P0303 on 2018-2019 GMC Terrain: Cylinder 3 Misfire Causes and Fixes

What's Unique About the 2018-2019 Gmc TERRAIN

For the 2018-2019 GMC Terrain with the 1.5L (LYX) engine, General Motors issued Technical Service Bulletin PIE0505A. This bulletin was created to gather information from the field about a recurring issue with P030x misfire codes. The key finding was that the ceramic insulator at the firing end of the spark plug could break. This loose piece, nicknamed a 'rattler,' can cover the plug gap and prevent a proper spark, directly causing a misfire. This makes a careful visual inspection of the removed spark plug a critical diagnostic step, beyond just checking for normal wear.

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
• Noticeable loss of engine power
• Solid Check Engine Light on the dashboard
• Flashing Check Engine Light (indicating a severe misfire)
• Decreased fuel economy
• A smell of gasoline from the exhaust

Most Likely Causes

1. Faulty Ignition Coil 🔴 High Probability Ignition coils are a common failure item on most modern engines due to constant exposure to heat and vibration. While not specific to the Terrain, it's the most logical first part to check.
   How to confirm: Swap the ignition coil from cylinder 3 with another cylinder (e.g., cylinder 2). Clear the codes and drive the vehicle. If the code changes to P0302, the coil is bad. A YouTube video on a 2018 Equinox demonstrates this exact process. 🎬 Watch: A mechanic's guide to diagnosing and fixing a P0303 code.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all coils if they are original and have high mileage.
   Est. part cost: $40-$80
2. Worn or Damaged Spark Plug 🔴 High Probability A GM TSB for the 1.5L engine, PIE0505, notes a specific issue where the spark plug's ceramic insulator can crack and break, directly causing a misfire. This makes the spark plugs a primary suspect on these engines.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 3. Inspect it for wear, fouling (black, oily, or white deposits), or physical damage like a cracked ceramic insulator. You can also swap it with another cylinder's plug to see if the misfire follows.
   Typical fix: Replace the spark plug. It is best practice to replace all spark plugs at the same time with OEM-spec plugs (e.g., ACDelco Iridium). For the 1.5L (LYX), the specified part is ACDelco 41-166. 🎬 See this step-by-step spark plug replacement guide for your Terrain.
   Est. part cost: $10-$25 per plug
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability Direct injection fuel injectors operate under high pressure and can be susceptible to clogging over time, though it's less common than ignition component failure.
   How to confirm: If the misfire remains on cylinder 3 after swapping the coil and plug, the injector is a likely suspect. A professional can test the injector's flow rate and electrical function. A DIY method involves listening to the injector with a mechanic's stethoscope for a consistent 'clicking' sound. You can also swap injectors between cylinders, but this is a more involved job.
   Typical fix: Replace the faulty fuel injector. This is a more involved job than coils or plugs and may require special tools.
   Est. part cost: $70-$150

Rare But Worth Checking

• Internal Engine Mechanical Issue: While rare for a single-cylinder misfire, issues like low compression from a bad piston ring, leaking valve, or a burnt valve can cause P0303. A Reddit thread on a platform-mate 2018 Chevy Equinox with the 1.5L engine mentions that early versions were known for piston ring issues. A compression test and cylinder leak-down test are needed to diagnose this.
• Vacuum Leak: A vacuum leak near the intake manifold runner for cylinder 3 could cause a lean condition and misfire. This is less common than ignition issues and often triggers other codes, like lean condition codes. A smoke test is the most effective way to find a vacuum leak.
• Moisture in Charge Air Cooler (CAC): For the 1.5L Turbo engine, GM has acknowledged an issue where moisture can build up in the charge air cooler, especially in humid or cold weather. This can lead to water being ingested by the engine, causing random misfires (P0300) but could potentially manifest as a single cylinder misfire as well. TSB 18-NA-020 addresses this.

Diagnosis Steps

1. Read the codes with an OBD-II scanner to confirm P0303 and check for any other codes.
2. Swap the ignition coil from cylinder 3 to a different cylinder (e.g., cylinder 2). The cylinders on the 1.5L and 2.0L engines are numbered 1-2-3-4 from front (passenger side) to back (driver side).
3. Clear the codes using the scanner, then start the engine and let it run or drive it briefly.
4. Re-scan for codes. If P0302 appears, the ignition coil is faulty and needs replacement.
5. If P0303 returns, the coil is likely good. Move to the spark plug. Remove the spark plug from cylinder 3. Crucially for the 1.5L engine, inspect the ceramic insulator for any cracks or breaks as per TSB PIE0505.
6. Swap the spark plug from cylinder 3 to the cylinder you just moved the coil to (cylinder 2).
7. Clear codes and re-run the test. If P0302 appears, the spark plug is the issue.
8. If P0303 returns again, the problem is not the coil or the plug. Inspect the wiring and connector for the cylinder 3 ignition coil and fuel injector for damage or looseness.
9. If wiring is intact, the next likely cause is a faulty fuel injector or a mechanical engine problem. A fuel injector balance test or swapping injectors can be performed.
10. As a final step for persistent issues, perform a compression test and a cylinder leak-down test to check for mechanical problems like bad rings, valves, or head gasket.

