Technician A says that the PCM will go into limp-home mode if both APP sensors become faulty. Technician B says the PCM may shut down the alternator if TPS voltage exceeds a certain limit. Which technician is correct?

Technician A says that the PCM will go into limp-home mode if both APP sensors become faulty. Technician B says the PCM may shut down the alternator if TPS voltage exceeds a certain limit. Which technician is correct?

• A. Tech B
• B. Tech A
• C. Neither
• D. Both

Answer: (B)

When a system uses two accelerator pedal position sensors, the PCM compares their readings to ensure reliable throttle input. If both sensors fail or their outputs diverge beyond what the PCM considers acceptable, there’s no trustworthy pedal signal, so the PCM typically reduces engine power and other nonessential functions to allow the vehicle to be driven safely to a service location—this is limp-home mode. That makes the claim about the PCM entering limp-home when both APP sensors are faulty the correct one. The idea that the PCM would shut down the alternator simply because the throttle position sensor voltage is too high isn’t a standard protective action. The throttle input affects engine control, not the charging system directly, and alternator regulation is handled by the charging system and battery voltage protections, not by a TPS overvoltage condition. So that scenario isn’t a typical or reliable behavior.

When a system uses two accelerator pedal position sensors, the PCM compares their readings to ensure reliable throttle input. If both sensors fail or their outputs diverge beyond what the PCM considers acceptable, there’s no trustworthy pedal signal, so the PCM typically reduces engine power and other nonessential functions to allow the vehicle to be driven safely to a service location—this is limp-home mode.

That makes the claim about the PCM entering limp-home when both APP sensors are faulty the correct one.

The idea that the PCM would shut down the alternator simply because the throttle position sensor voltage is too high isn’t a standard protective action. The throttle input affects engine control, not the charging system directly, and alternator regulation is handled by the charging system and battery voltage protections, not by a TPS overvoltage condition. So that scenario isn’t a typical or reliable behavior.