P0308 on 2000-2007 GMC Sierra 1500: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2000-2007 Gmc SIERRA 1500

For the GMT800 and early GMT900 Sierra, the LS-based V8 engines are generally reliable, but the individual coil-on-plug ignition systems are a frequent failure point as they age. While a P0308 code is most often a simple fix like a coil or spark plug, this platform has two well-documented, more severe potential causes. First, GM has issued Technical Service Bulletins (TSBs) acknowledging that persistent misfires can be a symptom of a worn camshaft lobe or a failing lifter. This is particularly relevant for the later 5.3L/6.0L engines equipped with Active Fuel Management (AFM), which are known for lifter failures. Second, a subset of these engines was built with 'Castech' branded cylinder heads which are prone to cracking and causing misfires. This makes proper diagnosis crucial before simply replacing parts.

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 the end of the first generation GMT800 platform (2000-2006 and 2007 'Classic') and the start of the second generation GMT900 (2007). The underlying 4.8L, 5.3L, and 6.0L V8 engines are very similar, and the causes for P0308 are generally consistent. However, the 2007+ GMT900 models with 5.3L and 6.0L engines introduced Active Fuel Management (AFM), which adds complexity and a higher likelihood of lifter-related failures causing this code.

Symptoms You May Notice

• Rough or shaking idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Solid or flashing Check Engine Light
• Decreased fuel economy
• Smell of raw fuel or rotten eggs from the exhaust
• Engine jerking or bucking under load

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability → Shop Ignition Coil The individual coil packs are subjected to constant heat cycles and vibration, leading to eventual electronic failure. It's a very common failure item on LS-based engines. Cylinder 8's location at the rear of the engine bay can subject it to extra heat.
   How to confirm: Swap the cylinder 8 ignition coil with another cylinder (e.g., cylinder 6 on the same bank). Clear the codes and run the engine. If the code changes to P0306, the coil is confirmed bad. This is the most common and effective diagnostic step mentioned by owners.
   Typical fix: Replace the faulty ignition coil. Many owners on forums like GMTrucks.com recommend sticking with ACDelco or Delphi brands for reliability. If one coil fails and the others are original with high mileage, replacing all 8 is a common preventative measure.
   Est. part cost: $25-$50
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug
   How to confirm: Remove the spark plug from cylinder 8. Inspect for wear on the electrode, cracks in the porcelain, or heavy carbon/oil/coolant fouling. A worn plug cannot produce a strong spark. One owner on a Chevrolet forum found a bad plug was the simple fix after the truck would jerk and lose power.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 spark plugs at the same time for consistent performance. Owners strongly recommend using the specified ACDelco Iridium or Platinum plugs. 🎬 Watch: Step-by-step spark plug and wire replacement guide.
   Est. part cost: $5-$15 per plug
3. Bad Spark Plug Wire 🟡 Medium Probability → Shop Spark Plug The wire for cylinder 8 is long and routed in a hot area. In a well-documented issue, coolant from leaking heater core quick-connect fittings at the firewall can drip directly onto the #8 plug wire and coil, causing corrosion and failure.
   How to confirm: Visually inspect the wire for cracks, burns, or corrosion on the contacts. Check the firewall above cylinder 8 for signs of coolant leaks. With the engine running in the dark, you can sometimes see the spark arcing from the wire to a metal part of the engine.
   Typical fix: Replace the spark plug wire. Wires are typically sold in a set of 8. If a coolant leak is the cause, the leaking heater hose connectors must be replaced to prevent repeat failure. 🎬 See how to fix these common heater hose leaks.
   Est. part cost: $50-$90 for a set
4. Clogged or Failing Fuel Injector ⚪ Low Probability → Shop Fuel Injector
   How to confirm: Use a mechanic's stethoscope or a long screwdriver to listen for a consistent clicking sound from the cylinder 8 injector. If it's silent or has an irregular pattern, it may be faulty. A more definitive method is to swap the cylinder 8 injector with another cylinder and see if the misfire code follows. Some owners have had success running a concentrated fuel system cleaner as a first step.
   Typical fix: Replace the faulty fuel injector. It's also recommended to replace the injector O-rings. One owner on a Reddit forum noted that a P0308 was accompanied by an injector circuit code (P124F), pointing directly to an electrical issue with the injector or its wiring.
   Est. part cost: $40-$100

