P0303 on 2018-2021 Chevrolet Suburban: Cylinder 3 Misfire Causes and Fixes

What's Unique About the 2018-2021 Chevrolet SUBURBAN

The 2018-2021 Suburban, equipped with 5.3L or 6.2L V8 engines, uses Active Fuel Management (AFM) or Dynamic Fuel Management (DFM). These systems deactivate cylinders to save fuel but are widely known to cause premature failure of the specialized valve lifters. A collapsed or stuck lifter is a very common cause for a misfire code like P0303 on this platform, making it a much more serious potential issue than on many other vehicles. This problem is so prevalent that GM has issued technical service bulletins, such as TSB 23-NA-043, specifically to address diagnosis and repair of failed lifters causing misfires.

Diagnostic Flowchart

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

Generation note: This range covers two generations: the 11th (2018-2020) and the 12th (2021). The 2018-2020 models primarily use Active Fuel Management (AFM) on the L83/L86 engines, while the 2021 models introduced the more advanced Dynamic Fuel Management (DFM) on the L84/L87 engines. Both systems are linked to valve lifter failures that can cause a P0303 code.

Symptoms You May Notice

• Flashing or solid Check Engine Light
• Rough or shaky idle
• Engine hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Reduced fuel economy
• Engine ticking or tapping noise, especially on a cold start or that changes with RPM
• Smell of fuel from the exhaust

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems are common failure points due to constant exposure to engine heat and vibration. It's a frequent and relatively simple cause for a single-cylinder misfire.
   How to confirm: Swap the ignition coil from cylinder 3 with another cylinder (e.g., cylinder 1). Clear the codes and drive the vehicle. If the code changes to P0301, the ignition coil is faulty. Also inspect the coil's electrical connector for loose or damaged wires, as this can mimic a failed coil.
   Typical fix: Replace the faulty ignition coil. 🎬 See this walkthrough for replacing a GM 5.3 ignition coil. Many owners choose to replace the corresponding spark plug at the same time.
   Est. part cost: $40-$95
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a regular maintenance item. A worn electrode, cracked porcelain, or carbon/oil fouling can prevent a proper spark. Oil fouling can be a symptom of other issues like PCV problems or failing piston rings.
   How to confirm: Remove the spark plug from cylinder 3 and inspect it for wear, damage, or fouling (black, sooty, or oily deposits). Compare it to a new plug or a plug from a working cylinder.
   Typical fix: Replace the spark plug. It is best practice to replace all 8 spark plugs at the same time to ensure even performance and prevent future misfires.
   Est. part cost: $8-$20 per plug
3. Failed AFM/DFM Valve Lifter 🟡 Medium Probability → Shop Engine Valve Lifter The Active/Dynamic Fuel Management systems in GM's 5.3L and 6.2L engines are a known major weak point. The specialized lifters can collapse, get stuck, or have their internal locking pin fail, preventing the valve from opening and causing a persistent misfire. TSB #23-NA-043 directly addresses this issue, indicating its prevalence.
   How to confirm: This is an advanced diagnosis. A distinct ticking noise from the engine is a strong indicator. A compression test on cylinder 3 will likely show low or zero compression. A borescope may reveal the valve is not moving. A mechanic can use a scan tool to monitor valve operation if the feature is available. A collapsed lifter can also damage the camshaft, requiring further inspection once the engine is opened up.
   Typical fix: This is a significant and expensive repair. Per TSB #23-NA-043, the procedure involves replacing all lifters and guides on the affected bank (or both banks if under 16k miles). If the camshaft is damaged, it must also be replaced. Many owners opt to perform an 'AFM/DFM delete', replacing the specialized components with standard lifters, a new camshaft, and an ECU tune to prevent future failures. 🎬 Watch: A mechanic explains the permanent fix for GM lifter failure.
   Est. part cost: $500-$1500 for lifters, guides, gaskets, and potentially a camshaft.
4. Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Direct injection fuel injectors operate under high pressure and can become clogged or fail electronically. While less common than ignition or lifter issues for a P0303 code, it is a known failure point.
   How to confirm: Listen for a consistent clicking sound from the injector with a mechanic's stethoscope. A more definitive test is to swap the fuel injector from cylinder 3 with another cylinder, but this is significantly more labor-intensive than swapping a coil. A scan tool can also be used to perform a fuel injector balance test.
   Typical fix: Replace the faulty fuel injector. It's often recommended to replace the seals for adjacent injectors at the same time.
   Est. part cost: $50-$120

