P0303 on 2010-2013 GMC Sierra Denali: Cylinder 3 Misfire Causes & Fixes

What's Unique About the 2010-2013 Gmc SIERRA DENALI

The 2010-2013 GMC Sierra Denali, typically equipped with the 6.2L V8 engine (RPO codes L9H, L94), has two well-documented issues that can cause a P0303 misfire. First, a manufacturer Technical Service Bulletin (TSB PIP4819A) points directly to loose spark plug wires as a potential cause. More critically, these engines feature an Active Fuel Management (AFM) system, which is known for lifter failures. While cylinder 3 is a non-AFM cylinder, a collapsed or stuck AFM lifter (on cylinders 1, 4, 6, or 7) can cause widespread valvetrain instability or, more commonly, damage the camshaft. A worn camshaft lobe for cylinder 3, resulting from debris from a failed AFM lifter, can cause a persistent P0303 code.

Diagnostic Flowchart

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

Symptoms You May Notice

• Solid or flashing Check Engine Light
• Rough or shaky idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of power
• A ticking, chirping, or squealing sound from the engine bay, which may indicate a lifter or camshaft issue
• Reduced fuel economy
• Fuel smell from the exhaust
• Service StabiliTrak message appearing on the dash

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability The coil-on-plug ignition system is subjected to high heat and vibration, leading to eventual failure. It's a very common failure item on modern engines.
   How to confirm: Swap the ignition coil from cylinder 3 with another cylinder (e.g., cylinder 1). Clear the codes and run the engine. If the code changes to P0301, the coil is faulty.
   Typical fix: Replace the faulty ignition coil. It's often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $35-$75
2. Worn or Fouled Spark Plug 🔴 High Probability Spark plugs are a routine maintenance item. Carbon buildup, oil fouling, or simple electrode wear can prevent a strong spark. Be cautious of counterfeit ACDelco plugs sold online, which have been reported to fail prematurely.
   How to confirm: Remove the spark plug from cylinder 3 and inspect it. Look for a cracked porcelain insulator, heavy carbon deposits, oil fouling, or excessive wear on the electrode. A wet, sooty plug indicates it is not firing.
   Typical fix: Replace the spark plug. It is highly recommended to replace all 8 spark plugs at the same time 🎬 See this easy DIY walkthrough for replacing all 8 spark plugs. for even performance. Use of the OEM recommended ACDelco 41-110 Iridium plug is common, though it has been superseded by 41-162, with some owners preferring the quality of the older 41-110.
   Est. part cost: $8-$20 per plug
3. Loose or Damaged Spark Plug Wire 🔴 High Probability A specific TSB (PIP4819A) was issued for these models, citing loose spark plug wires as a direct cause for misfires. The connection can loosen from vibration over time, or the wire can degrade internally.
   How to confirm: Inspect the spark plug wire for cylinder 3. Ensure it is securely connected ('clicked' on) at both the coil and the spark plug. Check for any signs of physical damage, burns, or arcing (white marks).
   Typical fix: Securely reconnect the wire or replace it if it's damaged. Wires are often replaced as a set.
   Est. part cost: $50-$100 for a set
4. Failed Active Fuel Management (AFM) Lifter / Camshaft Damage 🟡 Medium Probability The 6.2L V8 engine's AFM system lifters are a known common failure point. While cylinder 3 is not an AFM cylinder, a failing AFM lifter can send metal debris through the engine, wiping out lobes on the camshaft, including the lobe for cylinder 3. This is a widely discussed issue in owner forums and is addressed in TSB 19-NA-219. 🎬 Watch: Why your 6.2L misfire might not be a lifter failure.
   How to confirm: A persistent ticking or chirping noise from the engine that changes with RPM is a strong indicator. After ruling out ignition and fuel issues, a mechanic must remove the valve cover to inspect valve movement. If valve lift on cylinder 3 is visibly less than others, a damaged camshaft is likely. A borescope can be used to inspect the cam lobes through the lifter bores.
   Typical fix: This is a major repair. It requires replacing the damaged camshaft, all lifters (both AFM and standard), and often the lifter guides and oil pump. Many owners opt to install an AFM-delete kit at this time, which involves replacing the AFM lifters with standard lifters and reprogramming the ECU.
   Est. part cost: $500-$2500 for parts, with full AFM delete kits costing more.
5. Clogged or Faulty Fuel Injector ⚪ Low Probability
   How to confirm: Listen to the injector with a mechanic's stethoscope; it should have a consistent clicking sound. A more definitive test is to swap the fuel injector from cylinder 3 with another cylinder (e.g., cylinder 5) and see if the misfire code moves to P0305.
   Typical fix: Replace the faulty fuel injector. It's good practice to replace the O-rings and seals on adjacent injectors during the service.
   Est. part cost: $50-$120

