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“Circuit high” means pedal sensor output is higher than expected. A “circuit low” would mean the opposite. Derating (C) is a result, not the fault itself. Turbos (D) are unrelated. ASE stresses checking redundant pedal sensors for plausibility.

If soot accumulates but regeneration doesn’t occur, the DPF temperature sensor may be faulty, preventing correct regeneration logic. Turbo (B), crankshaft (C), and fuel pumps (D) don’t directly prevent regen initiation. ASE stresses verifying temperature sensors in aftertreatment diagnostics.

Whistling and reduced boost almost always point to intake piping leaks. Pump (B), glow plugs (C), and regulators (D) don’t produce this symptom. ASE stresses boost leak testing as a first step.

If temp is stuck low, ECM enriches fueling, thinking engine is still cold, creating smoke. Reduced fueling (B) and increased boost (C) are opposite responses. ASE stresses cross-checking scan vs. thermometer.

An EGR stuck open dilutes intake with inert gases at idle, leading to roughness and stalling. Higher NOx (B) occurs with EGR closed. Excess boost (C) and better economy (D) don’t happen with excessive EGR flow. ASE stresses EGR flow control checks during idle complaints.