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.
Set Condition
A signal toggle bit and a signal parity bit are sent by the PCM within the CAN message. In order to be sure the message content is continuously updated, the value of the toggle bit changes from 0 to 1 and vice versa every two messages. In order to be sure that the message content is valid, the parity bit value is set so the sum of specific signal bits of the message content, plus the toggle bit, plus the parity bit is an even number. If the toggle bit value does not change for a calibrated amount of time or the signal parity is not verified for a calibrated amount of time then DTC U0401 is set.
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Use the Manual With the Right Hardware
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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.