C0042 is a chassis diagnostic code indicating a fault in the Brake Pedal Position Sensor (BPPS) circuit, also known as the brake light switch. This sensor monitors brake pedal travel and communicates pedal position to the ABS module, traction control system, and other controllers. When the circuit reads out of expected parameters—too high, too low, or intermittent—the code sets and the system may default to a safe mode.
The sensor itself wears out over time from millions of pedal actuations, causing inconsistent or no signal output to the vehicle's computers.
Wiring near the brake pedal assembly flexes constantly and can break, pull apart, or corrode at connector pins, causing open or high-resistance circuits.
An open circuit in the brake light bulb or socket (including wrong bulb wattage) can cause the sensor circuit to register a fault due to circuit monitoring.
A weak or corroded ground for the brake pedal switch can cause voltage fluctuations that the module interprets as a sensor fault.
The control module that reads the sensor can fail internally, though this is considerably less common than sensor or wiring failures.
The engine control module rarely fails solely for a brake pedal sensor code; this is usually a last resort diagnosis after all other causes are ruled out.
Minor driving may be possible, but brake light functionality is compromised—other drivers cannot see you braking. Additionally, stability control and traction control may be impaired or inoperable. The vehicle should be diagnosed and repaired promptly.
In the majority of cases, yes—the brake pedal position sensor (brake light switch) is the most common failure point for this code. It is also the least expensive repair, which is why it should be investigated first.
ABS modules and control computers rarely fail for this code alone. These are expensive repairs that should only be pursued after the sensor, wiring, grounds, bulbs, and fuses have all been verified good. Many shops replace modules unnecessarily when the fix is a $15 brake light switch.
Yes. If the brake light circuit is open due to a burned-out bulb, missing bulb, or corroded socket, the sensor circuit monitoring may set a fault code. Always verify proper brake light operation before replacing sensors.