* Loose or Damaged Spark Plug Wires: The most common culprit. Ensure all spark plug wires are securely attached to both the spark plugs and the distributor cap (if applicable) or coil packs. A damaged wire (cracked insulation, etc.) can prevent proper spark delivery. Even a slightly loose connection can interrupt the circuit.
* Incorrect Spark Plug Gap: If the gap on the new spark plugs is incorrect (too wide or too narrow), it can prevent a strong enough spark. Check the gap with a feeler gauge and adjust if necessary according to the manufacturer's specifications.
* Damaged Coil Packs (or Ignition Coil): A faulty coil pack prevents spark delivery to one or more cylinders. This could have been coincidentally damaged during the spark plug replacement or pre-existed but was only revealed after the plugs were changed.
* Crossed or Mis-wired Spark Plug Wires: If the wires are connected to the wrong cylinders, the engine won't fire correctly. Double-check the firing order against your Trailblazer's specifications.
* Ignition Switch Problem: Though less likely directly related to the plug change, a faulty ignition switch can prevent the engine from cranking altogether.
* Battery Issues: A weak or dying battery might not have enough power to turn the engine over, even if everything else is working correctly. Check the battery voltage.
* Damaged Crankshaft Position Sensor (CKP): This sensor tells the engine's computer where the crankshaft is, which is essential for proper ignition timing. A damaged CKP sensor prevents the engine from starting.
* Security System Issues: Some vehicles have security systems that can prevent cranking if there's a problem with the key, immobilizer, or alarm system.
Troubleshooting Steps:
1. Recheck Spark Plug Wires: This is the *first* thing to examine carefully. Make sure they're firmly seated and undamaged.
2. Verify Spark Plug Gap: Use a feeler gauge to check the gap.
3. Visually Inspect Coil Packs (or Ignition Coil): Look for any signs of damage.
4. Check Battery Voltage: Make sure the battery has a sufficient charge.
5. Check for error codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the vehicle's computer. This can provide valuable clues.
If you've checked all of the above and the Trailblazer still won't crank, it's best to take it to a qualified mechanic for diagnosis. Attempting further troubleshooting without the proper tools and knowledge could lead to more damage.
