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Circuit low = sensor output voltage below spec. Circuit high (B) is opposite. Pump (C) and turbo (D) don’t cause electrical faults. ASE stresses confirming with scan and mechanical gauge.

Blue smoke under load with normal compression suggests oil leaking into the intake via turbo seals. Injector faults (B) cause fuel-related smoke. Pump wear (C) lowers rail pressure. Glow plugs (D) affect starting. ASE stresses visual turbo inspection for oil leaks.

“Circuit low” means sensor voltage is too low, often shorted to ground. “Circuit high” (B) is the opposite. Pumps (C) and turbo actuators (D) don’t cause circuit-level faults. ASE stresses sensor resistance checks against temperature charts.

Blue smoke during deceleration typically comes from oil leaking past worn valve stem seals. Air filters (B) and injectors (C) don’t create this condition. Turbo actuators (D) affect boost, not oil consumption. ASE stresses valve inspections when smoke is load-dependent.

A clogged DPF causes excessive backpressure, forcing ECM to derate. Pumps, glow plugs, and regulators don’t raise backpressure. ASE stresses aftertreatment differential pressure monitoring.