Q: The gas pedal needs to be fully pushed in order to touch the Button sensor. It does "click".

There should be stored trouble codes that once downloaded will narrow the focus of the diagnostic. If you suspect that the accelerator pedal sensor (aka, "pedal value sensor") is faulty, that component has to be tested to rule it in or out as a cause of any problem. Once the sensor is tested, if it is performing as required, the diagnostic moves on to the other many causes of limp mode. If you want a professional mechanic to resolve this, I would suggest you just request a Check Engine Light diagnostic with the idea that trouble codes will first be sought (downloaded). If no codes were stored, the standard MB protocol for diagnosing limp mode would be followed. The only way this will be resolved is by pinpointing the faults and that requires specific tests of vehicle components and systems. If you request the Check Engine Light diagnostic, the responding certified mechanic will get this resolved right at your location. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.

Replacing the button probably won’t help you. The car has an electronically controlled transmission that takes into account vehicle speed, engine load, accelerator position and other factors. The button under the accelerator is only for kick-down (passing). The accelerator position sensor itself is built into the throttle body. You need to have the car scanned by someone with the appropriate equipment. A generic scanner will not do the trick in this case. The car uses information from all of the controllers communicating over a network to make shifting decisions so your transmission problem could be in the transmission, the control system or it could be anywhere else in the car. Really! The last time I had a problem with a MB that wouldn’t shift I solved it by replacing the brake module! You would do well to have this sorted out by a pro. You can contact YourMechanic to have a technician come out to your home or office to check out your Mercedes ML and let you know what it will take to solve it.