Diagnostic Overview
Detroit SPN 1187 is commonly associated with turbocharger compressor outlet temperature, charge-air temperature, or an air-management temperature signal depending on calibration. The ECM uses this information to protect the engine and calculate air density. A fault can affect boost control, fueling, and derate strategy.
Likely Causes
Failed temperature sensor, damaged wiring, poor connector contact, abnormal charge-air temperature, charge-air cooler restriction, intake restriction, or sensor contamination.
What To Check First
Compare temperature readings cold and hot, inspect the sensor and harness, check charge-air cooler condition, and confirm the FMI to determine whether the issue is high, low, or rationality related. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.