Benz A170D automatic "HIGH HAND" needed

Apologize not typing Chinese – public library computer – no input, sorry. Please reply either in Chinese or English at convenience - Neng Kan Dong.

Year 2003 Mercedes Benz Avangard A170D automatic 52,000 miles. “F” flashing out while driving on “D” and still can drive hundreds yards.After pull over, switch off the engine and start engine again. Car is back to normal. Strange enough that it only happens when I drive. My hu*****and does not have such problem, never seen “F”. He drove the car to Benz Service Garage and was told that the “sensor” was faulty need to be changed. It will cost £500. We did not change and only paid £50 for checking the car. Now the car is normal when my hu*****and drives. However “F” turn up when I drive. 2 weeks already, what should we do, please any “High Hand” know automatic German car (used to drive Toyota automatic, no problem) how serious the “F” sign mean? Please

Thank you very much.