Rare But Worth Checking

• Intake Manifold Gasket Leak: A vacuum leak near the intake port for cylinder 3 can lean out the air/fuel mixture and cause a misfire. This can be diagnosed with a smoke test.
• Low Engine Compression: Internal engine damage such as a worn piston ring, a bad valve, or a leaking head gasket can cause a loss of compression. A compression and leak-down test is needed to confirm this. One owner reported a P0303 with only 100 psi in the cylinder versus 180 psi in others, indicating internal failure.
• Broken Valve Spring: Though less common than a lifter failure, a broken valve spring on cylinder 3 will prevent the valve from closing properly, causing a loss of compression and a persistent misfire. This is often diagnosed during a visual inspection with the valve cover removed.

Diagnosis Steps

1. Read the codes with an OBD-II scanner to confirm P0303 and check for any other codes 🎬 Watch: A breakdown of what P0303 really means for your engine. like P0300.
2. Listen for a persistent ticking or chirping noise from the engine bay, which strongly suggests a valvetrain issue like a lifter or camshaft.
3. Inspect the spark plug wire for cylinder 3 (driver's side, second from front). Ensure it is securely connected at both ends as per TSB PIP4819A.
4. Swap the ignition coil from cylinder 3 with an adjacent cylinder (like cylinder 1). Clear the codes and drive the vehicle. If the code changes to P0301, the ignition coil is bad.
5. If the misfire remains on cylinder 3, remove and inspect the spark plug. Look for wear, fouling, or damage. Swap the spark plug with another cylinder and see if the code follows.
6. If ignition components are confirmed good, consider swapping the fuel injector from cylinder 3 with another cylinder. This is more involved but will isolate a faulty injector.
7. If the misfire still persists on cylinder 3, the cause is likely mechanical. Perform a compression test and a cylinder leak-down test on cylinder 3. Low compression points to an internal problem.
8. If compression is low, remove the driver's side valve cover. Visually inspect for a broken valve spring or a rocker arm that is not moving as much as the others, which indicates a collapsed lifter or wiped camshaft lobe.

Parts You'll Likely Need

• Ignition Coil (OEM #12611424) — A common failure point for misfires. Swapping coils is a key diagnostic step.
  Trusted brands: ACDelco, Delphi
  OEM price range: $50-$80
  Aftermarket price range: $30-$60
• Spark Plug (OEM #41-110) — A standard maintenance item that is a frequent cause of single-cylinder misfires. This part number is often preferred by enthusiasts over its replacement, 41-162, due to perceived quality differences.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$15
  Aftermarket price range: $8-$12
• Spark Plug Wire Set (OEM #9748RR) — GM issued a TSB (PIP4819A) specifically pointing to loose wires as a cause for misfires on these trucks.
  Trusted brands: ACDelco, NGK
  OEM price range: $70-$100
  Aftermarket price range: $40-$80
• Standard (Non-AFM) Lifter (OEM #12648846) — If performing an AFM delete during a camshaft replacement, these standard lifters are used to replace the failure-prone AFM lifters.

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or affects multiple cylinders, a P0300 (Random Misfire) code may appear alongside P0303. This is common when an AFM lifter begins to fail, causing instability before a single cylinder becomes the primary issue.

Technical Service Bulletins (TSBs) & Recalls

• PIP4819A: Points to loose spark plug wires as a cause for various misfire codes, including P0303.
• 19-NA-219: A detailed diagnostic guide for engine noises and misfires (P0300-P0308), listing potential causes as worn camshaft lobes, sticking valves, broken valve springs, and collapsed AFM lifters. It provides crucial context for how a non-AFM cylinder can misfire due to these related failures.

