. A related code, P124F, may point directly to an injector circuit fault.
   Typical fix: Replace the fuel injector for cylinder 8.
   Est. part cost: $50-$150

Rare But Worth Checking

• Worn Camshaft Lobe: → Shop Engine Camshaft Often occurs in conjunction with a failed lifter. TSB #PIP4138R notes that a worn camshaft lobe can cause misfire codes P0300-P0308. If a lifter has failed, the camshaft should always be inspected, which may require a borescope or physical removal.
• Low Engine Compression: Can be caused by internal engine issues like worn piston rings, a bent pushrod, or a bad head gasket, though less common than lifter failure for a single-cylinder misfire on this platform.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the cylinder 8 intake runner can lean out the air/fuel mixture and cause a misfire.

Diagnosis Steps

1. Read the codes with an OBD-II scanner to confirm P0308 and check for any other codes like P0300 or P050D.
2. Inspect the spark plug wire for cylinder 8. Ensure it is securely connected and not damaged. Per TSB #PIP5159C, check for a missing 'C' clip on the wire end that connects to the coil.
3. Remove and inspect the spark plug from cylinder 8. Look for signs of wear, fouling (oil or fuel), or damage. Replace if necessary.
4. To test the ignition coil, swap the coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and drive. If the code changes to P0306, the coil is faulty and needs replacement.
🎬 Watch: How to properly install a new ignition coil.
5. If the misfire persists, check the fuel injector. Listen for a steady clicking sound. You can also test its resistance or swap it with another cylinder's injector to see if the problem moves.
6. Listen for a distinct ticking, chirping, or squeaking noise from the engine, which could indicate a valvetrain problem as described in TSB #19-NA-219.
7. If ignition and fuel components are good, suspect a mechanical issue. Perform a compression test on cylinder 8 and compare it to a known good cylinder.
8. If compression is low, the problem is likely internal. Given the history of these engines, a failed AFM lifter is a strong possibility. This requires removing the valve cover and potentially the cylinder head for inspection, a job best left to experienced mechanics.

Parts You'll Likely Need

• Spark Plug — Worn or fouled spark plugs are a primary cause of single-cylinder misfires.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$20
  Aftermarket price range: $5-$15
• Ignition Coil (OEM #12611424 (GMT900), 12699383 (K2XX)) — A failed ignition coil cannot provide enough voltage to create a spark, leading to a misfire. Heat in the cylinder 8 position can contribute to failure.
  Trusted brands: ACDelco, Delphi
  OEM price range: $60-$120
  Aftermarket price range: $40-$80
• Spark Plug Wire — A damaged or loose wire can prevent spark from reaching the plug. TSB #PIP5159C specifically mentions this as a known issue.
  Trusted brands: ACDelco
  OEM price range: $15-$30
  Aftermarket price range: $10-$20
• AFM/DOD Valve Lifter Kit (OEM #12645725 (Superseded), 12740071) — This is the most common major mechanical failure causing a P0308 on this platform. A collapsed lifter prevents the valve from opening correctly.
  Trusted brands: ACDelco, Melling
  OEM price range: $300-$500
  Aftermarket price range: $200-$400

Related Codes That Often Appear With This One

• P0300 — P0300 indicates a random or multiple cylinder misfire. It can appear alongside P0308 if the misfire is intermittent, starting to affect other cylinders, or if the underlying cause (like a failing VLOM) is systemic.
• P050D — TSB #PIP5628G links this code (Cold Start Rough Idle) with misfire codes P0300-P0308, suggesting a complex diagnostic situation that may require dealer assistance. It can sometimes be caused by leaking fuel injectors.
• P124F — This code indicates 'Injector 8 Positive Voltage Control Circuit Shorted to Control Circuit' and points directly to a problem with the fuel injector or its wiring harness for cylinder 8, as seen in a forum discussion.

Technical Service Bulletins (TSBs) & Recalls

• PIP5159C: Addresses misfire codes P0300-P0308 caused by loose spark plug wires, potentially from a missing 'C' clip on the coil-side terminal.
• 19-NA-219 (replaces PIP4138R): Details diagnosis for engine noise (tick, chirp, squeak) and misfires (P0300-P0308) that may be caused by a worn camshaft lobe or a collapsed AFM lifter.
• PIP4138R: Predecessor to 19-NA-219, linking misfires and engine noises to worn camshaft lobes and/or lifter rollers.
• PIP5628G: Relates cold start rough idle (P050D) to misfire codes and suggests a complex diagnostic path.

Platform-Specific Known Issues

• Active Fuel Management (AFM) Lifter Failure: The most significant issue for these trucks. The lifters used on AFM cylinders are prone to collapsing or getting stuck. This is a well-documented problem that GM has addressed with multiple TSBs, including #PIP4138R and #19-NA-219. A failure often requires replacing the lifters, lifter guides, and sometimes the camshaft. One owner on a forum reported a $6,500 dealer repair for a collapsed lifter on cylinder 8 that included a new camshaft.
• Loose Spark Plug Wires: TSB #PIP5159C highlights that a loose spark plug wire, potentially due to a missing 'C' clip on the wire end, is a known cause for misfires on these trucks.

