P0305 on 2019-2022 Chevrolet Blazer 3.6L: Cylinder 5 Misfire Causes and Fixes

What's Unique About the 2019-2022 Chevrolet BLAZER

For the 2019-2022 Blazer, the P0305 code is exclusive to the 3.6L V6 LGX engine. While typical causes like ignition coils and spark plugs are common, this platform has a specific Technical Service Bulletin (TSB #21-NA-036) that links P0305 to a potentially damaged Active Fuel Management (AFM) Oil Control Valve (OCV) solenoid. Cylinder 5 is one of the two cylinders used for AFM (cylinder deactivation). A failure in the OCV solenoid can cause improper oil pressure, leading to unintended cylinder deactivation and a persistent misfire code. This makes checking the AFM system a unique and critical diagnostic step for this specific code 🎬 Watch: Why cylinder 5 misfires on this specific engine on this engine.

Diagnostic Flowchart

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

Symptoms You May Notice

• Rough or shaky engine idle
• Hesitation or stumbling during acceleration
• Reduced engine power
• Flashing or solid Check Engine Light
• Poor fuel economy
• Fuel odor from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Coil-on-plug systems are a common failure point on many modern engines as they age and are subjected to heat cycles.
   How to confirm: The easiest way to confirm a bad coil is to swap the ignition coil from cylinder 5 with another cylinder (e.g., cylinder 3). Clear the codes and drive the vehicle. If the code changes to P0303, the ignition coil is faulty. This is a widely accepted diagnostic first step.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $35-$60
2. Worn or Fouled Spark Plug 🔴 High Probability
   How to confirm: After ruling out the ignition coil, inspect the spark plug for cylinder 5. Look for excessive wear on the electrode, carbon buildup, oil fouling, or incorrect gap. The OEM ACDelco Iridium plugs are pre-gapped and should not be adjusted.
   Typical fix: Replace the spark plug. If one is worn, it's best practice to replace all six spark plugs as a set 🎬 See this step-by-step spark plug replacement walkthrough to ensure consistent performance and avoid future misfires on other cylinders.
   Est. part cost: $15-$25 per plug
3. Damaged AFM Oil Control Valve (OCV) Solenoid 🟡 Medium Probability A specific GM Technical Service Bulletin (#21-NA-036) identifies an issue with damaged AFM OCV solenoids causing oil leaks and misfires specifically on cylinders 2 and 5, which are the AFM cylinders. The bulletin notes the end of the solenoid can break off and get stuck in the camshaft cover.
   How to confirm: Visually inspect the AFM OCV solenoid for the bank containing cylinder 5 (Bank 1, the rear bank) for signs of oil leakage or physical damage. The TSB provides images of a failed solenoid. A diagnostic step can be to swap the solenoids between the front and rear banks to see if the misfire moves.
   Typical fix: Replace the faulty AFM OCV solenoid and clean up any resulting oil residue. 🎬 Watch: How to replace the VVT solenoid on a GM 3.6L If the tip is broken off in the camshaft cover, it must be carefully removed with a pick tool.
   Est. part cost: $45-$75
4. Clogged or Faulty Fuel Injector ⚪ Low Probability Direct injection systems like the LGX operate under high pressure and can be susceptible to clogging from fuel contaminants or carbon buildup over time, which can lead to misfires.
   How to confirm: This is more difficult to diagnose. A professional may need to perform a fuel injector balance test using a high-end scan tool. Swapping injectors between cylinders is possible but is significantly more labor-intensive than swapping coils as it requires removing the fuel rail.
   Typical fix: Replace the faulty fuel injector. It is often recommended to replace them as a set.
   Est. part cost: $70-$120

Rare But Worth Checking

• Internal Engine Issues (Low Compression): Though uncommon on newer engines, a misfire can be a symptom of a more serious mechanical problem like a worn piston ring, a failing valve, or a damaged cylinder wall. A compression test followed by a cylinder leak-down test is needed to diagnose this.
• Leaking Spark Plug Tube: Another TSB on some GM 3.6L engines (PIP5095G, superseded by 21-NA-120) mentions that porosity in the cylinder head can cause oil to leak into the spark plug tube, fouling the plug and causing a misfire. This is a major repair that requires cylinder head replacement as the tubes are not serviced separately.
• AFM Lifter / Rocker Arm Failure: Related TSBs (like PIP5706B and 21-NA-036) for the LGX engine family describe scenarios where a misfire on an AFM cylinder (like #5) is caused by faulty Switching Roller Finger Followers (rocker arms) or Stationary Hydraulic Lash Adjusters (lifters). This can cause a ticking noise and may require removing the camshaft cover to inspect valve operation and replacing the lifters and rockers for that cylinder.

Diagnosis Steps

1. Read the codes with an OBD-II scanner to confirm P0305 and check for any other codes (e.g., P0300, P0302).
2. Identify cylinder 5 on the 3.6L V6 engine. In the transverse-mounted Blazer, Bank 1 is the rear bank against the firewall. Cylinders are numbered 1-3-5 from passenger to driver side on Bank 1. Cylinder 5 is the rear-most cylinder on the driver's side.
3. Swap the ignition coil from cylinder 5 to another cylinder on the same bank (e.g., cylinder 3). This is more accessible than swapping to the front bank. Clear the codes and run the engine to see if the misfire code moves to P0303. If it does, the coil is bad.
4. If the misfire stays on cylinder 5, inspect the spark plug. Remove it and check for wear, fouling, or damage. Since access is difficult, it's wise to replace it while it's out. The OEM plug is an ACDelco Iridium #12683541.
5. If the code remains, inspect the wiring and connector for the ignition coil and fuel injector on cylinder 5 for any damage or loose connections.
6. Following TSB #21-NA-036, inspect the Bank 1 (rear) AFM Oil Control Valve solenoid for signs of oil leakage or damage. This is a known issue for this specific misfire code.
7. If the above steps do not identify the problem, more advanced diagnostics are needed. This includes testing the fuel injector, performing an engine compression test, and a cylinder leak-down test to rule out mechanical engine problems.

