Start with what you noticed
You do not need a scanner to begin. Pick the symptom that matches, answer a few narrowing questions, and work through the causes in the order that costs least to rule out.
The symptom troubleshooter is temporarily unavailable. You can still look up a trouble code.
Read the stored code first. It tells you what a control module actually measured, which is a far narrower starting point than a symptom. Look up a code →