Car no starting After removing black box

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Mk3 (2000–2007): Fuse No. 22 is located in the passenger-side fuse box, which is under the glove compartment. Also check fuse No. 17 under the bonnet, as this powers fuse No. 22.

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Mk4 (2007–2014): Fuse No. 22 (20 Ampere) in the passenger side fuse box under the glove compartment. I don't know if you also need to check fuse No 17. I seem to remember that fuse No. 17 wasn't under the bonnet for this model.

1.

Battery: Grab a new battery first. They only cost a few pounds and, to be honest, they solve the problem about 90% of the time. Even if your key still kinda works, a dying battery can cause all sorts of weird issues.

2.

Look Inside: Pop it open and take a peek. You're looking for anything funky - green crusty stuff on the contacts (wipe it off with some rubbing alcohol), cracks in the board, or any signs water got in there.

3.

Distance Test: Try unlocking from different spots. Start close, then walk further back. If it used to work from across the parking lot but now only works up close, that's your key acting up. But if it won't work even when you're standing right at the door? That's probably the car's receiver.

4.

Metal Check: Try using it away from your keys, phone, or anything metal. Sometimes that stuff messes with the signal.

5.

Button Test: Put your key in and turn it, but don't start the car. Try all the buttons. If none of them do anything, the key's probably the issue. If some buttons work, it could be a car problem.

If everything checks out with the key after all this, then yeah, you're probably looking at the antenna or engine control unit (ECU) in the car itself. That's when you'd need to take it to someone who can hook it up and figure out what's going on.