A technician is diagnosing a vehicle with an intermittent 'Check Engine' light and a stored DTC for 'MAP Sensor Circuit Low Input'. Using a scan tool, the technician observes the Manifold Absolute Pressure (MAP) sensor reading. With the engine off (KOEO), the MAP sensor reads 1.5V. When the engine is started and idling, the MAP sensor reads 0.9V. The atmospheric pressure at the technician's shop is 14.7 psi (101 kPa, 29.92 inHg). What do these readings MOST likely indicate?
- AA restriction in the intake manifold or exhaust system.
- BA faulty MAP sensor providing an inaccurate signal.
- CA vacuum leak in the intake manifold.
- DA faulty MAP sensor circuit with an intermittent open.
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty MAP sensor providing an inaccurate signal.
A healthy MAP sensor should read close to atmospheric pressure (around 4.5-5.0V or 14.7 psi) with the engine off (KOEO), indicating the pressure inside the manifold is equal to the outside air pressure. A reading of 1.5V (which typically corresponds to very low pressure, like extreme vacuum) with the engine off is incorrect and indicates the sensor itself is faulty, providing an inaccurate signal. The 0.9V at idle is also inappropriately low, reinforcing the sensor's inaccuracy.
Why the other options are wrong
- A. A restriction in the intake or exhaust would affect engine performance and vacuum, but the primary indicator of a faulty sensor is the incorrect reading at KOEO compared to atmospheric pressure.
- C. A vacuum leak would cause a *higher* than normal idle MAP reading (less vacuum), not a lower one. For example, a healthy idle MAP might be 0.8-1.2V in a normal engine, but the KOEO reading of 1.5V is the key here.
- D. An intermittent open in the circuit would likely cause the voltage to fluctuate wildly or drop to 0V entirely, not consistently low readings.
MAP Sensor KOEO Test
With the engine off (Key On, Engine Off - KOEO), a Manifold Absolute Pressure (MAP) sensor should output a voltage signal corresponding to atmospheric pressure. A significantly lower or incorrect reading at KOEO indicates a faulty MAP sensor.
- KOEO MAP voltage should reflect atmospheric pressure (e.g., 4.5-5.0V for 14.7 psi).
- Lower voltage indicates higher vacuum (or faulty sensor).
- Higher voltage indicates lower vacuum/higher pressure (or faulty sensor).
Memory trick: KOEO MAP, if it's low, the sensor's bad, it won't go.