Diagnostic Overview
Cummins fault code 691 commonly points to turbocharger actuator, boost control, or air-handling system performance. The ECM may detect low boost, high boost, or poor turbo response. Common causes include a failed actuator, sticking VGT vanes, boost leaks, charge-air cooler damage, exhaust leaks, restricted intake, faulty boost pressure sensor, or wiring issues. A technician should inspect the turbo actuator, pressure-test the charge-air system, verify sensor data, and check exhaust piping before replacing the turbo.