When a patient experiences an underdose of insulin due to an insulin pump malfunction, the primary reason for the encounter would be the malfunction itself, which is coded first. The resulting hyperglycemia or hypoglycemia due to the pump failure is a secondary condition. According to ICD-10-CM guidelines, the code for the mechanical complication of the pump (T85.633-) is sequenced first, followed by a code for the diabetes with complication (E11.65 for type 2 diabetes with hyperglycemia).References: ICD-10-CM (current year), Chapter 19: Injury, Poisoning and Certain Other Consequences of External Causes (S00-T88), ICD-10- CM Official Guidelines for Coding and Reporting, Section I.C.4.