P0308 on 2017-2020 GMC Sierra Denali: Cylinder 8 Misfire Causes & Fixes

What's Unique About the 2017-2020 Gmc SIERRA DENALI

The 2017-2020 GMC Sierra Denali, equipped with 5.3L and 6.2L V8s, features Active Fuel Management (AFM) or Dynamic Fuel Management (DFM). The specialized valve lifters used in these systems are a well-documented, widespread failure point. A collapsed or stuck lifter is a very common cause for a P0308 code on this platform, often turning what would be a simple ignition issue on other vehicles into a major mechanical repair that may require camshaft replacement. This issue is so prevalent that it has led to multiple class-action lawsuits against GM.

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 K2XX platform (2017-2018) and the beginning of the T1XX platform (2019-2020). Both generations use Gen V EcoTec3 V8 engines (5.3L and 6.2L) and are known to be susceptible to the same AFM/DFM lifter-related failures that cause misfires. TSBs like 19-NA-219 cover models from both generations, indicating a persistent issue.

Symptoms You May Notice

• Rough or shaking engine idle
• Engine hesitation or loss of power during acceleration
• Flashing or steady Check Engine Light
• Engine noise described as a 'tick', 'chirp', or 'squeak', often more noticeable on a cold start
• Vehicle lurching or shuddering

Most Likely Causes

1. Failed AFM/DFM Lifter 🔴 High Probability → Shop Engine Valve Lifter The Active/Dynamic Fuel Management systems in the 5.3L and 6.2L V8s use specialized lifters that are a known, widespread failure point. The internal locking pin or spring can break, causing the lifter to collapse or become stuck. This is a systemic issue across GM's V8 truck lineup.
   How to confirm: After ruling out ignition and fuel issues, a persistent ticking noise is a strong indicator. Diagnosis requires removing the valve cover to inspect for bent pushrods or collapsed lifters. A technician may measure valve lift or perform a relative compression test with a PicoScope as outlined in TSB PIP5628G.
   Typical fix: Requires extensive engine work to replace the lifters on the affected bank of cylinders. Due to the high failure rate, many mechanics and GM itself recommend replacing all 16 lifters (both AFM/DFM and standard) and the lifter guides. The camshaft must also be inspected for damage and replaced if any lobes are worn, which is a common consequence.
   Est. part cost: $400-$1500
2. Faulty Ignition Coil or Spark Plug 🔴 High Probability → Shop Ignition Coil Heat and vibration can cause ignition components to fail. Cylinder 8 is at the rear of the engine bay on the passenger side, a location prone to high heat and potential fluid leaks from above.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6). Clear the codes and drive the vehicle. If the misfire code changes to P0306, the coil is bad. If not, inspect the spark plug for wear, fouling, or incorrect gap. A user on Reddit noted their P0308 started after replacing the coil, suggesting the next step is the plug and wire.
   Typical fix: Replace the faulty ignition coil or all 8 spark plugs. It is best practice to replace all spark plugs at once, especially if near the 100,000-mile service interval.
   Est. part cost: $20-$150
3. Worn Camshaft Lobe 🟡 Medium Probability → Shop Engine Camshaft This is almost always a direct result of a failed lifter. The roller on the lifter stops spinning and grinds against the camshaft lobe, causing rapid wear. TSB 19-NA-219 (which supersedes PIP4138R) explicitly lists this as a cause for misfires from P0300-P0308.
   How to confirm: This can only be confirmed by physically inspecting the camshaft after removing the lifters, which is a labor-intensive job. A technician might first measure cam lobe lift at the rocker arm to spot a discrepancy.
   Typical fix: Replace the camshaft, all 16 lifters, and associated gaskets and seals. This is a major engine repair.
   Est. part cost: $300-$800
4. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: After ruling out ignition issues, a technician can perform a fuel injector balance test using a GM diagnostic tool (GDS2) to check flow rates. Swapping injectors between cylinders is possible but labor-intensive on these direct-injection engines.
   Typical fix: Replace the faulty fuel injector.
   Est. part cost: $50-$120

Rare But Worth Checking

• Coolant Leak onto Ignition Components: Owners have reported leaks from the heater core quick-connect fittings on the firewall dripping directly onto the cylinder 8 ignition coil and spark plug wire, causing corrosion and misfires. A Reddit user on an r/ChevyTrucks thread specifically cited this as the cause for their P0308 on a similar GM truck.
• Engine Harness Chafing: TSB #21-NA-149 notes that on 2019-2022 models, the engine wiring harness can rub against specific points like the ECM/TCM bracket, the upper control arm, or a shock tower bolt, potentially shorting the wires for the cylinder 8 ignition coil or injector.
• Broken Valve Spring: TSB 19-NA-219 also lists a broken valve spring as a potential cause for a single-cylinder misfire. This can be intermittent and may only occur at higher RPMs. Diagnosis requires removing the valve cover for inspection.

