P0300 on 2020-2025 Cadillac CT5: Random Misfire Causes and Fixes

What's Unique About the 2020-2025 Cadillac CT5

For the 2020-2025 Cadillac CT5, the P0300 code is not just about typical spark plug or coil failures. General Motors has issued several Technical Service Bulletins (TSBs) that point to more specific underlying causes. TSB #21-NA-036 for the 3.0L V6 identifies a damaged Active Fuel Management (AFM) Oil Control Valve (OCV) solenoid that can leak oil and cause misfires specifically on cylinders 2 and 5. Other bulletins like #18-NA-115 address persistent cold-start misfires which can stem from serious issues like engine block porosity causing coolant to seep into a cylinder. These platform-specific issues require diagnostics that go beyond a simple tune-up.

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 idle, especially on a cold start.
• Hesitation or stumbling during acceleration.
• Flashing or solid Check Engine Light.
• Reduced engine power.
• Decreased fuel economy.
• The StabiliTrak warning light may also illuminate as a secondary effect.

Most Likely Causes

1. Worn or Fouled Spark Plugs 🔴 High Probability → Shop Spark Plug Spark plugs are a standard maintenance item. The CT5's direct-injected and turbocharged engines are sensitive to spark quality for complete combustion. Bad fuel can also foul plugs and cause a temporary P0300 code.
   How to confirm: Remove and inspect the spark plugs for wear, carbon buildup, or oil/coolant fouling. Check the electrode gap. A scan tool can also help identify which cylinders are misfiring most frequently.
   Typical fix: Replace all spark plugs as a set. 🎬 See this walkthrough for replacing CT5 3.0L spark plugs. Use high-quality Iridium plugs as specified by the manufacturer.
   Est. part cost: $80-$150 for a set of 4 or 6
2. Faulty Ignition Coil(s) 🔴 High Probability → Shop Ignition Coil Ignition coils can fail with heat and age. Since P0300 is a random misfire, one failing coil can intermittently cause issues before failing completely and setting a cylinder-specific code (e.g., P0301).
   How to confirm: Use a scan tool to monitor misfire data per cylinder. Swap the suspected ignition coil with one from a known good cylinder and see if the misfire follows the coil.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace the corresponding spark plug at the same time.
   Est. part cost: $50-$110 per coil
3. Damaged AFM Oil Control Valve (OCV) Solenoid 🟡 Medium Probability → Shop Engine Variable Valve Timing (VVT) Oil Control Valve TSB #21-NA-036 specifically calls out a damaged OCV solenoid that can cause an oil leak and misfires on 2020-2022 models with the 3.0L V6 engine. This issue primarily affects cylinders 2 and/or 5, which are the Active Fuel Management (AFM) cylinders.
   How to confirm: Visually inspect the OCV solenoids for oil leaks. The solenoid may break upon removal, with the end stuck in the camshaft cover bore. A diagnostic tool can be used to command the solenoids on and off to test for proper mechanical response.
   Typical fix: Replace the faulty OCV solenoid and clean up any leaked oil. If the end is broken off in the bore, it must be carefully removed with a pick tool.
   Est. part cost: $50-$150 per solenoid
4. Carbon Buildup on Intake Valves 🟡 Medium Probability Like many direct injection engines, the LSY and LGY do not have fuel washing over the intake valves. This can lead to the accumulation of carbon deposits over time, which can disrupt airflow into the cylinders and cause misfires, especially on cold starts.
   How to confirm: This requires a physical inspection using a borescope inserted into the intake ports. TSB #21-NA-278 outlines this diagnostic step for related engine noise and misfire issues.
   Typical fix: The valves must be cleaned manually, typically through walnut blasting or another abrasive media 🎬 Watch: How to professionally clean intake valves using walnut blasting. cleaning process. This is a labor-intensive job.
   Est. part cost: $500-$1000+ for professional cleaning service
5. Vacuum Leak 🟡 Medium Probability Turbocharged engines have a complex network of hoses and gaskets. A leak in the intake system after the mass airflow sensor introduces unmetered air, leading to a lean air-fuel mixture and misfires.
   How to confirm: Perform a smoke test on the intake system to visually identify the source of any leaks. Listen for hissing sounds around the engine bay.
   Typical fix: Replace the leaking hose, gasket, or seal.
   Est. part cost: $10-$100 depending on the part
6. Fuel System Issues (Injectors, Pumps) ⚪ Low Probability Direct injection fuel systems operate under very high pressure. A failing high-pressure fuel pump or dirty/clogged fuel injectors can disrupt fuel delivery. GM has issued bulletins regarding fuel injector diagnostics for misfires.
   How to confirm: Monitor fuel pressure with a scan tool (both low and high pressure sides). A fuel injector balance test (AFIT) can identify poorly performing injectors.
   Typical fix: Clean or replace faulty fuel injectors. Replace the high or low-pressure fuel pump if it fails diagnostic tests.
   Est. part cost: $150-$500+ for injectors or pumps