Rare But Worth Checking

• Failed AFM Lifter: → Shop Engine Valve Lifter On 2007+ models with the 5.3L or 6.0L engine, Active Fuel Management (AFM) lifters can collapse or get stuck. This results in a loss of compression and a persistent misfire on an AFM cylinder (1, 4, 6, or 7), but can cause issues on others as well. This is a major mechanical failure often preceded by a ticking noise and requires significant engine work to repair.
🎬 Watch: How to diagnose and replace collapsed AFM lifters.
• Worn Camshaft Lobe: → Shop Engine Camshaft Per GM TSB #PIP4138R, if common diagnostics don't resolve the misfire, a worn camshaft lobe should be investigated. This often happens in conjunction with a failed lifter, where the lifter's roller stops spinning and grinds down the cam lobe. This is a severe mechanical failure.
• Cracked 'Castech' Cylinder Head: → Shop Engine Cylinder Head A known defect exists in some cylinder heads made by 'Castech', identifiable by a logo under the valve cover. They can develop a porosity crack, often leading to a slow coolant leak into the engine, which can foul the spark plug in cylinder 8 and cause a misfire. Symptoms include unexplained coolant loss and sometimes milky oil.
• Low Cylinder Compression: Caused by issues like a worn piston ring, a leaking valve, or a blown head gasket (potentially from a cracked head). A compression test is needed to diagnose this, followed by a cylinder leak-down test to pinpoint the source of the leak.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the cylinder 8 intake runner can lean out the air/fuel mixture enough to cause a misfire. This was a more common issue on the 96-99 Vortec engines but can still occur on these models. A smoke test is the best way to diagnose this.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0308. Note any other codes present and check freeze-frame data to see the engine conditions when the misfire occurred.
2. Use the scanner's live data function to monitor misfire counts for each cylinder. Confirm that cylinder 8 is the primary offender.
3. Identify cylinder 8 (rearmost cylinder on the passenger side).
4. Perform a visual inspection. Look for obvious issues like a disconnected spark plug wire, damaged wiring harness, or signs of coolant leaking from the heater hoses onto the cylinder 8 coil area.
5. Swap the ignition coil from cylinder 8 with cylinder 6 (front passenger side). Clear codes and test drive. If the code changes to P0306, the coil is bad.
6. If the code remains P0308, inspect the spark plug. Remove it and check for wear, fouling (oil, fuel, or coolant), or incorrect gap. A fouled plug can indicate other problems (bad rings, leaking valve seals, cracked head).
7. Inspect and test the spark plug wire. Check its resistance with a multimeter (should be within spec, typically under 1,000 ohms per foot).
8. If ignition components are good, check the fuel injector. Listen for a steady clicking sound. Consider swapping the injector with another cylinder to see if the misfire follows.
9. If the cause is still not found, perform a cylinder compression test. A healthy cylinder should be around 120-150 PSI or higher, with minimal variation from other cylinders. Low compression points to a mechanical problem.
10. If compression is low, perform a cylinder leak-down test to determine if the leak is from intake valves, exhaust valves, piston rings, or the head gasket.
11. If all else fails, the issue could be related to the camshaft or lifters as described in TSB #PIP4138R, which may require professional diagnosis.

Parts You'll Likely Need

• Ignition Coil (OEM #12573190 (ACDelco D514A)) — This is the most common failure point for a single-cylinder misfire on this engine.

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or starting to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0308.
• P0171/P0174 — If the misfire is caused by a vacuum leak (like an intake gasket), the engine may run lean, triggering 'System Too Lean' codes for one or both banks.

Technical Service Bulletins (TSBs) & Recalls

• PIP5628G: A general diagnostic aid for technicians contacting GM's Technical Assistance Center (TAC) for misfire codes P0300-P0308. It outlines a template of information to collect, including freeze-frame data, PicoScope results, and fuel trim values, to expedite diagnosis.
• PIP4138R: Advises technicians to inspect for a worn camshaft lobe and/or a failed lifter if standard ignition and fuel system diagnostics do not resolve a P0300-P0308 misfire. This points to a known mechanical weakness.

Platform-Specific Known Issues

• AFM/DOD Lifter Failure: → Shop Engine Valve Lifter On 2007+ 5.3L and 6.0L engines, the Active Fuel Management (AFM) system lifters are a known major failure point. A lifter can become stuck or collapse, causing a complete loss of valve function for that cylinder and a persistent misfire code. This is a costly repair often requiring camshaft replacement as well.
• Cracked Castech Cylinder Heads: → Shop Engine Cylinder Head Some engines were assembled with cylinder heads from a supplier named 'Castech'. These heads are known to develop porosity cracks, often allowing coolant to seep into the oil passages or directly into a cylinder, which can foul the spark plug and cause a P030X misfire code.
• Heater Hose Coolant Leak: Leaking plastic heater core quick-connect fittings at the firewall can drip coolant directly onto the cylinder 8 ignition coil, spark plug wire, and wiring harness connector. This causes corrosion and electrical shorts, leading specifically to a P0308 code.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.4 to 2.0 ohms. Failure: A reading outside this range, especially an open circuit (infinite resistance) or a short (zero resistance).
• Ignition Coil Secondary Resistance — expected: 6,000 to 10,000 ohms (6k-10k Ω). Failure: A reading significantly outside this range indicates a faulty coil winding.
• Fuel Pressure (Key On, Engine Off) - Return-Style System (approx. 2000-2004) — expected: 55-62 PSI. Failure: Pressure below this range may indicate a weak fuel pump or clogged filter. Pressure should drop when the vacuum line is removed from the rail-mounted regulator.
• Fuel Pressure (Key On, Engine Off) - Returnless System (approx. 2004-2007) — expected: Consistently 55-65 PSI (typically stable around 58 PSI). Failure: Low or fluctuating pressure points to a failing in-tank fuel pump module, as the regulator is part of that assembly.
• Scan Tool Misfire Counters (Current) — expected: Zero or very few, infrequent counts on a healthy cylinder at idle.. Failure: A number that rapidly and consistently increases on cylinder 8 confirms an active misfire.

