It can only be tested with the help of schematic circuit of the membrane switch. It is usually the shift knob that goes wrong. If there is no schematic circuit, you can only find out the problem by yourself. Membrane switch is actually a trigger signal to the IC. It is mostly a line or an array. This problem is rather obvious and easy to find out, but generally when membrane switch goes wrong, there is no way to repair but switch to a new one.
Maintenance method of membrane switch: Just replace the membrane switch panel of the same model and specification. When replacing the membrane switch, you just need to detach its control plate, which can be done as long as you have basic practical ability (but if you don't have the ability, there is no way to do so). If you can't buy the membrane switch panel, you can replace it with a micro-switch. The circuit wiring should be in accordance with the old membrane switch panel, and the panel needs drilling, installing the micro-switch, fixing with silica gel and wiring.
