These devices usually have a list of compatible vehicles. If yours is not on the list, it may not work.
If it does not have a list, don't buy it. If you bought it already, your only option left may be to sell it.
If I followed this or similar guide. I would never get OBD2 computers that suit my needs working with my car, because my car is usually not on a "list". At the beginning of my struggles I tried several OBD2 displays which could not connect with the car at all. Here is my hardware:
Car engine is VAG made, 1.6L gasoline engine, codename BSE.
ECU is Siemens Simos 7.1
ECU broadcasts obd2 data using specific obd2 protocol, which is ISO14230-4 (KWP2000) (5 baud init,10.4 Kbaud). The "init" means it needs a signal from OBD2 display to start a connection.
Here is the path I walked to get my OBD2 displays working:
KW681 diagnostic tool helped me to find out which protocol my ECU uses.
Then I saw on the Internet, on customers added pictures, that there is one OBD2 display that supports this protocol.
It was Autool X60 - mini, round OBD2 screen. When I tried it, it automatically recognised OBD2 protocol and set itself to use it every time. It tests every protocol at the first startup, displaying tested protocol, then remembers which one works and does not search for it later.
Another one I used with success was Actisafety A401, AKA AP-1/AP1, AKA P8 HUD, however it did not connect automatically. It required me to set protocol manually and/or set car make manually (I am about to determine the role of car make setting in this success, because Actisafety support told me it is intended to solve car make-specific problems). I prefer A401 over X60 because it can be conveniently mounted at the top of windshield (for some reason they do not suggest this place but its the most ergonomic place in my opinion). I need it do display ECT (+ high temperature alarm), battery voltage (+ low battery alarm), and OBD2 vehicle speed, calibrated according to actual tire wear.
So to get given OBD2 computer working with a car, you need to know OBD2 protocol used by its ECU and know, whether the computer supports this protocol. Different protocols use different cable pairs. Some hints on this are in the attachment and under this link:
http://www.obdtester.com/obd2_protocols