Rare But Worth Checking

• Bent Pushrod: Often a direct consequence of a collapsed or seized lifter. If a lifter fails, the force from the camshaft can bend the pushrod, which also prevents valve actuation and causes a misfire. This is usually discovered during a lifter replacement job.
• Broken Valve Spring: Though less common than a lifter failure, a broken valve spring can cause a P0303. This will prevent the valve from closing properly, leading to a loss of compression and a misfire. GM TSB 19-NA-219 lists this as a possible cause for misfires on this engine family.
• Intake Manifold Gasket Leak: → Shop Engine Intake Manifold A vacuum leak near the intake port for cylinder 3 can lean out the air/fuel mixture and cause a misfire. This would typically be accompanied by other codes, like a P0171 or P0174 (System Too Lean).

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner. Confirm P0303 is present and note any other codes (e.g., P0300, P0203). Use a scanner with live data to view misfire counts for each cylinder.
🎬 Watch: How to diagnose misfire codes and test ignition components.
2. Clear the codes and perform a test drive to confirm the P0303 code returns.
3. Locate cylinder 3 (driver's side, second cylinder from the front).
4. Swap the ignition coil from cylinder 3 with the coil from cylinder 1. Clear codes and test drive. If the code changes to P0301, the ignition coil is bad. Inspect the wiring connector for damage.
5. If the code remains P0303, remove the spark plug from cylinder 3. Inspect for damage, wear, or fouling (especially wetness from fuel or oil). If the plug is suspect, replace it. Swapping plugs is also an option.
6. If ignition components are good, listen to the fuel injector on cylinder 3 with a stethoscope to confirm it is clicking. Compare the sound to an adjacent injector.
7. If the misfire persists and especially if a ticking noise is present, perform an engine compression test on cylinder 3. A reading significantly lower than other cylinders points to a mechanical problem.
8. If compression is low, the most likely cause on this engine is a failed AFM/DFM lifter, bent pushrod, or broken valve spring. This is a complex repair that requires cylinder head removal and likely professional service.

Parts You'll Likely Need

• Ignition Coil (OEM #12619161) — This is one of the most common failure items for a single-cylinder misfire. It's relatively easy to diagnose by swapping cylinders.
  Trusted brands: ACDelco, Delphi, Duralast
  OEM price range: $60-$95
  Aftermarket price range: $40-$70
• Iridium Spark Plug (OEM #41-168) — Spark plugs are a maintenance item and can cause misfires when worn. It's often replaced along with the ignition coil.
  Trusted brands: ACDelco, NGK
  OEM price range: $10-$20
  Aftermarket price range: $8-$15
• Valve Lifter Kit (AFM/DFM Repair or Delete)

Related Codes That Often Appear With This One

• P0300 — If the misfire is severe or affects multiple cylinders, a P0300 (Random/Multiple Cylinder Misfire) may be stored alongside P0303. This is common with systemic issues like lifter failures.
• P0203 — This code for 'Cylinder 3 Injector Circuit Malfunction' points specifically to an electrical problem with the injector or its wiring. It will almost always be accompanied by a P0303 misfire code.
• P0301, P0305, P0307, etc. — If the cause is multiple failing lifters, which is common, codes for other AFM/DFM cylinders (1, 4, 6, 7) may appear as well.

Technical Service Bulletins (TSBs) & Recalls

• Poor Engine Performance, Engine Misfires, Malfunction Indicator Lamp (MIL) Illuminated: This TSB directly addresses DTCs P0300 through P0308. It provides guidelines for replacing failed AFM/DFM lifters. If the vehicle has under 16,000 miles, GM recommends replacing all lifters and guides on both banks. If over 16,000 miles, only the lifters on the affected bank should be replaced. The bulletin confirms that a bent pushrod can be a consequence of a collapsed lifter.
• Diagnostic Tip for Misfire, Chirp, Squeak, Squeal or Tick Noise: This bulletin provides diagnostic tips for various engine noises and misfires (P0300-P0308). It lists potential causes including worn camshaft lobes, sticking valves, broken valve springs, and collapsed AFM lifters, which are all relevant to a P0303 diagnosis.
• Engine Misfire/Tick Noise, Malfunction Indicator Lamp (MIL) Illuminated with DTC P0300 Set: An earlier series of bulletins that also addresses engine misfires and ticking noises caused by mechanically collapsed or stuck AFM lifters. It instructs technicians to inspect the camshaft for damage and replace it along with the lifters if wear is found.

