P0308 on 2012-2016 GMC Sierra Denali: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2012-2016 Gmc SIERRA DENALI

The 2012-2016 GMC Sierra Denali, spanning the GMT900 and K2XX platforms, primarily uses the 5.3L and 6.2L V8 engines. A notable issue, confirmed by GM Technical Service Bulletin PIP5159A, is the tendency for spark plug wires to become loose at the ignition coil due to a missing 'C' clip on the wire end, leading to misfires. Cylinder 8 is located at the rear passenger side of the engine, a location prone to high heat and, as reported by owners, potential coolant drips from heater core fittings that can damage ignition components. Furthermore, on higher mileage engines, issues with the Active Fuel Management (AFM) system's lifters or worn camshaft lobes can manifest as a persistent P0308 code after more common ignition-related causes have been ruled out.

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 second generation (GMT900, 2012-2013) and the beginning of the third generation (K2XX, 2014-2016). The engines (5.3L and 6.2L V8s) are similar, but the K2XX models feature the updated EcoTec3 direct-injection engines. While the fundamental causes for P0308 are the same, TSBs specifically mention issues like worn camshafts and loose plug wires across these years.

Symptoms You May Notice

• Rough or shaking engine idle
• Loss of power and sluggish acceleration
• Flashing or solid Check Engine Light
• Hesitation or stumbling during driving
• Reduced fuel economy
• Noticeable fuel smell from the exhaust
• "Stabilitrak off" or "Service Traction Control" message appearing, especially at idle

Most Likely Causes

1. Faulty or Worn Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a regular maintenance item. Cylinder 8 is located at the rear of the engine on the passenger side, where it can be exposed to more heat, potentially shortening plug life.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it for wear, fouling (black, oily, or white deposits), or damage such as a cracked ceramic insulator.
   Typical fix: Replace the spark plug. It is highly recommended to replace all 8 spark plugs at the same time for even performance. The OEM recommended part is an ACDelco Iridium plug. 🎬 Watch: How to replace the cylinder 8 spark plug.
   Est. part cost: $8-$25 per plug
2. Loose or Faulty Spark Plug Wire 🔴 High Probability → Shop Spark Plug GM has issued multiple Technical Service Bulletins (TSBs) for this exact issue. Bulletin #PIP5159A specifically mentions loose spark plug wires at the ignition coil, sometimes due to a missing 'C' clip on the wire end, as a direct cause for P0300-P0308 codes.
   How to confirm: Visually inspect the spark plug wire for cylinder 8 for any signs of damage, arcing (burn marks), or looseness. Firmly pull and push on the boot at the ignition coil to ensure the 'C' clip is engaged and it 'clicks' securely into place.
   Typical fix: Reseat the wire firmly. If the connection is still loose or the wire is damaged, replace the spark plug wire set. ACDelco is the OEM brand.
   Est. part cost: $60-$120 for a set
3. Defective Ignition Coil 🟡 Medium Probability → Shop Ignition Coil Heat and vibration can cause ignition coils to fail over time. Cylinder 8's rear position can subject the coil to significant heat soak. Note that there are two different coil designs for the 2014+ models (square vs. round).
   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 faulty.
   Typical fix: Replace the faulty ignition coil. Ensure you are purchasing the correct design (square or round) for your specific vehicle. Brands like Delphi and ACDelco are common choices.
   Est. part cost: $40-$100
4. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Direct injection (2014+) and standard fuel injectors can become clogged with deposits over time, preventing the correct amount of fuel from being delivered to cylinder 8.
   How to confirm: A professional can perform a fuel injector balance test. A DIY method is to listen to the injector with a mechanic's stethoscope; it should have a consistent clicking sound. One owner reported fixing an intermittent P0308 by using a quality fuel system cleaner like Techron over two tanks of gas.
   Typical fix: First, try a quality fuel system cleaner. If that fails, the injector may need to be professionally cleaned or replaced.
   Est. part cost: $70-$150