Hidden / Shadow Codes Worth Checking

• Misfire History/Current Counters: While not a 'code' in the traditional sense, dealer-level scan tools like the GM Tech 2 provide access to detailed misfire data. It shows 'Current Misfires' to identify which cylinder is failing now, and 'History Misfires' to show past events. This is more granular than a simple P0308 code and is essential for diagnosing intermittent issues. (see via GM Tech 2, or advanced aftermarket scanners with GM-specific software. Basic code readers may not show this level of detail.)

Scan Tool Commands That Help

• GM Tech 2 / GDS2 (or equivalent): Crankshaft Position (CKP) System Variation Learn Procedure — Use this after replacing the crankshaft position sensor, PCM, or on a high-mileage engine where false misfires are suspected. It calibrates the PCM to minor variations in the crankshaft reluctor wheel, improving misfire detection accuracy and reducing false counts.
• GM Tech 2 / GDS2 (or equivalent): Injector Balance Test / Cylinder Power Balance — After confirming the issue is not spark-related, this function allows the technician to disable the fuel injector for cylinder 8 and observe the change in engine RPM. If there is little or no change when cylinder 8's injector is disabled, it confirms that cylinder was not contributing, pointing to a fuel or mechanical issue on that cylinder.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine block/cylinder head.. This is the primary ground point for all eight ignition coils. Corrosion or a loose connection at this specific ground can cause weak or intermittent spark on one or more cylinders, including cylinder 8, leading to a misfire that is difficult to diagnose.
• G104 — Located on the rear of the passenger side cylinder head.. This ground is critical for the Vehicle Control Module (VCM/PCM) and multiple sensors. A poor connection here can cause erratic sensor readings and incorrect PCM calculations, potentially leading to false misfire detection or other driveability problems.
• Heater Core Quick-Connects — On the firewall, passenger side, above the rear of the engine.. These plastic coolant fittings are known to develop leaks. The dripping coolant falls directly onto the cylinder 8 ignition coil and spark plug wire, causing corrosion and shorting out the ignition components, leading directly to a P0308 code.

Real Owner Repair Stories

• Reddit user in r/ChevyTrucks (2011 Suburban (GMT900 platform with similar 5.3L engine)) — P0308 code.
  ❌ Tried (didn't work) Initial diagnosis pointed towards common ignition parts.
  ✅ What actually fixed it The heater core quick-connect fittings at the firewall were leaking coolant directly onto the #8 spark plug wire. Fixing the coolant leak and cleaning the wire resolved the P0308 code.
• TriStateTuners.com forum user (1998 Pontiac Trans Am (LS1 engine, similar architecture)) — Severe misfire, blinking CEL, codes for misfires on cylinders 5, 7, and 8.
  ❌ Tried (didn't work) Swapping MAF sensor.
  ✅ What actually fixed it The final diagnosis was a broken valve spring on one of the affected cylinders. This is a mechanical failure that requires removing the valve cover to inspect the valvetrain.

"I Checked Everything" — The Actual Cause

• If a smoke test is performed to check for vacuum leaks and comes back clean, but the P0308 misfire persists, the cause is likely not an unmetered air leak. The problem is almost certainly isolated to the three core areas for that specific cylinder: ignition (spark plug, wire, coil, coil ground), fuel (injector), or a mechanical engine fault. A common 'actual cause' in this scenario is a mechanical issue like a broken valve spring, a worn camshaft lobe, or a failing lifter, none of which would be detected by a smoke test. A compression test and cylinder leak-down test are the correct next steps.

OEM Part Supersession History

• 12573190 → 12699382 (also supersedes 12739916) — Standard part evolution for performance and reliability.
  Heads up: These part numbers refer to the round-style D514A ignition coil used on these engines. They are not compatible with the earlier flat-style ignition coils found on some other GM engines.

Model Year Variations Within This Range

• 2000-2004 (approx.) vs 2004-2007 (approx.): The fuel delivery system changed mid-generation. Earlier models (approx. '00-'04) use a 'return-style' fuel system with a vacuum-actuated fuel pressure regulator on the fuel rail. Later models (approx. '04-'07) use a 'returnless' system where the fuel pressure regulator is inside the fuel tank as part of the pump assembly, delivering constant pressure to the rail. This changes fuel pressure diagnosis; on return-style systems, you test the regulator on the rail, while on returnless systems, a pressure issue points to the in-tank pump module.