P0301 on 2017-2021 Ford GT: Cylinder 1 Misfire Causes and Fixes

What's Unique About the 2017-2021 Ford GT

The Ford GT uses a high-performance, high-output version of the 3.5L EcoBoost V6 engine, shared with the F-150 Raptor. While the root causes of a misfire are universal (spark, fuel, compression), this engine's high cylinder pressures from turbocharging can be particularly demanding on ignition components, leading to a higher likelihood of spark plug or coil failure. Notably, Ford has issued multiple Technical Service Bulletins (TSBs) for the GT, indicating that a rough run or hard start with a P0301 code might occur due to specific "vehicle operating conditions." This suggests a sensitivity that may not be present in other applications of this engine. Forum discussions on similar engines suggest these conditions can include repeated short trips without reaching full operating temperature, which can lead to fouled plugs. A very common, but often missed, issue on this engine family is a hairline crack in the spark plug's porcelain insulator, which can cause a misfire under load that is difficult to diagnose with a simple coil swap.

Diagnostic Flowchart

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

Symptoms You May Notice

• Rough running engine, especially at idle
• Hard starting
• Hesitation or stumbling during acceleration
• Flashing or solid Check Engine Light
• Noticeable loss of power
• Vibration from the engine that can be felt in the gas pedal

Most Likely Causes

1. Worn or Fouled Spark Plug 🔴 High Probability Turbocharged, direct-injection engines like the high-output 3.5L EcoBoost are known to be harder on spark plugs, leading to faster wear of the electrode or cracking of the porcelain insulator. High cylinder pressures can also 'blow out' the spark if the gap has widened even slightly from wear.
   How to confirm: Remove the spark plug from cylinder 1. Inspect for a widened gap, carbon fouling, oil residue, or a cracked insulator. The most effective diagnostic is to swap the cylinder 1 spark plug with one from another cylinder (e.g., cylinder 2). If the misfire code changes to P0302, the spark plug is the culprit.
   Typical fix: Replace the faulty spark plug. It is best practice to replace all six spark plugs at the same time to ensure consistent performance and prevent future misfires.
   Est. part cost: $15-$30 per plug
2. Faulty Ignition Coil (Coil-on-Plug) 🔴 High Probability Ignition coils are subjected to significant heat and vibration, which can cause their internal windings to fail over time, resulting in a weak or non-existent spark. This is a very common failure point on modern engines. For the 2020 model year, the Ford GT was upgraded with higher-energy ignition coils, which may affect failure rates compared to earlier models.
   How to confirm: After confirming the spark plug is not the issue, swap the ignition coil from cylinder 1 with a coil from another cylinder (e.g., cylinder 2). Clear the codes and drive the vehicle. If the code returns as P0302, the ignition coil is faulty.
   Typical fix: Replace the failed ignition coil. While some owners replace all coils at once as a preventative measure, it is only necessary to replace the failed unit.
   Est. part cost: $70-$120 per coil
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability As a direct-injection (GDI) engine, carbon deposits can build up on the injector tips, affecting the spray pattern and fuel volume. This can lead to a lean condition in the cylinder and cause a misfire.
   How to confirm: This is more difficult to diagnose. It involves checking the wiring to the injector and may require a professional scan tool like Ford's IDS to perform a fuel injector balance test or power balance test. Swapping injectors is possible but is a more labor-intensive job than swapping coils or plugs.
   Typical fix: First, try a quality fuel system cleaner designed for GDI engines. If that fails, the injector may need to be professionally cleaned or replaced.
   Est. part cost: $100-$250 per injector

