Trailblazer, Trax, Encore GX & Envista 1.2L Ignition Coil: Misfire Diagnosis & Replacement

Critical: 1.2L vs. 1.3L Engine Compatibility

Is It Really a Bad Coil? Check for Emission Recall A242435780

Before you spend money on new parts, be aware of GM Voluntary Emission Recall #A242435780, issued in August 2024. This recall affects certain 2024 models of the Chevrolet Trax, Trailblazer, Buick Encore GX, and Buick Envista with the 1.2L engine. The engine's computer (ECM) may fail to control ignition timing correctly after the auto start/stop system restarts the engine.

The symptoms of this software issue — a Check Engine Light, rough running, and engine knock — are nearly identical to a failing ignition coil. The fix is a free software update performed by a GM dealer. Check with a dealer to see if your vehicle is affected by this recall before proceeding with a diagnosis.

Symptoms of a Failing Ignition Coil

When an ignition coil starts to fail, the signs are usually obvious. Because each coil is responsible for one of the engine's three cylinders, losing one is like losing a third of your engine's power.

• Check Engine Light: This is often the first sign. The onboard computer detects a misfire and stores a trouble code. A solid light means a fault has been recorded, but a flashing Check Engine Light signals a severe misfire that is actively dumping unburned fuel into the exhaust, which can quickly destroy the expensive catalytic converter.
• Engine Misfire (Rough Idle, Sputtering): The engine will shake, vibrate, or sound uneven, especially at idle or low speeds. It may feel like the car is stumbling or hesitating when you try to accelerate.
• Loss of Power: With one cylinder not firing, the vehicle will feel noticeably sluggish and weak, especially when climbing hills or accelerating.
• Poor Fuel Economy: The engine's computer tries to compensate for the misfire, often by using more fuel, causing your gas mileage to drop.
• Hard Starting: The engine may take longer than usual to start or may not start at all if multiple coils have failed.
• Engine Backfire: In some cases, unburned fuel can detonate in the exhaust pipe, causing a loud popping or banging sound.

Common Diagnostic Trouble Codes (DTCs)

Using an OBD-II scanner, you may find one or more of the following codes:

• P0300: Random/Multiple Cylinder Misfire Detected.
• P0301: Cylinder 1 Misfire Detected.
• P0302: Cylinder 2 Misfire Detected.
• P0303: Cylinder 3 Misfire Detected.
• P0351, P0352, P0353: Ignition Coil 'A', 'B', or 'C' Primary/Secondary Circuit Malfunction.

Diagnosing a Bad Coil: The Swap Test

The easiest way to confirm a bad coil without special tools is the 'swap test'. This is especially simple on this engine.

1. Identify the misfiring cylinder using an OBD-II scanner (e.g., code P0302 points to cylinder 2).
2. Label the coils with their cylinder numbers (1, 2, 3) with a marker.
3. Swap the ignition coil from the misfiring cylinder with a coil from a known good cylinder. For example, swap the coils from cylinder 2 and cylinder 1.
4. Clear the diagnostic codes with your scanner and start the engine. Let it run until the Check Engine Light comes back on.
5. Re-scan for codes. If the code has 'moved' to the new cylinder (e.g., you now have a P0301 code), you have confirmed the ignition coil is bad. If the code stays on the original cylinder (P0302), the problem is likely the spark plug, fuel injector, or a mechanical issue with that cylinder.

Buying a Used Ignition Coil: What to Look For

Ignition coils are a simple and reliable part, making good, low-mileage used OEM coils an excellent value. They often outlast cheap aftermarket alternatives. Here's what to inspect before buying:

• Verify the Part Number: Ensure the part is a genuine GM 55595166 or an equivalent for the 1.2L engine. Do not buy a coil for the 1.3L engine.
• Check for Physical Damage: Inspect the hard plastic housing on top. Look for any cracks, melting, or swelling. A swollen top is a sign of internal overheating and failure.
• Inspect the Stem and Boot: Gently flex the rubber boot and stem. It should be pliable with no cracks or brittleness. Cracks can allow spark to escape to the engine block instead of going to the spark plug.
• Look for Corrosion: Check the electrical connector pins for any green or white corrosion. Look down into the spark plug boot and check the metal contact spring; it should be clean and free of rust.
• Consider Mileage and Source: A coil from a low-mileage, late-model vehicle is ideal. Purchase from a reputable seller that offers a warranty and a clear return policy.

Replacement Guide (DIY Difficulty: 1/5)

Replacing the ignition coils on the 1.2L engine is one of the easiest repairs you can perform. It requires only basic tools and about 15-30 minutes.

1. Remove the Engine Cover: The plastic cover on top of the engine simply pulls straight up and off.
2. Locate the Coils: The three ignition coils are in a row, right on top of the engine.
3. Disconnect the Electrical Connector: Each coil has a connector with a red locking tab. Slide the red tab away from the coil to unlock it. Then, press down on the black release tab and pull the connector straight off.
4. Remove the Retaining Bolt: Use an 8mm socket with a ratchet to remove the single bolt holding the coil in place.
5. Remove the Coil: Gently twist and pull the ignition coil straight up and out of the spark plug tube. It may be seated snugly.
6. Install the New Coil: Apply a small amount of dielectric grease to the inside of the new coil's boot. This prevents it from getting stuck to the spark plug and seals out moisture. Push the new coil firmly into place until it seats on the spark plug.
7. Reinstall Bolt and Connector: Reinstall the 8mm bolt (do not overtighten) and reconnect the electrical connector, making sure to push the red locking tab back into place.
8. Repeat for any other coils you are replacing, then put the engine cover back on.

Cost of Replacement

Frequently Asked Questions (FAQ)