P0302 on 2018-2019 GMC Terrain: Cylinder 2 Misfire Causes and Fixes

What's Unique About the 2018-2019 Gmc TERRAIN

The 2018-2019 GMC Terrain uses small displacement, turbocharged, direct-injection gasoline engines (1.5L LYX or 2.0L LTG). While the primary causes for P0302 are typical (plugs, coils), these engines have specific issues noted in GM Technical Service Bulletins. GM issued TSB 21-NA-255, which applies to the 1.5L LYX engine, addressing engine misfires caused by a broken spark plug center insulator. Another TSB, #22-NA-065, addresses moisture building up in the charge air cooler, which can be ingested during acceleration and cause random misfires (P0300), but could potentially manifest as a single cylinder misfire. The 2.0L LTG engine is also known for carbon buildup on intake valves, which can disrupt airflow to a specific cylinder and cause a misfire over time.

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
• Hesitation or stumbling during acceleration
• Reduced engine power and acceleration
• Flashing Check Engine Light
• Poor fuel economy
• Strange smell from the exhaust (like rotten eggs or gasoline)
• Engine stalling

Most Likely Causes

1. Failing Ignition Coil 🔴 High Probability → Shop Ignition Coil Coil-on-plug systems are common failure points due to constant high-temperature cycles and vibration in the engine bay. Aftermarket coils are also a frequent source of repeat failures if not high quality.
   How to confirm: Swap the ignition coil from cylinder 2 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. This is the most common and effective initial diagnostic step.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all coils at the same time for preventative maintenance, but it's not required.
   Est. part cost: $40-$80
2. Worn or Defective Spark Plug 🔴 High Probability → Shop Spark Plug GM issued TSB 21-NA-255 specifically for the 1.5L (LYX) engine in these model years due to misfires caused by a broken center insulator on the spark plug. This can cause the loose ceramic piece to block the spark gap. Standard wear also reduces plug effectiveness over time, with a typical service interval of 60,000-80,000 miles.
   How to confirm: After ruling out the coil, remove the spark plug from cylinder 2. Inspect it for wear, carbon fouling, oil, or a cracked/broken ceramic insulator. A key diagnostic step from TSB 21-NA-255 is to invert the spark plug and see if the center insulator slides down the electrode. Compare it to a plug from another cylinder.
   Typical fix: Replace the spark plug. It is best practice to replace all spark plugs as a set to ensure even performance. Ensure they are gapped correctly, although OEM Iridium plugs often come pre-gapped and should not be adjusted. For the 1.5L, use ACDelco 41-166 or equivalent. 🎬 Watch a DIY walkthrough on changing spark plugs in this engine.
   Est. part cost: $10-$25 per plug
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability → Shop Fuel Injector Direct injection (DI) fuel injectors operate under very high pressure and can be susceptible to clogging from carbon deposits or fuel contaminants. This can disrupt the fuel spray pattern and volume, leading to a misfire.
   How to confirm: Listen for a consistent ticking sound from the injector with a mechanic's stethoscope. A more definitive but advanced test is to swap the fuel injector from cylinder 2 with another cylinder, similar to the coil swap test. This requires relieving fuel pressure and is a more involved job.
   Typical fix: Replace the faulty fuel injector. A professional fuel system cleaning service may resolve minor clogs, but replacement is often necessary for a failed injector.
   Est. part cost: $70-$150

