P0308 on 2004-2008 GMC Sierra: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2004-2008 Gmc SIERRA

On the V8 engines in this generation of GMC Sierra (GMT800/GMT900 platforms), a P0308 code requires careful diagnosis. While the cause is often a simple ignition part, these Vortec engines have documented valvetrain issues. A manufacturer TSB (PIP4138L and its successors) specifically warns that if standard checks for spark and fuel don't solve the misfire, the cause could be a worn camshaft lobe or a failed lifter. This turns a potentially simple fix into a major mechanical repair, a known issue for this platform. Additionally, a common point of failure is the plastic heater core quick-connect fittings at the firewall 🎬 Watch: How to fix these common heater core fitting leaks., which can leak coolant directly onto the cylinder 8 ignition coil and spark plug wire, causing corrosion and electrical shorts that specifically trigger a P0308.

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 GMT800 generation (2004-2006 and 2007 'Classic' models) and the beginning of the GMT900 generation (2007-2008). While the core causes are similar, the 2007-2008 models with 5.3L and 6.0L engines introduced Active Fuel Management (AFM), which has its own set of lifter-related issues. However, P0308 is on a non-AFM cylinder in most configurations, so the cause is more likely related to general ignition wear, the valvetrain issues described in TSB PIP4138L that affect both generations, or coolant leaks onto cylinder 8 components.

Symptoms You May Notice

• Check Engine Light is on or flashing
• Rough idle or engine vibration and shaking
• Noticeable loss of engine power and acceleration
• Engine stumbling or hesitating
• Engine may be hard to start
• Smell of unburnt fuel from the exhaust
• Engine noise described as a tick, chirp, or squeak, especially on a cold start (if related to cam/lifter issues)

Most Likely Causes

1. Bad Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug ignition systems are common failure points due to heat and vibration. Cylinder 8 is at the rear of the engine bay, which can be a high-heat area. Coolant leaks from heater core fittings are known to drip directly on the #8 coil, causing it to fail.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 6 on the same bank). Clear the codes and drive the truck. If the code changes to P0306, the ignition coil is faulty. This is the most common and effective diagnostic step.
   Typical fix: Replace the failed ignition coil. Many owners on forums recommend sticking with ACDelco or Delphi brands for reliability and consider replacing all 8 coils if they are original and high-mileage.
   Est. part cost: $30-$70 per coil
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a standard maintenance item. Cylinder 8 can be difficult to access, so it may be neglected 🎬 See this trick for reaching the difficult cylinder 8 spark plug. during tune-ups. GM also revised the spark plug gap specification from 0.060" to 0.040" for some iridium plugs to improve idle, and using the wrong plug or gap can cause issues.
   How to confirm: Remove the spark plug from cylinder 8 and inspect it. Look for a worn electrode, cracks in the porcelain, or heavy fouling from oil, fuel, or carbon. Compare it to a plug from a known good cylinder.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 spark plugs at the same time using the recommended ACDelco Iridium plugs (like 41-110) for consistent performance. Iridium plugs are pre-gapped and should not be adjusted.
   Est. part cost: $8-$20 per plug
3. Faulty Spark Plug Wire 🟡 Medium Probability → Shop Spark Plug The wire for cylinder 8 is long and routed in a hot area near the firewall. More specifically, coolant leaks from plastic heater core quick-connect fittings have been known to drip directly onto the wire, causing corrosion, shorts, and failure. GM also issued TSBs like #PIP4819A for loose spark plug wires causing misfires.
   How to confirm: Visually inspect the wire for cracks, burns, or signs of arcing (white marks). Check for a secure 'click' at both the coil and plug. With the engine running in the dark, you may see a spark jumping from the wire to the engine block. Test resistance with a multimeter (should be under 1,000 ohms per foot).
   Typical fix: Replace the spark plug wire. Wires are typically sold as a complete set for the engine.
   Est. part cost: $50-$100 for a set
4. Clogged or Failed Fuel Injector ⚪ Low Probability → Shop Fuel Injector While less common, the injector can fail. A Reddit user with a 2014 Sierra reported a P0308 accompanied by a P124F (Injector 8 circuit short), which they traced back to corrosion from the known heater hose coolant leak affecting the injector wiring.
   How to confirm: After ruling out ignition issues, listen to the injector with a mechanic's stethoscope for a consistent clicking sound. A more definitive test is to swap the injector with another cylinder and see if the misfire code follows, though this is more labor-intensive. A 'noid light' can also be used to confirm the injector is receiving an electrical pulse.
   Typical fix: Replace the faulty fuel injector. It's often recommended to replace the injector O-rings at the same time.
   Est. part cost: $50-$120

