Diagnostic errors, including missed diagnoses of adolescent depression, elevated blood pressure, and delayed response to abnormal lab results, are common in pediatric primary care. Building upon previous work, this study used root cause analyses to identify the failure points and contributing factors to these errors. Omitted process steps included failure to screen for adolescent depression, failure to recognize and act on abnormal blood pressure values, and failure to notify families of abnormal lab results. Factors contributing most commonly to these errors were patient volume, inadequate staffing, clinic environment, electronic and written communication, and provider knowledge.