Diagnosis Steps

1. Check if the Check Engine Light is flashing. If so, avoid driving to prevent catalytic converter damage.
2. Use a scan tool to confirm P0308 and check for any other stored codes like P0300 or P050D.
3. Swap the ignition coil from cylinder 8 with an adjacent cylinder (like 6). Clear the codes and drive the vehicle. If the code changes to P0306, the ignition coil is faulty.
4. If the misfire remains on cylinder 8, remove and inspect the spark plug for wear, damage, or fouling. Replace if necessary. Check the spark plug wire for damage.
5. Listen to the fuel injector on cylinder 8 with a mechanic's stethoscope to ensure it is clicking. Compare the sound to a known good injector.
6. Inspect for external issues: Check the heater core connections on the firewall for coolant leaks dripping onto the #8 coil pack area. Also, inspect the engine wiring harness for chafing at the points identified in TSB 21-NA-149.
7. If ignition and fuel checks do not reveal the cause, listen for a persistent ticking noise from the engine, which strongly suggests a valvetrain problem.
8. Perform a cylinder compression test and a cylinder leak-down test. Low compression on cylinder 8 points to a mechanical issue like a bent valve, broken valve spring, or a failed lifter that is not allowing the valve to open.
9. If a mechanical issue is suspected, refer to TSBs 19-NA-219 and PIP5628G. The next step is to remove the valve cover for cylinder 8 to inspect the pushrods and valve springs, and ultimately, remove the cylinder head to inspect the lifters and camshaft.

Parts You'll Likely Need

• AFM/DFM Valve Lifter Kit (OEM #12698946 (Individual AFM/DOD Lifter)) — This is the most common major failure causing a P0308 code on this platform. A failed lifter must be replaced, and it's recommended to replace them as a set.
  Trusted brands: ACDelco, General Motors, Delphi
  OEM price range: $300-$600
  Aftermarket price range: $200-$450
• Ignition Coil (OEM #12699383) — A common and simple cause of a single-cylinder misfire. It's a primary diagnostic step to check and replace this part.
  Trusted brands: ACDelco, NGK
  OEM price range: $40-$70
  Aftermarket price range: $25-$50
• Spark Plug (OEM #41-166 (Iridium)) — Spark plugs are a routine maintenance item and a frequent cause of misfires. A fouled or worn plug on cylinder 8 will trigger this code.
  Trusted brands: ACDelco, NGK
  OEM price range: $8-$15
  Aftermarket price range: $5-$10
• Camshaft (OEM #12689035 (Example for L86)) — Often required if a lifter has failed and damaged a lobe. The camshaft must be inspected whenever lifters are replaced.

Related Codes That Often Appear With This One

• P0300 — A P0300 (Random/Multiple Misfire) code often accompanies or precedes a single-cylinder misfire code as the underlying problem (like a failing lifter) begins to affect engine stability. Multiple TSBs group P0300 with P0301-P0308.
• P050D — This code for 'Cold Start Rough Idle' is mentioned in TSB #PIP5628G in conjunction with misfire codes P0300-P0308, indicating a related diagnostic path that often points toward valvetrain or lifter issues.

Technical Service Bulletins (TSBs) & Recalls

• TSB 19-NA-219: Supersedes PIP4138R. Addresses engine misfire (P0300-P0308) and/or engine noises like chirp, squeak, or tick. Points to potential causes like worn camshaft lobes, collapsed AFM lifters, sticking valves, or broken valve springs.
• TSB 21-NA-149: Details how the engine wiring harness can chafe against brackets or the shock tower, causing various electrical issues including misfires. Provides specific inspection points.
• TSB PIP5628G: Provides diagnostic aids for technicians dealing with misfires (P0300-P0308) and/or P050D (Cold Start Rough Idle), recommending the use of tools like a PicoScope for relative compression tests.

