CAN is an international communication standard used In automotive control system.CAN XL is the so called third generation CAN protocol developed based on CAN.It should satisfy strict scrutiny for functional safety to satisfy requirement of zero road casualties.CAN XL uses two incompatible transceiver mode in its different functional frame segment.The key bit error in CAN XL frame at FDF or XLF will cause receiving node explaining that it is receiving a CAN or CAN FD frame.Error will happen lately due to transceiver working mode difference.Error reporting of receiving node will cause two transceiver mode in two node working simultaneously.Voltage conflict will happen.The CAN XL protocol prohibits error reporting in data field.This make error situation lasting longer.The key bit caused frame failure rate is about 6*ber.If downward compatibility is a strictly design item,route-depending error caused single fault at key bit can not be relieved.