Rare But Worth Checking

• Worn Camshaft Lobe and/or Lifter: → Shop Engine Camshaft This is a known issue documented in GM TSB #PIP4138L and its successors (like 19-NA-219). If ignition and fuel system checks do not resolve the P0308 code, this mechanical failure is a strong possibility, especially if accompanied by a ticking or squeaking engine noise. A failed lifter can seize and grind down the camshaft lobe, requiring replacement of both components.
• Low Engine Compression: Caused by more serious issues like a leaking head gasket, worn piston rings, or a problem with the cylinder head valves (e.g., broken valve spring or bent valve). TSB 19-NA-219 lists broken valve springs as a potential cause for P030x codes. A compression test is needed to diagnose this.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak from a failed intake manifold gasket near the cylinder 8 runner can cause the air/fuel mixture to become too lean, resulting in a misfire on just that cylinder.

Diagnosis Steps

1. Read the code with an OBD-II scanner and note any other codes present, such as P0300 or P124F.
2. Visually inspect the area around cylinder 8 (rear passenger side). Look specifically for signs of coolant leaks from the plastic heater core quick-connect fittings on the firewall dripping onto the coil, plug wire, and wiring harnesses.
3. Swap the ignition coil from cylinder 8 with cylinder 6 (an easier-to-access cylinder on the same bank). Clear the codes and run the engine. If the code changes to P0306, the coil is bad.
4. If the code remains P0308, remove the spark plug. Inspect it for wear, damage, or fouling (oil, fuel, carbon). Replace if necessary.
5. Inspect the spark plug wire for any signs of damage, burns, or corrosion. Ensure it 'clicks' securely onto the coil. Test its resistance with a multimeter (should be under 1,000 ohms per foot).
6. If ignition components are good, listen to the fuel injector on cylinder 8 with a stethoscope to ensure it is clicking consistently.
7. If a ticking or squeaking noise is present and the misfire persists, the issue may be related to the valvetrain (lifter/camshaft) as noted in TSB PIP4138L.
8. If other steps don't identify the cause, perform a compression test on cylinder 8 to check for mechanical engine problems like bad rings, valves, or head gasket.

Parts You'll Likely Need

• Ignition Coil (OEM #12570616) — A failed ignition coil is one of the most common causes of a single-cylinder misfire. This part number is widely cross-referenced for these V8 engines.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $45-$70
  Aftermarket price range: $25-$50
• Spark Plug (OEM #41-110) — Spark plugs are a routine wear item and a primary cause of misfires. Cylinder 8 can be hard to reach and may be overlooked. This ACDelco Iridium plug is specified for many of these engines.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$15
  Aftermarket price range: $5-$10
• Spark Plug Wire Set (OEM #9748RR) — The wire for cylinder 8 is long and exposed to heat and potential coolant leaks, making it prone to failure over time.

Related Codes That Often Appear With This One

• P0300 — If the condition causing the P0308 is intermittent or beginning to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may also be stored.
• P124F — This code, for 'Injector 8 Positive Voltage Control Circuit Shorted to Control Circuit', can appear with P0308 if wiring corrosion, often from a coolant leak, affects the fuel injector circuit for cylinder 8.

Technical Service Bulletins (TSBs) & Recalls

• PIP4138L: Notes that Diagnostic Trouble Codes P0300-P0308 can be caused by a worn camshaft lobe and/or lifter if standard diagnostics do not isolate the cause.
• 19-NA-219: Successor to PIP4138, providing diagnostic tips for engine noises and misfires (P0300-P0308) caused by worn cam lobes, lifters, sticking valves, or broken valve springs.
• PIP4819A: Cites loose spark plug wires as a potential cause for misfires across all cylinders, including P0308.

Platform-Specific Known Issues

• TSB #PIP4138L: This bulletin from GM (and its successors like 19-NA-219) states that after ruling out common ignition or fuel issues, a persistent P0308 code could be caused by a worn camshaft lobe and/or a failed lifter, which may also produce a ticking or squeaking noise.
• Heater Core Hose/Fitting Leak: A widely reported issue where plastic heater core fittings on the firewall crack and leak coolant. The location is directly above the rear of the passenger-side cylinder head, causing the coolant to drip onto the cylinder 8 ignition coil, spark plug wire, and wiring harnesses, leading to corrosion and misfires.