Platform-Specific Known Issues

• AFM/DFM Lifter Failure: → Shop Engine Valve Lifter The most prominent issue for the 2017-2020 Sierra Denali V8s is the failure of the valve lifters associated with the cylinder deactivation system. These lifters can become mechanically stuck or collapse internally, leading to a misfire and often a distinct ticking noise. The failure can also cause the pushrod to bend and, in severe cases, lead to the destruction of the camshaft lobe, necessitating a much larger repair. This problem is the subject of numerous TSBs and class-action lawsuits.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.4 to 2.0 ohms. Failure: A reading outside this range suggests a faulty coil winding.
• Ignition Coil Secondary Resistance — expected: 6,000 to 15,000 ohms (6.0 kΩ–15.0 kΩ). Failure: A reading outside this range, especially an open circuit (infinite resistance), indicates a failed coil.
• Fuel Injector Coil Resistance (5.3L V8) — expected: Around 12.4 to 14.6 ohms. Failure: A reading significantly outside this range suggests a faulty injector coil.

Hidden / Shadow Codes Worth Checking

• Mode $06 Misfire Counter: This function on advanced scan tools shows misfire counts per cylinder, even for misfires not yet frequent enough to set a P030X code. It can confirm an intermittent misfire on cylinder 8 before it becomes constant. (see via Advanced OBD-II Scan Tool or GM GDS2 software. Look for 'Misfire History' or 'Misfire Graphic' data PIDs.)

Scan Tool Commands That Help

• GM GDS2: Cylinder Power Balance Test — This active test disables one cylinder at a time and measures the RPM drop. If disabling cylinder 8 causes little or no change in RPM, it confirms that cylinder is not contributing power, helping to isolate the fault without disassembly.
• GM GDS2: Fuel Injector Balance / Kill — After confirming a power loss on cylinder 8 with the power balance test, this function can disable just the fuel injector. If there's no additional change in engine operation, it strongly suggests the injector was the non-functioning component.

Wiring & Ground Locations

• Engine to Firewall Ground Strap — Runs from the back of the passenger-side cylinder head or engine block to the vehicle's firewall.. A corroded or loose ground strap at the back of the engine can cause a host of electrical issues, including weak spark or erratic sensor readings that could contribute to a misfire on the rearmost cylinder, #8.
• G110 / G114 — On 2019-2020 T1XX models, these are key ground points for the body and engine harnesses.. GM has an engineering inquiry (PIE0548) for loose connections at these specific grounds, which can cause various electrical faults. A poor ground for the engine harness (G114) could affect the ignition or injector circuit for cylinder 8.
• PCM Ground — A ground wire from the main wiring harness connects to the top rear of the engine block.. This is a critical ground for the Powertrain Control Module (PCM). If this wire is frayed or broken, it can cause erratic sensor codes and misfires as the computer receives unreliable data.

Real Owner Repair Stories

• ScannerDanner Forum (2016 GMC Sierra 5.3L) — Flashing P0308 check engine light.
  ❌ Tried (didn't work) A dealership replaced the camshaft and all lifters for $6500 under warranty.
  ✅ What actually fixed it The P0308 code returned two days after the major lifter/cam replacement. The owner's father, a mechanic, suspected a bent or sticking valve was missed during the initial repair, or that the catalytic converter on that bank was damaged by the original misfire. The final resolution was not posted, but highlights that even a full lifter job may not be the fix if other damage occurred.
• Reddit r/gmcsierra (2018 Sierra 1500) — P0308 misfire, especially when cold and raining/damp.
  ❌ Tried (didn't work) Replacing all spark plugs., Replacing all spark plug wires.
  ✅ What actually fixed it The final repair was replacing the fuel injector for cylinder 8. The owner reported the dealership found 'a ton of buildup in the cylinder' and also claimed the spark plug wire wasn't seated correctly. The final cost was around $500.

OEM Part Supersession History

• 12639516, 12645725, 12680871 → 12698946, now 12740071 — GM has updated the AFM/DOD lifter design multiple times to improve reliability and address high failure rates.
  Heads up: While all are intended for AFM/DOD systems, it is critical to use the latest specified part number (currently 12740071) when performing repairs to get the most updated design.

Model Year Variations Within This Range

• 2017-2018 (L83/L86 engines): These models use Active Fuel Management (AFM), which deactivates a fixed bank of 4 cylinders (including #8) under light load.
• 2019-2020 (L84/L87 engines): These models use Dynamic Fuel Management (DFM), a more advanced system that can deactivate any number of cylinders in 17 different patterns. While the lifters are still a failure point, the system operates differently. The 2019+ T1XX platform also has different ground locations (G110/G114) that are noted as potential problem areas.