Land Rover Repair: range rover headlights, main stays, becm

Question
QUESTION: Hi, we have a 2002 range rover just failed its mot for main beam staying on. I've replaced the dip switch but still no good. If you turn the sidelights on then main is on, but goes out if you push to main postion or flash. If you turn to dip beam main stays on whtever you do!
Thanks for your help!!

ANSWER: It sounds like you have a failure within the BECM   In that truck the dimmer switch signals the BECM, and the BECM controls the lights

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QUESTION: Thanks for your reply, can you tell me where the BECM is located and is it possible for us to test it before buying a new one as I fear this is expensive!

Answer
The BECM is under the passenger seat.  You can test inputs to it (like the headlamp switch) with a meter, and look for output, but to test the unit itself you need a dedicated Land Rover test system like the Autologic or T4.  You also need that dedicated tester to configure a new BECM after installation; otherwise the vehicle may not start and run and its electrics will not work properly or at all.  You can't simply swap the part