Parts You'll Likely Need

• Ignition Coil (OEM #12707304) — This is one of the most common failure points for a single-cylinder misfire and the easiest to diagnose via the 'swap test'.
  Trusted brands: ACDelco (OE), Delphi, NGK
  OEM price range: $40-$60
  Aftermarket price range: $30-$50
• Iridium Spark Plug (OEM #12683541) — Spark plugs are a routine maintenance item and can wear out or become fouled, causing misfires. The OEM plug is an ACDelco Iridium, which is pre-gapped.
  Trusted brands: ACDelco (OE), NGK, Denso
  OEM price range: $15-$25
  Aftermarket price range: $10-$20
• Engine Variable Valve Timing (VVT) Solenoid / AFM Oil Control Valve (OCV) (OEM #12732244) — A known issue documented in TSB #21-NA-036 for this vehicle that can cause oil leaks and a P0305 misfire due to its role in the Active Fuel Management system.
  Trusted brands: ACDelco (OE), Standard Motor Products (SMP), Dorman
  OEM price range: $50-$75
  Aftermarket price range: $35-$60

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 P0305. This can sometimes point towards a systemic issue like a vacuum leak or fuel pressure problem.
• P0302 — TSB #21-NA-036 specifically calls out P0302 and P0305 together, as cylinders 2 and 5 are the designated Active Fuel Management (AFM) cylinders. A fault in the AFM system could potentially trigger codes for both.

Technical Service Bulletins (TSBs) & Recalls

• Bulletin #21-NA-036

Platform-Specific Known Issues

• TSB #21-NA-036: Special Coverage for a possible issue with a damaged AFM Oil Control Valve (OCV) solenoid that may cause an oil leak and misfire, setting DTCs P0300, P0302 and/or P0305. The bulletin notes the end of the solenoid can break off in the camshaft cover.
• TSB #PIP5706B / 21-NA-174: Addresses engine misfires (P0300-P0306) possibly accompanied by ticking noises, which may be caused by faulty AFM lifters (SHLAs) or rocker arms (SRFFs), especially after a hot soak.

Mechanic-Grade Diagnostic Values

• Fuel Injector Resistance (3.6L LGX GDI) — expected: Approximately 1.5 - 2.2 Ohms. Failure: A significant deviation or an open/short circuit reading indicates a faulty injector coil.
• AFM/VVT Solenoid Resistance (3.6L LGX) — expected: 8 - 12 Ohms. Failure: Readings outside this range suggest a faulty solenoid.
• Low-Pressure Fuel System (Key On, Engine Off) — expected: 55-60 PSI. Failure: Pressure below this range indicates a problem with the in-tank fuel pump.
• High-Pressure Fuel System (Under Load) — expected: Can exceed 2,000 PSI. Failure: Low pressure, which may set other codes like P0087, can contribute to lean misfires. Requires a scan tool to monitor.

Hidden / Shadow Codes Worth Checking

• Misfire Type A / Type B: GM classifies misfires by severity. A Type A misfire is severe enough to damage the catalytic converter (occurs within 200 revolutions) and will cause the Check Engine Light to flash. A Type B is less severe (occurs within 1,000 revolutions) and will illuminate a solid Check Engine Light. (see via This is an internal ECM classification. A flashing vs. solid MIL is the primary indicator. A professional scan tool (like GDS2) can display misfire counters and history.)

Scan Tool Commands That Help

• GDS2 / Professional Bidirectional Scanner: Crankshaft Position Variation Learn (CASE Relearn) — This procedure is mandatory after replacing the crankshaft position sensor, ECM, or making significant engine repairs. It synchronizes the sensor with the actual crankshaft position to ensure accurate misfire detection. Failure to perform this can cause false misfire codes or prevent the system from detecting real misfires.
• GDS2 / Professional Bidirectional Scanner: Fuel Injector Balance Test — This function allows a technician to command each injector to fire for a set duration and measures the corresponding pressure drop in the fuel rail. It can identify a clogged or weak injector on a specific cylinder without requiring physical removal.
• GDS2 / Professional Bidirectional Scanner: AFM Solenoid Control — Allows a technician to manually command the Active Fuel Management solenoids on and off to verify their mechanical and electrical operation, helping to diagnose AFM-related misfires as described in TSB #21-NA-036.

OEM Part Supersession History

• 12666339 → 12707304, and subsequently 12741091 — Standard part evolution for improved durability or manufacturing process changes.
  Heads up: The newer part numbers are generally backward compatible for this application.
• 12647900, 12671336, 12687167, 12690515, 12707054 → 12732244 — Multiple revisions consolidating into a final part number for the VVT/AFM solenoid.
  Heads up: 12732244 is the current recommended replacement for the specified LGX engine applications.

Model Year Variations Within This Range

• 2019: The 2019 model year was the launch year and also offered a base 2.5L 4-cylinder engine on lower trims, which was discontinued for 2022. The 3.6L V6 LGX engine, however, remained mechanically consistent across the 2019-2022 range.
• 2020-2021: A 2.0L Turbo engine was added as an option. No significant changes were made to the 3.6L V6 powertrain that would affect the diagnosis of P0305.
• 2022: The base 2.5L engine and its corresponding L and 1LT trims were dropped, making the 2.0L Turbo the new standard engine. The 3.6L V6 remained the optional and standard engine on higher trims like the RS.