Rare But Worth Checking

• Internal Engine Mechanical Failure: While rare on newer vehicles, TSBs point to serious underlying issues. TSB #18-NA-115 describes cold start misfires caused by engine block porosity at the deck face, allowing coolant to leak into a cylinder. This requires engine replacement. Other issues can include faulty AFM/DFM lifters or rocker arms that fail to latch, deactivating a cylinder. TSB #21-NA-147 also discusses misfires from oil/coolant leaking into spark plug tubes due to porous cylinder heads, which requires cylinder head replacement.
• Weak Battery or Electrical Issues: A failing battery can provide inconsistent voltage to the ECM and ignition system, which can sometimes trigger a P0300 code under specific conditions.
• Moisture in Charge Air Cooler (CAC): → Shop Intercooler Especially in cold climates, moisture from the PCV system can accumulate and freeze inside the charge air cooler, restricting airflow and causing P0300, often accompanied by P0299 (underboost).

Diagnosis Steps

1. Read all stored fault codes with an OBD-II scanner. Note any other codes present alongside P0300, such as P050D or lean codes.
2. Check the freeze frame data to see the engine conditions (speed, load, temperature) when the misfire was first detected.
3. Use the scanner's live data function to monitor misfire counts for each cylinder. This can help determine if the misfire is truly random or if it's more frequent on certain cylinders (e.g., Cylinders 2 & 5 on the V6).
4. Perform a thorough visual inspection of the engine bay. Look for cracked vacuum hoses, loose electrical connectors, and any signs of oil leaks, especially around the valve covers and OCV solenoids.
5. Inspect the spark plugs. Their condition is a key indicator of engine health. Look for wear, oil fouling, or coolant contamination (a sign of issues from TSB 18-NA-115 or 21-NA-147).
6. Test the ignition coils. The easiest method is to swap a coil from a frequently misfiring cylinder with one from a cylinder that is not misfiring to see if the problem moves.
7. If TSB #21-NA-036 is relevant (3.0L V6 with misfires on Cyl 2/5), inspect the AFM OCV solenoid for damage or being broken off in the cam cover.
8. If a vacuum leak is suspected, perform a smoke test to pinpoint the source.
9. Test fuel pressure and perform a fuel injector balance test (AFIT) as recommended in TSB #21-NA-278 if an injector issue is suspected.
10. For persistent cold start misfires, investigate TSB #18-NA-115. This may involve adding coolant dye and using a borescope to check for coolant intrusion into the cylinders.
11. If all else fails, a mechanical compression and cylinder leak-down test may be necessary to check for internal engine problems like faulty lifters or piston ring issues.

Parts You'll Likely Need

• Spark Plugs (OEM #ACDelco 41-162 (for 3.0L LGY) / ACDelco 41-166 (for 2.0L LSY)) — This is the most common maintenance item related to misfires. Worn plugs are a primary cause of inefficient combustion.
  Trusted brands: ACDelco, NGK
  OEM price range: $13-$25 per plug
  Aftermarket price range: $11-$21 per plug
• Ignition Coil (OEM #ACDelco 12716099 (for 3.0L LGY) / ACDelco 12699539 (for 2.0L LSY)) — Ignition coils provide the high voltage needed for spark. They are a frequent failure point leading to misfires.
  Trusted brands: ACDelco, Delphi
  OEM price range: $65-$110 per coil
  Aftermarket price range: $45-$80 per coil
• Oil Control Valve (AFM Solenoid) (OEM #12687167 (for 3.0L V6)) — A known failure point on the 3.0L V6 engine as per TSB #21-NA-036, causing misfires on cylinders 2 and 5.
  Trusted brands: ACDelco
  OEM price range: $50-$150 per solenoid
  Aftermarket price range: $40-$100 per solenoid