Mechanic-Grade Diagnostic Values

• Fuel Injector Resistance — expected: 11 - 14 ohms. Failure: A reading significantly outside this range, or a variance of more than 1 ohm from other injectors, indicates a faulty injector.
• Ignition Coil Secondary Resistance — expected: 6,000 - 10,000 ohms (6k-10k Ω). Failure: A reading significantly outside this range suggests a bad secondary winding. Note: Swapping coils is often a more reliable test.
• Fuel Pressure (Key On, Engine Off) — expected: 55 - 62 PSI. Failure: Pressure below 55 PSI can indicate a weak fuel pump or clogged filter, which typically causes multiple/random misfires but can sometimes manifest on a single cylinder.
• Scan Tool Misfire Counters (Mode $06 or specific GM data) — expected: Near zero for all cylinders.. Failure: A rapidly increasing count for cylinder 8 confirms a live misfire that the PCM is detecting.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $A9: Provides the raw misfire count for cylinder 8 during the current and last 10 driving cycles. This data can show an intermittent misfire before it's severe enough to keep the Check Engine Light on. (see via Most advanced OBD-II scan tools with Mode $06 capability.)

Scan Tool Commands That Help

• GM Tech2 / GDS2 or equivalent professional scanner: Cylinder Power Balance Test — This is used to confirm a loss of contribution from cylinder 8. The tool deactivates the cylinder, and the resulting RPM drop is measured. If there is little to no drop in RPM when cylinder 8 is deactivated, it confirms it was not contributing power.
• GM Tech2 / GDS2 or equivalent professional scanner: Misfire History / Misfire Graphic — To view real-time and historical misfire counts for each cylinder. This helps confirm the misfire is isolated to cylinder 8 and can show if it occurs under specific conditions (e.g., high RPM, idle).

Wiring & Ground Locations

• G103 — On the front of the right cylinder head.. This is a primary ground point for components on the passenger side of the engine. Corrosion or a loose connection here can cause erratic behavior for sensors, the ignition coil, or the fuel injector on cylinder 8.
• G104 — On the front of the left cylinder head (driver's side).. While on the opposite bank, this ground is part of the main engine ground scheme. A problem here can introduce electrical noise or voltage drops that may affect the PCM's ability to correctly interpret sensor data, including the crankshaft position sensor used for misfire detection.
• Engine to Firewall Ground Straps — Braided straps typically running from the back of the cylinder heads to the firewall.. These grounds are critical for the ignition system. A corroded or broken strap can lead to weak spark and misfires, particularly affecting the rearmost cylinders like #7 and #8.
• PCM Connector Pins — At the Powertrain Control Module (PCM).. The specific pins for the cylinder 8 ignition coil control and fuel injector control can be tested for continuity and voltage, ruling out a wiring break between the PCM and the component.

Real Owner Repair Stories

• silveradosierra.com forum user (2008 GMC Sierra) — P0308 code with a persistent ticking noise, especially on cold starts.
  ❌ Tried (didn't work) Replacing spark plug, Replacing spark plug wire, Replacing ignition coil
  ✅ What actually fixed it The final repair was replacing a collapsed lifter and the worn camshaft lobe on cylinder 8, as described in the manufacturer TSB.
• gmtrucks.com forum user (2005 GMC Sierra) — P0308 misfire, visible coolant residue on passenger side of engine bay.
  ❌ Tried (didn't work) Initial diagnosis focused only on ignition parts without checking for external factors.
  ✅ What actually fixed it The root cause was a coolant leak from the plastic heater core quick-connect fittings at the firewall dripping onto the cylinder 8 coil pack. The fix involved replacing the plastic fittings with metal aftermarket ones and replacing the coolant-damaged ignition coil and wire.
• YouTube video / Forum post (GMC Sierra 5.3L) — P0308 misfire that persisted after ignition and fuel system checks.
  ❌ Tried (didn't work) Spark plug, coil, and wire replacement, Fuel injector swap
  ✅ What actually fixed it After removing the valve cover for inspection, a broken valve spring was discovered on cylinder 8. Replacing the spring and associated seals resolved the misfire.

OEM Part Supersession History

• Various (e.g., 12571595) → 12648846 (for AFM lifters) — GM has updated the lifter design multiple times to address failure rates.
  Heads up: Active Fuel Management (AFM) lifters (for cylinders 1, 4, 6, 7 on V8s) are not interchangeable with standard lifters (cylinders 2, 3, 5, 8). Using the wrong type will cause immediate valvetrain failure.

Model Year Variations Within This Range

• 2004-2006 (GMT800): These models use the Gen III Vortec V8s (e.g., LM7) which do not have Active Fuel Management (AFM). A P0308 on these trucks is almost never an AFM-related lifter issue and points more directly to standard ignition, fuel, or the base mechanical valvetrain wear described in TSB PIP4138L.
• 2007-2008 (GMT900): These models introduced the Gen IV Vortec V8s (e.g., LY5, LMG) which included Active Fuel Management (AFM). While cylinder 8 is a non-AFM cylinder, the entire engine system, including the oil pressure regulation (VLOM), is more complex. Issues with the AFM system can sometimes lead to oiling problems or other symptoms that might be misdiagnosed.