Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Theory Of Operation
The vehicle speed sensor rationality is a continuous test that monitors the vehicle speed sensor for lack of activity. The Powertrain Control Module (PCM) receives this information from the Anti-Lock Brake System (ABS) module over the CAN C bus. The CAN C bus is a high speed communication bus that allows real time control capability between various controllers. The rationality test will not run if the brakes are applied, a Brake Switch code is set, or a limp-in condition exists for Manifold Absolute Pressure (MAP) Sensor, Throttle Position, Crankshaft Position (CKP) Sensor, Camshaft Position (CMP) Sensor, or Engine Coolant Temperature (ECT) Sensor. If vehicle speed signal is below a minimum threshold for a calibrated period of time after the vehicle has been operating within the modeled parameters that would indicate it is being driven, a failure will be indicated.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.