Rare But Worth Checking

• Vacuum Leak: A leak in a vacuum hose or intake gasket near cylinder 1 can lean out the air/fuel mixture, causing a misfire. This would typically be accompanied by other codes or a rough idle.
• Low Engine Compression: This is a serious mechanical issue, such as a worn piston ring, a leaking valve, or even a damaged cylinder head. A compression test would be needed to confirm this, which is usually a last resort after checking spark and fuel.
• Intake Valve Carbon Buildup: Direct injection engines spray fuel directly into the cylinder, not over the intake valves. As a result, oil vapor from the PCV system can bake onto the back of the intake valves, restricting airflow into the cylinder and causing misfires. This is a known issue on many GDI engines.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0301 and check for any other codes.
2. Note if the Check Engine Light is solid or flashing. A flashing light indicates a severe misfire requiring immediate attention to prevent catalytic converter damage.
3. Locate cylinder 1, which is the front-most cylinder on the passenger side of the engine.
4. Swap the ignition coil from cylinder 1 with the coil from cylinder 2. Clear the codes and drive the car to see if the fault code changes to P0302. If it does, the ignition coil is bad.
5. If the code remains P0301, swap the spark plug from cylinder 1 with the plug from cylinder 2. VERY CAREFULLY inspect the insulator on the removed plug for hairline cracks. Clear the codes and drive again. If the code changes to P0302, the spark plug is bad.
6. If the misfire still remains on cylinder 1, the issue is likely related to fuel delivery (injector) or a mechanical problem.
7. Inspect the wiring and connector for the fuel injector and ignition coil on cylinder 1 for any visible damage or corrosion.
8. If the above steps do not resolve the issue, professional diagnosis is recommended to test the fuel injector, check for vacuum leaks, and perform a compression test.

Parts You'll Likely Need

• Spark Plug (OEM #Motorcraft SP-596) — Spark plugs are a common wear item and the most frequent cause of a single-cylinder misfire on high-output EcoBoost engines.
  Trusted brands: Motorcraft, NGK
  OEM price range: $15-$30
  Aftermarket price range: $10-$25
• Ignition Coil (OEM #HL3Z-12029-D (replaces HL3Z-12029-A)) — The second most common cause of a P0301 misfire, ignition coils fail from heat and vibration over time, leading to insufficient spark.
  Trusted brands: Motorcraft (OEM), NGK, Denso
  OEM price range: $70-$120
  Aftermarket price range: $50-$90

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) may be stored alongside P0301.
• P0316 — This code indicates a 'Misfire Detected on Startup (First 1000 Revolutions)'. It often appears with P0301, pointing to the misfire being present immediately from a cold start. The official Ford TSBs for the GT specifically mention this code appearing with P0301.

Technical Service Bulletins (TSBs) & Recalls

• SSM 50616: Some 2017-2022 Ford GT vehicles may exhibit a runs rough or hard start with or without an illuminated MIL with DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306 and/or P0316.
• SSM 49536: Some 2017-2021 Ford GT vehicles may exhibit a runs rough or hard start with or without an illuminated MIL with DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306 and/or P0316.
• SSM 49028: Some 2017-2020 Ford GT vehicles may exhibit a runs rough or hard start with or without an illuminated MIL with DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306 and/or P0316.

Platform-Specific Known Issues

• Ford issued Technical Service Bulletins #SSM 50616, #SSM 49536, and #SSM 49028 for 2017-2022 Ford GT models. These bulletins note that the vehicle may exhibit a 'runs rough or hard start' with misfire codes including P0301. Crucially, they instruct technicians to 'determine if the vehicle operating conditions may have resulted' in the code before performing diagnostics. This suggests certain driving patterns, such as repeated short trips without the engine fully warming up, could trigger a temporary misfire condition, potentially by fouling spark plugs.

Mechanic-Grade Diagnostic Values

• Ignition Coil Primary Resistance — expected: ~0.5 Ohms. Failure: A reading significantly higher than 0.5 Ohms or an open circuit (infinite resistance) indicates a faulty coil.
• Ignition Coil Secondary Resistance — expected: ~6.0k - 6.2k Ohms. Failure: A reading far outside this range or an open circuit points to a failure in the secondary windings.
• Low-Pressure Fuel System — expected: 40-60 PSI at idle. Failure: Pressure below this range could indicate a weak in-tank fuel pump, affecting the high-pressure pump's ability to function.
• High-Pressure Fuel System (Direct Injection) — expected: ~500 PSI during cranking, up to 2150 PSI under load. Failure: If the actual pressure on a scan tool lags more than 200 PSI behind the desired pressure, it can indicate a failing high-pressure fuel pump (HPFP) or a leaking injector.
• Mode $06 Misfire Count (TID $53, CID $01) — expected: A count at or near zero.. Failure: A non-zero count that is significantly higher than the counts for other cylinders indicates an intermittent or developing misfire on cylinder 1, even if a P0301 code has not been set.