Platform-Specific Known Issues

• Active Fuel Management (AFM) Lifter Failure Leading to Camshaft Damage: This is the most significant issue for this platform. The system deactivates cylinders 1, 4, 6, and 7 to save fuel, but the specialized lifters are prone to collapsing or sticking. When an AFM lifter fails, it can be destroyed by the camshaft, sending metal particles throughout the engine. These particles can then cause rapid wear on other components, most notably the camshaft lobes for other cylinders. A persistent P0303, after ruling out ignition/fuel, is often caused by a wiped cam lobe on cylinder 3 due to a failure that originated in an AFM lifter.

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: Approx. 65 psi. Failure: Significantly lower pressure indicates a weak fuel pump or leak.
• Fuel Pressure (Engine Idling) — expected: Approx. 45 psi. Failure: Pressure below this range can cause lean conditions and misfires.
• Fuel Injector Resistance — expected: Approx. 12.5 Ohms. Failure: A reading far outside this value (open circuit or short) indicates a failed injector coil.
• Ignition Coil Secondary Resistance — expected: 6,000 - 10,000 Ohms (6kΩ - 10kΩ). Failure: A reading outside this range suggests a faulty internal coil winding.

Hidden / Shadow Codes Worth Checking

• Misfire Counter Data: Professional scan tools can access live data showing the exact number of misfire events per cylinder. This is more granular than the P0303 code and can help confirm if the misfire is consistent or intermittent, even before the Check Engine Light is triggered for that cylinder. (see via GM Tech2 or GDS2 scan tool, or other high-end professional scanners with GM-specific software.)

Scan Tool Commands That Help

• GM Tech2 / GDS2: Crankshaft Position System Variation Learn — After replacing a crank/cam sensor, reprogramming the PCM, or on high-mileage engines to improve the accuracy of misfire detection. Failure to perform this can lead to false misfire codes.
• Professional Scan Tool: Cylinder Power Balance Test — To actively test the contribution of each cylinder. The tool will sequentially disable the fuel injector or spark for each cylinder and measure the corresponding drop in RPM. A cylinder with a smaller RPM drop is the one that is misfiring.

Wiring & Ground Locations

• G104 — On the front of the left (driver's side) cylinder head.. This is a primary ground point for the engine block. A poor connection here can affect the ignition system's performance, potentially causing a weak spark on the cylinders of that bank, including cylinder 3.
• G102 — Lower left front of the engine.. Another critical engine ground. Corrosion or a loose connection can introduce electrical noise and resistance, leading to intermittent misfires that are hard to diagnose.
• Engine Block to Chassis Ground — A ground strap typically located from the rear of the engine block (often near the passenger side head) to the firewall/chassis.. This is a main ground path. If it becomes corroded or broken, the ignition system may try to find other paths to ground, leading to erratic behavior and weak spark.
• Cylinder 3 Ignition Coil Connector — The 4-pin electrical connector plugging into the ignition coil for cylinder 3.. Check for corrosion on the terminals or a loose fit. The pins provide power, ground, and the trigger signal from the PCM. A bad connection here will directly cause a misfire on cylinder 3.

Real Owner Repair Stories

• PerformanceTrucks.net Forums (2009 GMC Sierra Denali with 6.2L V8) — Check Engine Light with a single cylinder misfire code and a 'Service StabiliTrak' message.
  ❌ Tried (didn't work) Swapped ignition coil to another cylinder, Swapped spark plug wire to another cylinder
  ✅ What actually fixed it The misfire code did not move with the coil or wire swap. The final repair was replacing a single failed Active Fuel Management (AFM) lifter on the affected cylinder. This indicates the misfire was mechanical (improper valve lift) rather than ignition-related.

OEM Part Supersession History

• ACDelco 41-985 (Platinum), then 41-110 (Iridium) → ACDelco 41-162 (Iridium) — Change in supplier (e.g., from NGK to Denso) and a slight design change.
  Heads up: While 41-162 is the official replacement, there are reports of it being physically shorter or longer than the 41-110. Many enthusiasts and some mechanics report that the original-spec 41-110 provides a smoother idle and is of higher manufacturing quality, leading them to seek out the older part number.

Model Year Variations Within This Range

• 2010-2013: The 6.2L V8 in the Sierra Denali could be either the L9H engine or the L94 engine. The L94 was introduced in 2010 and includes Active Fuel Management (AFM). The L9H does not have AFM. This is a critical difference, as a P0303 caused by camshaft damage from a failed lifter is a high-probability diagnosis on an L94, but extremely unlikely to be AFM-related on an L9H.