Parts You'll Likely Need

• Ignition Coil (OEM #12679077) — This is one of the most common failure points for a single-cylinder misfire. It is a relatively simple and inexpensive part to replace.
  Trusted brands: ACDelco (GM OE), Delphi, NGK
  OEM price range: $60-$90
  Aftermarket price range: $40-$70
• Spark Plug (OEM #12680023 (superseded by 41-166 for 1.5L LYX)) — Spark plugs are a routine maintenance item and can cause misfires when worn. On the 1.5L engine, they are a known point of failure due to potential ceramic cracking. 🎬 Watch: How to change spark plugs properly to avoid damage.
  Trusted brands: ACDelco (GM OE), NGK
  OEM price range: $15-$25
  Aftermarket price range: $10-$20

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders, a P0300 (Random Misfire) code may appear alongside P0303. This could point to a systemic issue like the Charge Air Cooler moisture problem.
• P0301, P0302, P0304 — If other cylinder-specific misfire codes are present, it points to a problem affecting the whole engine, like a major vacuum leak or fuel delivery issue, rather than an isolated component on cylinder 3.
• P0171 — A P0171 (System Too Lean) code could appear if a vacuum leak near cylinder 3 is the root cause of the misfire.

Technical Service Bulletins (TSBs) & Recalls

• PIE0505A: Engineering Information – Engine Running Rough and/or Check Engine Light Illuminated, DTCs P0300, P0301, P0302, P0303, P0304 Set.
• PIE0505: The precursor to PIE0505A, with the same purpose of gathering field information on the misfire condition.
• 18-NA-020: Addresses moisture accumulation in the Charge Air Cooler (CAC) on 1.5L engines, which can cause misfires.

Platform-Specific Known Issues

• TSB #PIE0505A and #PIE0505 document that GM was investigating a root cause for P030x codes on the 2018-2019 Terrain 1.5L engine. The bulletin specifically instructs technicians to check for cracked ceramics on the firing end of the spark plugs, as a broken piece (a 'rattler') can block the gap and cause a misfire.
• TSB #18-NA-020 addresses an issue on the 1.5L turbo engine where moisture can accumulate in the Charge Air Cooler (CAC), leading to misfires (often P0300) when ingested by the engine. While typically causing random misfires, it's a known engine issue that could contribute to single-cylinder misfires.

Mechanic-Grade Diagnostic Values

• Long-Term Fuel Trim (LTFT) at idle — expected: -10% to +10%. Failure: A value consistently above +10% suggests the engine is compensating for a lean condition (too much air/not enough fuel), which could be caused by a vacuum leak or weak fuel injector. A value below -10% indicates a rich condition.
• Spark Plug Gap (ACDelco 41-166 Iridium) — expected: Approx. 0.035 inches (pre-gapped from factory). Failure: While pre-gapped, verification is recommended. However, adjusting an iridium plug's fine-wire electrode is risky and can easily cause damage. Incorrect gap can lead to weak spark and misfires.

Scan Tool Commands That Help

• GDS2 (GM Tech Tool): Cylinder Power Balance Test — Used to determine if each cylinder is contributing equally to engine power. A significant drop from cylinder 3 would confirm the misfire location without having to swap parts.
• GDS2 (GM Tech Tool): Fuel Injector Balance Test — Used after ruling out ignition components. This test commands each injector to fire for a set duration and measures the corresponding drop in fuel pressure, identifying a weak or clogged injector.
• Professional OBD-II Scanner: Misfire Counter (Mode $06 Data) — To monitor current and historical misfire events for each specific cylinder. This can help determine if the misfire is constant or intermittent and confirm it is isolated to cylinder 3 before beginning repairs.

Wiring & Ground Locations

• ECM (Engine Control Module) — Left side of the engine compartment on the 1.5L engine.. The ECM is the source of all ignition and fuel commands. A poor connection at the ECM could cause issues for a single cylinder driver circuit.
• G104 — Top left of the engine block.. This is a primary engine ground. A loose or corroded ground at this location can cause a wide range of electrical issues, including erratic sensor readings and misfires.
• Cylinder 3 Fuel Injector Wiring (1.5L LYX) — The supply wire is Green/Gray at Pin 70 of connector X2 at the ECM. The control wire is Brown/White.. If ignition is confirmed good, testing for voltage and pulse at these specific wires can confirm or deny a wiring or ECM driver issue for the cylinder 3 fuel injector.

OEM Part Supersession History

• 12680023 → 41-166 — Updated design/revision for the 1.5L (LYX) engine spark plug.
• 12658331 → 28289962 — Part revision for the fuel rail pressure sensor.
  Heads up: Using the correct, updated part number is crucial for accurate fuel pressure readings.