Related Codes That Often Appear With This One

• P0301-P0306 — These codes indicate a misfire on a specific cylinder (e.g., P0301 for Cylinder 1). A random P0300 may eventually become a specific cylinder code as a component fails completely.
• P050D — TSB #PIP5628G links this code, "Cold Start Rough Idle," with misfire codes, indicating a known issue with cold engine performance that requires specific data collection for diagnosis.
• P0171/P0174 — These "System Too Lean" codes can be caused by a vacuum leak, which is also a common cause for random misfires.
• P0302/P0305 (with 3.0L LGY) — TSB #21-NA-036 specifically links these cylinder misfires with P0300, pointing directly to a faulty AFM Oil Control Valve solenoid.

Technical Service Bulletins (TSBs) & Recalls

• TSB #21-NA-036: Details diagnosis for P0300, P0302, and/or P0305 on the 3.0L V6, pointing to a damaged AFM Oil Control Valve solenoid leaking oil or broken rocker arms on cylinders 2 and 5.
• TSB #18-NA-115: Addresses cold start misfires and rough idle with P0300. The ultimate cause can be engine block porosity allowing coolant to leak into a cylinder, which requires engine replacement.
• TSB #PIP5628G: A preliminary bulletin instructing technicians on what data to collect (freeze frame records, fuel trims, etc.) when diagnosing P0300-P0308 and P050D, highlighting the complexity of the issue.
• TSB #21-NA-147: Warns of fluid (oil or coolant) found in spark plug tubes causing misfires due to porosity in the cylinder head casting. Requires cylinder head replacement.
• TSB #21-NA-278: Provides a diagnostic strategy for engine ticking/rattle noises that can be accompanied by P0300, suggesting checks for leaking fuel injectors, carbon on valves, and AFM/DFM hardware.

Platform-Specific Known Issues

• Cold Start Misfire and Coolant Loss (TSB #18-NA-115): A particularly severe but documented issue involves a persistent misfire only on cold starts that disappears as the engine warms up. This can be caused by microscopic porosity in the engine block at the cylinder liner interface, allowing a small amount of coolant to seep into the cylinder overnight. Upon starting, this coolant causes a misfire until it is burned off. Diagnosis involves adding dye to the coolant and inspecting the cylinder with a borescope after a hot soak and cool-down period. If confirmed, the only official fix is engine replacement.
• AFM Cylinder Misfires on 3.0L V6 (TSB #21-NA-036): Owners of the CT5 with the 3.0L LGY V6 may experience a P0300 code that is heavily biased towards cylinders 2 and 5. This is a strong indicator of the issue described in TSB #21-NA-036. The cause is often the Active Fuel Management (AFM) oil control valve solenoid failing or breaking, leading to improper oil pressure for the lifters on those cylinders, causing them to deactivate improperly and misfire.
• Moisture Freezing in Charge Air Cooler (Cold Climates): → Shop Intercooler In regions with freezing temperatures, the 2.0L LSY engine can be prone to collecting moisture in the charge air cooler (CAC). This moisture can freeze, creating a significant airflow restriction that leads to a rough running engine and a P0300 code, often with an underboost code (P0299). GM has issued bulletins for similar engines describing this exact scenario.

Mechanic-Grade Diagnostic Values

• Direct Fuel Injector Voltage — expected: The ECM initiates injector opening with a 65V boost from an internal capacitor, then holds the injector open with a standard 12V signal.. Failure: Failure of the ECM's boost capacitor or driver circuit can lead to incomplete injector opening and misfires.
• Live Misfire Data Counts (per cylinder) — expected: Using a scan tool, all cylinders should show a count of '0' on the misfire counter at idle and under load.. Failure: Any cylinder showing incrementing numbers indicates a misfire on that cylinder. A P0300 is set when multiple cylinders show counts without a consistent pattern.

