Fail safe or "limp mode" is the program in the ECU that kicks in when there is a malfunction in the automatic transmission. You need the codes read. I would go to the dealer for this one. They have a scanner that can read all the data for the transmission. A cheap OBD II scanner won't do the trick.
Fail safe keeps the transmission in 3rd gear when a malfunction occurs so you can still get home or to the repair shop.
I've seen input and output speed sensors, shift solenoids, and open circuits in the wiring cause fail safe conditions.
Look for chewed wiring in the engine compartment or signs of rodent activity such as nests in the air cleaner or sunflower seed shells.
Once again, go to the dealer so you get the correct diagnoses.
========================================================
consuming 1 gallon every 15,6 miles.

放狗搜的，偶也看不太明白。下面那个应该是油耗吧。上面是抓瞎了。。。。。。。。。。。。。。。。。。。

就偶这水平看这段东西就是。ecu行车电脑报错，推荐去维修点用专业的检测器去检测到底出啥毛病。速度探测输入输出有问题或者电线开路之类的导致这个报错？总之是要去专业的维修点找原因了