Rare But Worth Checking

• Worn Camshaft Lobe / Failed AFM Lifter: → Shop Engine Valve Lifter This is a serious mechanical issue, particularly on engines with Active Fuel Management (AFM). TSB #19-NA-219 (which supersedes #PIP4138R) advises checking for a worn camshaft lobe or a collapsed/stuck AFM lifter if common ignition diagnostics fail. Symptoms can include a persistent misfire and a distinct 'chirp, squeak, or tick' noise from the engine. A forum member with a 2016 Sierra 5.3L confirmed a P0308 was caused by a collapsed lifter on cylinder 8, requiring camshaft and lifter replacement.
🎬 See this walkthrough on diagnosing and fixing collapsed AFM lifters.
• Coolant Leak onto Ignition Components: A known issue reported by owners is a coolant leak from the plastic heater core quick-connect fittings located on the firewall, above the passenger-side rear of the engine. This leak can drip directly onto the cylinder 8 spark plug wire and coil, causing corrosion and ignition failure, leading to a P0308 code.
🎬 Watch: How to fix leaking heater core connectors.
• Low Cylinder Compression: Caused by internal engine issues like a worn piston ring, a bad valve, a broken valve spring, or a blown head gasket. This prevents proper combustion. A compression test and/or cylinder leak-down test is needed to diagnose this.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308.
2. Check if the Check Engine Light is flashing. If so, the misfire is severe and driving should be minimized to prevent catalytic converter damage.
3. Visually inspect the area around cylinder 8 (rear passenger side) for signs of coolant leaks from the heater core fittings on the firewall.
4. Inspect the spark plug wire for cylinder 8. Ensure it is securely connected at both the ignition coil and the spark plug. Per TSB #PIP5159A, confirm the wire 'clicks' onto the coil and is not loose.
5. Swap the ignition coil from cylinder 8 with another cylinder (like 6). Clear the codes and see if the misfire code moves to the new cylinder (P0306). If it does, the coil is bad.
6. If the misfire remains on cylinder 8, remove and inspect the spark plug for wear, fouling, a cracked insulator, or incorrect gap. Replace if necessary.
7. If ignition components are good, check the fuel injector on cylinder 8. Listen for a steady clicking sound with a stethoscope or try a quality fuel system cleaner.
8. If the problem persists, a mechanical issue is possible. Listen for a consistent ticking or chirping noise which could indicate a valvetrain issue. Perform a compression test on cylinder 8 to check for internal engine problems.
9. If compression is good but a mechanical noise is present, consider the possibility of a worn camshaft lobe or failed AFM lifter, as noted in TSB #19-NA-219.

Parts You'll Likely Need

• Spark Plug (OEM #ACDelco 12622441 (also sold as 41-114)) — This is the most common and basic wear item for an ignition system. It's often the first part to fail and cause a single-cylinder misfire.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$25
  Aftermarket price range: $8-$18
• Spark Plug Wire Set (OEM #ACDelco 19431551) — GM TSBs specifically identify loose or faulty spark plug wires as a primary cause for this code on these trucks.
  Trusted brands: ACDelco, Delphi
  OEM price range: $80-$150
  Aftermarket price range: $50-$100
• Ignition Coil (OEM #ACDelco 12619161 (1st Design, Square) or 12699383 (2nd Design, Round)) — A common failure item that prevents spark delivery. It is crucial to identify the correct design for your vehicle.
  Trusted brands: ACDelco, Delphi
  OEM price range: $60-$120
  Aftermarket price range: $40-$80

Related Codes That Often Appear With This One

• P0300 — P0300 is a 'Random/Multiple Cylinder Misfire' code. It often appears alongside a specific cylinder code like P0308 if the misfire is intermittent or beginning to affect other cylinders.
• P124F — This code indicates 'Injector 8 Positive Voltage Control Circuit Shorted to Control Circuit'. It can appear with P0308 if there is an electrical issue with the injector or its wiring, sometimes caused by corrosion from external sources like a coolant leak.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628E: Communicates information needed to diagnose misfire codes P0300-P0308.
• PIP5159A/B: Notes that a loose spark plug wire, potentially from a missing 'C' clip, can cause P0300-P0308 codes.
• 19-NA-219 (replaces PIP4138R): Provides advanced diagnostics for misfires caused by worn camshafts, lifters, and other valvetrain issues.
• PIP4819A: Instructs technicians to inspect and replace loose spark plug wires causing P0300-P0308 codes.

Platform-Specific Known Issues

• Technical Service Bulletin #PIP5159A/B points to loose spark plug wires due to a missin

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 (8.0 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 13.5 ohms. A general acceptable range is 10-18 ohms.. Failure: A reading significantly outside this range, or a reading that differs greatly from the other injectors, points to a bad injector coil.
• Fuel Pressure (Key On, Engine Off - GMT900 platform) — expected: 55 to 65 PSI. Failure: Low pressure (e.g., 30 PSI) can indicate a weak fuel pump, which could contribute to a lean misfire, although it would typically affect multiple cylinders.
• Cylinder Compression — expected: 120 to 150 PSI, with minimal variation between cylinders.. Failure: A reading significantly below 120 PSI or more than 15-20% lower than other cylinders indicates a mechanical engine problem.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Misfire Data / Misfire Graphic — To view live and historical misfire counts for each cylinder, confirming that cylinder 8 is the primary or only offender. This is more precise than a generic OBD-II scanner.
• GDS2 (GM Global Diagnostic System 2): Cylinder Power Balance — This test deactivates one cylinder at a time and measures the drop in engine RPM. A cylinder that causes little or no RPM drop when deactivated is not contributing properly, pinpointing the misfire.
• GDS2 (GM Global Diagnostic System 2): AFM (Active Fuel Management) Disable — To diagnose a potential AFM lifter issue. If disabling AFM via the scan tool makes the misfire or associated engine noise disappear, it strongly suggests a problem with the AFM lifter on cylinder 8.
• GDS2 (GM Global Diagnostic System 2): Fuel Injector Balance Test — To command each injector to fire for a set duration and measure the corresponding drop in fuel pressure. This can identify a clogged or weak injector on cylinder 8 if the pressure drop is less than that of other cylinders.