Hidden / Shadow Codes Worth Checking

• Mode $06, TID $53: This is not a code but a monitor test result. It provides raw misfire counts for each cylinder over recent driving cycles. A high count for Component ID (CID) $01 indicates misfires on cylinder 1 before they are frequent enough to set a persistent P0301 trouble code. (see via Requires a scan tool that can access and display Mode $06 data. The data is presented as a list of Test IDs (TID) and Component IDs (CID).)

Scan Tool Commands That Help

• Ford IDS (Integrated Diagnostic System) or equivalent high-level scanner: Power Balance Test — This is a primary diagnostic step when a misfire is suspected but the cause isn't obvious. The test graphically displays the RPM drop from each cylinder, immediately showing if cylinder 1 is contributing less power than the others. The technician can then disable the injector for cylinder 1 to confirm it has little to no effect on engine operation, verifying it as the source of the problem.

Wiring & Ground Locations

• Cylinder Head Ground Stud — Typically located on the rear of the passenger-side cylinder head (Bank 1).. This is a critical ground point for the engine harness, which includes wiring for ignition coils and fuel injectors. A loose or corroded ground here can cause erratic behavior and misfires on any cylinder in that bank.
• Injector Power Wire — In the harness connector for the fuel injector on cylinder 1. On many Fords, this is a common color wire (e.g., red) shared across all injectors.. Verifying 12V at this wire with the key on confirms the injector is receiving power. The other wire is the ground pulse from the PCM. A lack of power would point to a wiring or fuse issue, not a faulty injector.
• Main Frame/Chassis Ground — A primary ground strap is typically located on the frame rail near the battery on the passenger side.. While less likely to cause a single-cylinder misfire, a poor main ground can cause a host of electrical issues, including weak spark or erratic sensor readings that could contribute to misfire detection.

Real Owner Repair Stories

• Multiple Ford F-150 EcoBoost Forums and YouTube repair videos (Various 2011-2020 Ford F-150s with 3.5L EcoBoost engine (same engine family as the GT)) — Engine shudder or hesitation under load (highway acceleration), often with a flashing check engine light and code P0301.
  ❌ Tried (didn't work) Swapping the ignition coil from cylinder 1 to another cylinder. The P0301 code remained, incorrectly suggesting the coil was not the problem.
  ✅ What actually fixed it Upon removing the spark plug from cylinder 1, a hairline crack was discovered in the white porcelain insulator. The crack was often difficult to see without close inspection. Replacing the single cracked spark plug completely resolved the misfire. This is a very common failure pattern for this engine family.

"I Checked Everything" — The Actual Cause

• A common scenario for a P0301 on this engine is a misfire under load that is not caused by a vacuum leak. A smoke test would show the intake system is sealed, and a coil swap test might also show no change. The actual cause is often a hairline crack in the spark plug's porcelain insulator. This allows spark energy to arc directly to the cylinder head instead of jumping the plug gap, but only under high cylinder pressures (i.e., high engine load/boost). This is an electrical failure, not a vacuum leak, so a smoke test would not find it.

When the Usual Fixes Don't Work

• While the most common fixes for P0301 are the spark plug or ignition coil, numerous owner reports on the similar 3.5L EcoBoost in the F-150 show that a coil swap test can be misleading. In many documented cases, swapping the ignition coil did not move the misfire code to the new cylinder. The root cause was ultimately found to be a spark plug with a hairline crack in its porcelain insulator, which only caused a misfire under high boost. This demonstrates that even if a coil swap test points away from the ignition system, the spark plug itself should still be considered a primary suspect and be physically inspected.

OEM Part Supersession History

• HL3Z-12029-A → HL3Z-12029-D — Standard part revision and update by the manufacturer.
  Heads up: None. The new part number is a direct, backward-compatible replacement for the original.

Model Year Variations Within This Range

• 2020-2021: For the 2020 model year, Ford updated the 3.5L EcoBoost in the GT. The changes included gallery-cooled pistons and, most relevant to P0301, 'higher-energy ignition coils'. This may result in different longevity or failure characteristics for ignition coils on 2020-2021 models compared to 2017-2019 models.