Which sensor adjusts base duty cycle in response to ambient air pressure to compensate altitude?

• A. BARO sensor
• B. IAT sensor
• C. MAP sensor
• D. MAF sensor

Answer: (A)

The key idea is that ambient atmospheric pressure changes with altitude, starving the engine of oxygen unless fueling is adjusted. The sensor that provides this outside air pressure to the engine computer is the barometric pressure (BARO) sensor. When the BARO sensor detects lower atmospheric pressure, the ECU lowers the base injector duty cycle so fuel delivery is reduced accordingly, helping maintain the target air–fuel ratio as altitude increases.

The other sensors measure different things: the IAT sensor reads intake air temperature, which affects air density but not the ambient pressure directly; the MAF sensor gauges actual mass of air entering the engine, guiding fueling based on real flow; the MAP sensor measures manifold pressure under boost or vacuum to infer engine load, but it doesn’t supply the ambient pressure input used specifically for altitude compensation.