Rare But Worth Checking

• Low Engine Compression: This indicates a more serious internal engine problem, such as worn piston rings, a leaking head gasket, or a problem with the valves (e.g., a burnt exhaust valve). A YouTube video documented a P0302 on a GMC Terrain that was ultimately caused by a burnt exhaust valve, confirmed by a compression test 🎬 See how a burnt exhaust valve was diagnosed and fixed. showing zero compression and a leak-down test. A compression test is required to confirm.
• Intake Valve Carbon Buildup: Common on gasoline direct injection (GDI) engines like the 1.5L and 2.0L, as fuel is not sprayed over the back of the intake valves to clean them. Over time, this can restrict airflow to cylinder 2, causing a lean misfire. This usually occurs at higher mileage (80k-100k miles) and may require a borescope to verify and walnut blasting to clean.
• Collapsed or Stuck Valve Lifter: GM has issued TSBs like 19-NA-218 for other engines regarding ticking noises and misfires caused by faulty valve lifters that fail to pump up with oil, especially on a cold start. While not specific to the Terrain's engines, a failed lifter can prevent a valve from opening correctly, leading to a misfire code.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0302 and check for any other stored 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 2 on your engine. On these inline 4-cylinder engines, it is the second cylinder from the front (accessory belt side).
4. Swap the ignition coil from cylinder 2 with the coil from cylinder 1.
5. Clear the codes, start the engine, and drive the vehicle to see if the misfire code returns as P0301. If it does, the ignition coil is the problem.
6. If the code remains P0302, remove the spark plug from cylinder 2. Inspect for a cracked insulator (per TSB 21-NA-255), heavy carbon fouling, oil, or electrode wear. If found, replace the spark plugs (preferably as a set).
7. If the plugs and coils are good, the issue may be with the fuel injector. This is a more complex diagnosis. You can listen for the injector's ticking sound with a stethoscope. Swapping injectors between cylinders is a definitive test but is more labor-intensive.
8. If the P0302 code persists, the issue is likely a mechanical engine problem or a wiring fault.
9. Perform an engine compression test on all cylinders. A significant loss of compression in cylinder 2 points to issues like bad piston rings, a faulty head gasket, or a valve train problem (e.g., burnt valve).
10. If compression is good, inspect the wiring and connector for the cylinder 2 fuel injector and ignition coil for any damage or corrosion. Pay close attention to known harness chafe points, such as where the harness contacts the transmission mount bracket or transmission cover studs (per TSB 19-NA-181).
11. For higher mileage vehicles, consider inspecting for carbon buildup on the intake valves using a borescope, as this is a known issue on GDI engines.

Parts You'll Likely Need

• Ignition Coil (OEM #12654078) — This is one of the most frequent causes of a single-cylinder misfire and is a straightforward part to replace.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $60-$100
  Aftermarket price range: $40-$80
• Spark Plug (OEM #ACDelco 41-166 (replaces 12688183) for 1.5L LYX. ACDelco 41-125 for 2.0L LTG.) — Spark plugs are a regular maintenance item and a known failure point, especially the insulator on the 1.5L engine as per TSB 21-NA-255.
  Trusted brands: ACDelco, NGK
  OEM price range: $15-$25
  Aftermarket price range: $10-$20
• Fuel Injector (OEM #12677836 (1.5L LYX), 12662571 (2.0L LTG)) — A clogged or failed injector will prevent the proper amount of fuel from reaching cylinder 2, causing a misfire. Replacement is more costly and labor-intensive than plugs or coils.
  Trusted brands: ACDelco, Bosch
  OEM price range: $100-$180
  Aftermarket price range: $70-$150

Related Codes That Often Appear With This One

• P0300 — This is a 'Random Misfire' code. It may appear alongside P0302 if the misfire is intermittent or if other cylinders are also beginning to misfire. Issues like the charge air cooler moisture (TSB 22-NA-065) can start as P0300 and then be isolated to a single cylinder by the ECM.
• P0234 — This 'Engine Overboost' code can be seen on these turbo engines. While not directly causing a misfire, issues within the turbocharger and induction system can sometimes lead to drivability problems that may be interpreted as or accompanied by misfire codes.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-255: Engine Misfire, Malfunction Indicator Lamp (MIL) Illuminated - DTC P0300 Set. Caused by a broken spark plug insulator on 1.5L LYX engines.
• PIE0505A: Ends the need for information from the field on a Condition of Engine Running Rough and/or Check Engine Light Illuminated, DTCs P0300, P0301, P0302, P0303, P0304 Set.
• PIE0505: Request for information from the field to determine a root cause for a condition of Engine Running Rough and/or Check EngineLight Illuminated, DTCs P0300, P0301, P0302, P0303, P0304 Set.
• 22-NA-065: Malfunction Indicator Lamp Illuminated or Flashing - DTC P0300 Set When Driving In High Humidity Or In Heavy Rain, potentially caused by moisture in the charge air cooler.

