1. Did you do anything else besides changing the plugs?
* Loose connections: The most likely culprit is a loose connection. Double-check all the connections related to the ignition system:
* Spark plug wires: Make absolutely sure each wire is securely attached to both the spark plug and the distributor cap (if applicable). A mis-fired connection can cause a misfire code.
* Coil pack connection(s): If your Bronco has a coil pack (rather than individual coil wires per plug), verify the connections to the coil pack are tight and secure.
* Mass Airflow Sensor (MAF) connection: While less likely directly related, a bumped or disconnected MAF sensor can trigger the light.
* Battery terminals: While unlikely to cause a CEL directly after a spark plug change, ensure your battery terminals are clean and tight.
* Damaged components: While unlikely if you were careful, did you accidentally damage any wires or components during the process? Visually inspect everything.
2. Check the spark plug gap: If you changed the spark plug gap and made it too wide or too narrow, this can cause a misfire and trigger the check engine light. Ensure the gap is within the manufacturer's specifications (check your owner's manual or a reputable online source). Incorrect gap is the second most likely reason.
3. Clear the code (Caution!): You can try clearing the trouble code from the vehicle's computer. The method varies greatly depending on the year and model of your Bronco, but there are a couple common ways:
* Disconnecting the battery: Disconnect the negative terminal of your battery for at least 30 minutes. This often resets the computer but does not guarantee it will solve the underlying problem.
* OBD-II Scanner: If your Bronco is equipped with an OBD-II port (less likely for a '91, but possible), use an OBD-II scanner to read the trouble codes. Then you can clear them using the scanner. *This is the preferred method as it identifies the exact problem*. An inexpensive code reader can be bought online or at an auto parts store.
4. If the light returns after clearing: The problem persists. The light is likely indicating a real issue, possibly related to but not directly caused by your spark plug change. The code reader is crucial here to diagnose the problem. Possible causes:
* Faulty oxygen sensor: Common cause of check engine lights.
* Crank or camshaft position sensor: Issues here greatly affect ignition timing.
* Other sensor issues: Numerous other sensors can trigger the light.
Important Considerations:
* 1991 Bronco OBD System: Your 1991 Ford Bronco likely doesn't have a standard OBD-II port, making code reading more complex. You might need a specialized code reader for older Ford vehicles or consult a mechanic. Simple battery disconnection might be your only option to clear codes without specialized tools.
* Professional Help: If you're uncomfortable troubleshooting electrical systems, take your Bronco to a qualified mechanic. They have the tools and expertise to diagnose the problem properly.
In short, before assuming anything is broken *recheck every connection* you worked with. If that doesn't solve it, get a code reader (or have a mechanic read the codes). The check engine light is a symptom, not the problem itself.