Platform-Specific Known Issues

• AFM/DFM Lifter Failure: The most significant and costly issue for these engines. The system that deactivates cylinders to improve fuel economy puts stress on the valve lifters, leading to a high rate of failure. A failed lifter will cause a misfire and can lead to camshaft damage if not addressed. GM has issued multiple TSBs for this problem.
• Faulty Ignition Coil Connector: In some cases, a persistent P0303 misfire that isn't solved by a new coil or plug can be caused by a faulty electrical connector to the ignition coil. The wires can become loose or break internally, causing an intermittent connection that mimics a bad coil.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: 0.4 to 2.0 Ohms (typical). Failure: A reading outside of the specified range.
• Ignition Coil Secondary Resistance — expected: 6,000 to 10,000 Ohms (typical). Failure: A reading outside of this range suggests a faulty coil.
• Fuel Injector Coil Resistance — expected: 10 - 16 Ohms. Failure: A reading significantly outside this range, or showing an open (infinite) or short (near-zero) resistance.
• Low-Pressure Fuel System (at fuel rail test port) — expected: 55-62 PSI (for returnless systems 2005+). Failure: Pressure significantly below 55 PSI indicates a weak low-pressure fuel pump or clogged filter.
• High-Pressure Fuel System (Direct Injection, via scan tool) — expected: ~500-900 PSI at idle. Failure: Pressure that is erratic or significantly lower than expected can indicate a failing high-pressure fuel pump.
• Scan Tool Misfire Counter — expected: Zero or very few counts on all cylinders at idle.. Failure: A rapidly increasing count on cylinder 3 confirms the misfire location in real-time.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Misfire Graphic — To visually monitor current and historical misfire counts for each specific cylinder to confirm the fault is isolated to cylinder 3.
• GDS2 (GM Global Diagnostic System 2): Fuel Injector Balance Test — To command each injector to fire and measure the corresponding pressure drop in the fuel rail. A cylinder 3 injector with a pressure drop significantly different (>1.5 PSI) from the others is likely faulty.
• GDS2 (GM Global Diagnostic System 2): Cylinder Deactivation (AFM/DFM Control) — To manually command the AFM/DFM system on and off to see if it influences the misfire, which can help diagnose a sticking or faulty deactivation lifter on cylinder 3.

Wiring & Ground Locations

• G102 — Located at the left rear of the engine.. This is the primary ground point for all 8 ignition coils. A loose or corroded connection at G102 can cause intermittent or weak spark on one or more cylinders, including cylinder 3, leading to a misfire code.
• Ignition Coil Connector — On top of the valve cover, connected to the cylinder 3 ignition coil.. The 4-pin connector provides power, ground, and the trigger signal from the ECM. A loose pin, corrosion, or damaged wire at this specific connector will cause a P0303 by preventing the coil from firing correctly.

Real Owner Repair Stories

• YouTube video from 'MecanicoSoyDeCoraza' (2018 Chevrolet Silverado 5.3L) — Persistent P0306 (Cylinder 6 Misfire), but logic applies to P0303.
  ❌ Tried (didn't work) Swapping spark plugs, Swapping ignition coils, Testing and cleaning fuel injectors in a lab
  ✅ What actually fixed it After extensive diagnosis, the problem was identified as a failed valve lifter, a common issue with the AFM system. The repair involved removing the cylinder head to replace the faulty lifter.

OEM Part Supersession History

• 12619161 (Ignition Coil) → 12723610 — Standard part evolution and potential design improvements.
• 12639516, 12645725 (AFM Lifter) → 12698946, and subsequently 12740071 — Multiple revisions by GM to address the high failure rate of the AFM lifters.
  Heads up: The newest DFM lifter design (for 2019+ L84/L87 engines) is generally backwards-compatible with older AFM engines (L83/L86), but the original AFM lifters are NOT forward-compatible with DFM engines.

Model Year Variations Within This Range

• 2018-2020: These models primarily use the 5.3L L83 and 6.2L L86 engines with Active Fuel Management (AFM), which deactivates the same 4 cylinders (1, 7, 6, 4). Lifter failures are common on these specific cylinders.
• 2021: The 2021 model introduced the 5.3L L84 and 6.2L L87 engines with Dynamic Fuel Management (DFM). DFM is more complex and can deactivate any combination of cylinders, meaning a lifter failure is possible on any of the 8 cylinders, not just the 4 associated with AFM.