Mechanic-Grade Diagnostic Values

• Ignition Coil Secondary Resistance — expected: 6.1K Ohms. Failure: A reading significantly higher or lower than the specification, or an open circuit (OL).
• Ignition Coil Primary Resistance — expected: 0.660 Ohms. Failure: A reading significantly different from the specification.
• Fuel Injector Resistance (GMT900) — expected: 11 to 14 Ohms. Failure: A reading outside of this range indicates a faulty injector coil. A 2005 Silverado with a bad injector measured 4.61 Ohms while good injectors were around 14.35 Ohms.
• Fuel Injector Resistance (K2XX - Direct Injection) — expected: Direct injection injectors are high-voltage and are not typically tested with a standard multimeter for resistance. Diagnosis relies on scan tool data and specialized tools.. Failure: N/A for multimeter test.

Scan Tool Commands That Help

• GDS2/Tech2: Cylinder Power Balance Test — To graphically see the contribution of each cylinder to crankshaft speed, helping to confirm that cylinder 8 is the one underperforming.
• GDS2/Tech2: AFM Cylinder Deactivation — A technician can command the AFM system on and off to see if the misfire or associated engine noise changes, which can help isolate the issue to the AFM lifters or VLOM.
• GDS2/Tech2: Misfire History / Misfire Graphic — To view current and historical misfire counts for each cylinder. This can show if the misfire on cylinder 8 is consistent or intermittent, and if other cylinders are beginning to show misfires under certain conditions.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine.. This ground is specifically for the 8 ignition coils. A poor connection here could cause weak spark or intermittent ignition issues on any or all cylinders, including cylinder 8.
• G103 — On the front of the right side (passenger side) cylinder head on some models.. This is a primary ground for the Engine Control Module (ECM). A faulty ECM ground can cause a wide range of erratic engine behavior and codes, including misfires.
• Cylinder 8 Fuel Injector Connector — Top of the right cylinder bank, at the rear-most position.. This is the connection point for the cylinder 8 injector. The wiring harness in this area can be subject to heat and vibration, potentially leading to a poor connection or damaged wire causing a fuel-related misfire.
• ECM Connector X2 (2014 K2XX) — At the Engine Control Module, left front of engine compartment.. Pin 68 (L-GN/BK wire) is the control circuit and Pin 49 (L-GN/GY wire) is the high voltage supply for the cylinder 8 direct fuel injector. These pins would be checked for voltage/signal during an advanced electrical diagnosis.

Real Owner Repair Stories

• ScannerDanner Forum user (2016 GMC Sierra 5.3L) — Flashing P0308 check engine light.
  ❌ Tried (didn't work) A dealership replaced the camshaft and lifters for $6,500 to fix a collapsed lifter on cylinder 8.
  ✅ What actually fixed it The P0308 code returned just two days after the major repair. The owner's father, using a scan tool, noted the bank 2 catalytic converter was 50 degrees colder than bank 1, suggesting the converter may have been damaged by the original misfire or that a valve issue was missed during the initial repair. The final resolution was not posted, but it highlights that lifter replacement may not be the end of the story.
• Reddit user on r/gmcsierra (2008 GMC Sierra) — Misfire and occasional stalling only at idle after performing an AFM delete and mild cam swap.
  ❌ Tried (didn't work) Mechanical checks for compression and fuel delivery were reportedly fine.
  ✅ What actually fixed it The issue was unresolved in the thread, but the user suspected a fuel timing issue, a bad sensor, or a clogged injector. This story is a cautionary tale that performing an AFM delete can introduce new, complex idle-only misfire issues if the corresponding ECM tune is not perfect.

When the Usual Fixes Don't Work

• While a failed AFM lifter is a very common cause for P0308, simply replacing the lifters and camshaft is not a guaranteed final fix. In one documented case, a 2016 Sierra had the P0308 code return just two days after a $6,500 lifter and cam replacement. The persistent misfire had likely damaged the catalytic converter on bank 2, which was running significantly cooler, indicating it was no longer functioning. This shows that secondary damage from the initial misfire must also be diagnosed and addressed.
• Performing an AFM delete is often suggested as a permanent fix for lifter issues. However, this can introduce new problems. One owner reported a persistent idle misfire after an AFM delete and cam swap, which drove perfectly under load. This suggests that the required ECM tuning to disable the AFM system is critical and can cause its own set of misfire problems if not calibrated perfectly for the new engine hardware.

OEM Part Supersession History

• HL-7011 (Elgin aftermarket equivalent for original AFM lifter) → HL-7025 (Elgin aftermarket equivalent for DFM lifter) — GM released a new lifter design for the newer Dynamic Fuel Management (DFM) engines starting in 2019.
  Heads up: The newer DFM lifter (HL-7025) is backwards compatible and can be used in older AFM engines. However, the original AFM-only lifter (HL-7011) will NOT work in the newer DFM engines. For a 2008-2015 model, either the original AFM style or the newer DFM style lifter can be used.

Model Year Variations Within This Range

• 2008-2013 (GMT900): These models use V8 engines (like the 5.3L LC9/LMG/LY5) with Active Fuel Management and standard multi-port fuel injection. Diagnosis for fuel issues involves checking traditional injectors with a multimeter for resistance (11-14 Ohms).
• 2014-2015 (K2XX): These models introduced the EcoTec3 engine family (like the 5.3L L83), which features Direct Injection (DI) in addition to AFM. This makes fuel system diagnosis more complex, as the injectors are high-pressure, high-voltage components controlled by a separate driver module. They cannot be tested with a simple multimeter.