Wiring & Ground Locations

• G110 / G114 — On the K2XX platform (2019+ shown, but relevant), these are key engine and body harness grounds located on the frame or body, often near the front of the vehicle.. A loose or corroded engine harness ground can cause a variety of intermittent electrical issues, including weak spark or erratic injector signals that could lead to a misfire. GM has investigated issues with these specific grounds being loose.
• G218 — On the K2XX platform, this ground is located under the passenger side dash/kick panel area.. While primarily an interior ground, it can cause strange electrical behavior if compromised. A video guide shows insulation getting caught under the nut, causing a poor connection.
• Cylinder Head Grounds — Ground straps are typically bolted to the rear of each cylinder head.. These are critical grounds for the engine's sensor and ignition systems. A loose or corroded ground on the passenger side head (Bank 2) could specifically impact cylinder 8 ignition or injector performance.
• Cylinder 8 Injector Connector — On the fuel rail at the top of the right cylinder bank (passenger side), at the rearmost position.. This connector is the direct electrical interface for the fuel injector. It is susceptible to corrosion, especially if the heater core hoses above it leak coolant, which can lead to code P124F along with P0308.

Real Owner Repair Stories

• Reddit user in r/ChevyTrucks (2011 Suburban (similar GMT900 platform and engine)) — P0308 code
  ❌ Tried (didn't work) Initial suggestions included replacing all plugs and wires.
  ✅ What actually fixed it The user found that the heater core quick-connect fittings were dripping coolant directly onto the #8 spark plug wire. Fixing the coolant leak and cleaning the plug wire resolved the P0308 code.
• ScannerDanner Forum user (2016 GMC Sierra 5.3L) — Flashing P0308
  ❌ Tried (didn't work) The dealership replaced the camshaft and all lifters due to a collapsed lifter on cylinder 8, but the P0308 code returned two days later.
  ✅ What actually fixed it The story was ongoing, but the immediate professional advice was to perform a compression test to check for a bent or sticking valve that may have been missed during the initial repair, and to have the original shop re-check their work.
• YouTube video by 'Corey' at Race30 Fab (2014 GMC Sierra 1500 5.3L with 55k miles) — Misfire code on cylinder 4 (P0304), but the cause is identical for P0308.
  ✅ What actually fixed it Upon removing the valve cover, a visual inspection confirmed a collapsed AFM lifter on the misfiring cylinder. The final fix was to install an AFM delete kit with LS7-style lifters and a new camshaft.
• Reddit user in r/gmcsierra (2014 GMC Sierra) — Flashing check engine light, weak and shaking engine, codes P0308 and P124F.
  ❌ Tried (didn't work) Swapping the cylinder 8 ignition coil with cylinder 2 and then cylinder 6 did not move the misfire; the code remained P0308.
  ✅ What actually fixed it The user identified corrosion on the ignition coil wiring harness connector for cylinder 8, caused by a previous coolant leak from the firewall hoses. The diagnosis concluded the wiring harness connector itself was the fault.

OEM Part Supersession History

• ACDelco 12619161 → ACDelco 12723610 — Standard part lifecycle update by the manufacturer for design or material improvements.
  Heads up: 12619161 is the '1st Design' square-shaped coil used on early 2014 models. It was superseded by other numbers for service, including 12658183 and eventually 12699383 (round 'Melco' style coils). It is critical to visually confirm the shape of the original coils before ordering a replacement.

Model Year Variations Within This Range

• 2014: The 2014 model year was a transition year and can have two different ignition coil designs: an early '1st Design' square-shaped coil (PN 12619161) and a later '2nd Design' round-shaped coil (often marked 'Melco', serviced by PNs like 12658183 or 12699383). The coils are not interchangeable, so visual inspection is required before purchasing a replacement.
• 2014-2016: These K2XX platform trucks feature the Gen V EcoTec3 engine with direct injection (DI). Unlike the port injection on 2012-2013 models, a P0308 could also be related to DI-specific components like the high-pressure fuel system or carbon buildup on the injector tip inside the cylinder.