Hidden / Shadow Codes Worth Checking

• P0315: Crankshaft Position System Variation Not Learned. This code indicates that the ECM has not learned the precise position of the crankshaft reluctor wheel, which is necessary for accurate misfire detection. (see via This code may be set after replacing a crankshaft position sensor, ECM, or crankshaft balancer. It is visible with a professional scan tool like a GM Tech2 or GDS2.)
• P1336: Crankshaft Position (CKP) System Variation Not Learned. This is a GM-specific code that is stored if the 'Crankshaft Variation Relearn' procedure is attempted but fails to complete successfully. (see via Visible with a professional scan tool after a failed relearn procedure.)
• Pending Codes: The ECM can detect an anomaly, like an intermittent misfire, and log it as a 'pending' or 'history' code. The Check Engine Light will not illuminate until the fault is detected a specific number of times within a drive cycle. (see via Most OBD-II scan tools have a function to read 'Stored Codes', 'Pending Codes', and 'History Codes' separately.)

Scan Tool Commands That Help

• GDS2 / Tech2 (or equivalent professional scanner): Crankshaft Variation Relearn (CASE Relearn) — Required after replacing the crankshaft position sensor, ECM, crankshaft balancer, or performing major engine work. Failure to perform this can be a direct cause of a P0300 code, as the ECM cannot accurately detect misfires. The procedure involves warming the engine, applying the brakes, and accelerating to a fuel cut-off RPM (around 4000 RPM) to allow the ECM to learn the crankshaft's profile.
• GDS2 (or equivalent): AFM Oil Control Valve Test / Oil Pump Command — When diagnosing a P0300 on a 3.0L LGY engine, especially if misfires are noted on AFM cylinders (2 and 5). After replacing a suspect Rocker Arm or AFM Oil Control Valve (OCV), a technician can command the oil pump to high pressure mode to verify the misfire is resolved.
• GDS2 (or equivalent): Injector Balance Test (AFIT - Active Fuel Injector Tester) — If a fuel system issue is suspected, this function can identify poorly performing, leaking, or clogged injectors that may be causing random misfires.

Wiring & Ground Locations

• G102 (Plausible) — Typically located at the left rear of the engine or engine block on GM vehicles.. This ground point is commonly used for the ignition coils. A loose or corroded G102 ground can cause a weak or intermittent spark across multiple cylinders, leading directly to a P0300 code.

Real Owner Repair Stories

• Reko Auto Parts blog (Cadillac, model unspecified, with symptoms of body shaking and weak acceleration.) — Body shaking, weak acceleration, illuminated Engine and TCS lights, codes P0300 and P0299 (Turbo Underboost).
  ❌ Tried (didn't work) Swapping the ignition coil from the misfiring cylinder (Cylinder 2) with another cylinder.
  ✅ What actually fixed it The spark plug in cylinder 2 was found to be black and wet. The root cause was a faulty fuel injector on cylinder 2, which was replaced.
• YouTube channel 'Handy Val' (2011 Cadillac CTS V6 3.0L (Note: Not a CT5, but demonstrates a relevant edge case)) — Car shaking, StabiliTrak light on, Check Engine light on with code P0300.
  ❌ Tried (didn't work) Initial diagnosis focused on common causes like bad fuel, spark plugs, and vacuum leaks.
  ✅ What actually fixed it The issue was resolved by replacing the original 8-year-old battery. A weak battery can provide unstable voltage to the ECM and ignition system, causing random misfires even if it has enough power to start the engine.

OEM Part Supersession History

• 13502510 → Not specified, but part is noted as replaced. — GM regularly updates service part designs to integrate new materials and technologies.

Model Year Variations Within This Range

• 2020+: The CT5 was launched with GM's 'Next Generation Digital Vehicle Platform,' featuring faster data processing and 100Mbps/1Gbps Ethernet connections. This advanced architecture can affect diagnostic procedures and data availability compared to older GM platforms.
• 2020: At the start of the 2020 model year, the 2.0L LSY was the standard engine. The 3.0L LGY twin-turbo V6 became available later in the model year for Premium Luxury and V-Series trims, meaning early 2020 models are less likely to have the V6-specific issues mentioned in some TSBs.