Platform-Specific Known Issues

• TSB #21-NA-255: This bulletin specifically addresses engine misfires (including P0302) on the 1.5L LYX engine. It instructs technicians to inspect for a broken center

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel System — expected: 750 PSI (5,171 kPa) at idle, up to 2,250 - 2,900 PSI (15,513 - 20,000 kPa) under load/WOT.. Failure: Pressure does not match the scan tool's desired pressure reading, or pressure bleeds off rapidly after engine shutdown.
• Low-Pressure Fuel System (In-tank pump feeding the HPFP) — expected: 35-75 PSI (241-517 kPa).. Failure: Pressure below this range indicates a weak in-tank pump or a restriction in the fuel supply line.
• Ignition Coil Primary Winding Resistance — expected: 0.4 - 2.0 Ohms.. Failure: A reading of OL (open circuit) or a value significantly outside this range indicates a faulty coil.
• Ignition Coil Secondary Winding Resistance — expected: 6,000 - 10,000 Ohms (6kΩ - 10kΩ).. Failure: A reading of OL (open circuit) or a value significantly outside this range indicates a faulty coil.
• Scan Tool Misfire Counter (Live Data) — expected: Zero or very few counts on all cylinders at idle and under load.. Failure: A rapidly increasing number of misfire counts on cylinder 2 confirms the misfire location in real-time.

Scan Tool Commands That Help

• GDS2 (GM) or equivalent professional scan tool: Cylinder Power Balance Test — Used to confirm which cylinder is misfiring. The tool deactivates one cylinder at a time (by cutting fuel) and measures the RPM drop. If deactivating cylinder 2 causes little or no change in RPM, it confirms that cylinder was not contributing power.
• GDS2 (GM) or equivalent professional scan tool: Fuel Injector Balance Test — This is an automated test for GDI systems that measures the pressure drop in the fuel rail as each injector is commanded to fire. It helps identify a clogged or weak injector without having to remove it from the engine.

Wiring & Ground Locations

• Engine Harness Chafe Point (TSB 19-NA-181) — Inspect the engine wiring harness where it may contact the transmission mount bracket or the studs on the transmission control valve body cover.. A chafe in the harness can cause a short or open in the circuit for the cylinder 2 ignition coil or fuel injector, leading to a persistent or intermittent misfire. The TSB notes that it's common for only a single circuit to be affected.
• Engine Harness Chafe Point (General) — On similar GM platforms, chafe points have been identified where the harness contacts the ECM/TCM bracket, the driver's side upper control arm, and the shock tower.. Engine movement can cause the harness to rub against these sharp or hard points, eventually damaging the insulation on the wires for the cylinder 2 components.
• G105 / G109 — Main engine grounds typically located on the rear or top-rear of the engine block/cylinder head.. A loose or corroded engine ground can cause erratic voltage in the ignition and fuel systems, potentially leading to misfire codes. The ECM and its sensors require a stable ground reference to the engine block.
• Cylinder 2 Fuel Injector Control Wire — On the 2.0L LTG engine, this is a Green (GN) wire at Pin 48 of the ECM connector.. This is the specific wire to test for continuity and shorts if a wiring fault is suspected between the ECM and the fuel injector.

Real Owner Repair Stories

• YouTube channel 'Parts shooter' (GMC Terrain (year not specified, but 2nd gen body style) with a P0302 misfire.) — Persistent misfire on cylinder 2.
  ❌ Tried (didn't work) A previous shop had already replaced the spark plugs with no success.
  ✅ What actually fixed it The mechanic performed a compression test and found zero compression in cylinder 2. A subsequent leak-down test confirmed a 70%+ leak. Using a borescope, a burnt exhaust valve was visually identified as the root cause. The final repair was a complete cylinder head valve job.

OEM Part Supersession History

• ACDelco 41-166 (Spark Plug, 1.5L) → 12680072 — Part number consolidation and updates.
• ACDelco 12654078 (Ignition Coil) → N/A — This remains a current and widely used part number.
  Heads up: Cross-references to aftermarket brands include Delphi GN10682 and Denso 673-7205.