* Loose or improperly tightened spark plugs: This is the most common cause. If the spark plugs aren't tightened to the manufacturer's specified torque, it can cause misfires, which trigger the check engine light. Over-tightening can also damage the threads. Always refer to your owner's manual for the correct torque specification.
* Damaged spark plug threads: During installation, you might have cross-threaded a plug or damaged the threads in the cylinder head. This will cause a misfire and illuminate the check engine light.
* Incorrect gap on spark plugs: If the gap on the new spark plugs isn't set correctly, it can lead to misfires. The gap should match the manufacturer's recommendation, usually found in your owner's manual or stamped on the plug itself.
* Damaged or disconnected wires: While changing the plugs, you might have accidentally damaged or disconnected an ignition coil wire or boot. A poor connection or damaged wire will interrupt the spark to that cylinder.
* Coincidence: Sometimes, the check engine light might have been about to come on anyway, unrelated to the spark plug change. The underlying issue was already present, and the plug change simply triggered the light's activation, it's not uncommon for OBD codes to lag activation if a sensor detects a problem.
* Sensor issues: While less likely, a related sensor (like the mass airflow sensor or oxygen sensor) could be malfunctioning, and the spark plug change might have temporarily affected its reading.
What to do:
1. Recheck your work: Double-check that all spark plugs are properly seated and tightened to the correct torque. Inspect the wires and boots for any damage.
2. Use a code reader: Get an OBD-II code reader (available cheaply at auto parts stores). This will give you a specific trouble code (e.g., P0300 - random misfire). This code will pinpoint the likely cylinder or system causing the problem.
3. Consult your owner's manual: It contains crucial information about spark plug torque specifications and gap settings.
If you've double-checked everything and the light remains on, it's best to take your Envoy to a mechanic to diagnose the problem accurately. Ignoring a check engine light can lead to further damage.
