The locking action on the boot lock is purely mechanical so closing the boot normally will re-lock it and you'll have to spring it from the inside again. Now, your problem is diagnosing what is stopping the lock from opening normally and there are four likely causes:

  1. The lock itself: There's a small electric motor inside it, almost identical to a 12V Scalextric motor,  that drives the release mechanism and this can burn out or become weak. There's also a switch inside that tells the central locking computer whether the boot is open or closed and could go wrong although you might expect the alarm to go if this is faulty and you get the boot open.
  2. The boot release button above the rear number plate: This sends a signal to the central locking computer that tells it you want to open the boot. If the computer acknowledges that either the boot unlock or doors unlock signal has been sent from the Renault Keycard then it'll drive the release motor in the lock until the switch in the lock signals that it is open. It was this button that was the problem on my daughter's car.
  3. The wiring to the tailgate: Some earlier Megane II models apparently had the wiring loom between the body and tailgate up in the top left corner of the tailgate. This got flexed tightly each time the boot was opened or closed and resulted in wires breaking inside the rubber gaiter that covers the wiring. My daughter's car has the wiring loom between the hinges and only gets a gentle twist so is much less likely to fail in this way.
  4. The central locking computer: This is the UCH behind the dashboard and also controls electric windows, etc. - you don't want it to be this, because it's an expensive unit and needs to be coded to the car, which generally means it's a Renault dealer job!

In addition to these, there is the possibility of a bad or corroded connection at any of the connectors if they've picked up moisture. One thing to realise here is that a wiring diagram of the boot isn't a whole lot of use to you since signals are only present for very short periods of time and you kind of need to be inside and outside the car at the same time to measure anything useful!

Just for information

I'm going concentrate on what goes wrong with the release button, but in case you want to look at the lock: 

Lock, outside
Outside face of the lock.

To get the lock out you need to take all the trim off the inside of the tailgate, starting with the piece above the window. Prise it up carefully at both sides first then it should come off fairly easily. Then the two side trims, noting that there's a Torx screw at the bottom of each piece. You don't need to worry about the buttons for the parcel shelf hangers as these are only fitted into the plastic trims and not into the metalwork underneath. Finally, you can get the main panel off, after removing the Torx screw inside each of the two handgrips near the bottom edge. You'll now be able to see all the wiring runs and get the two larger Torx screws out that hold the lock in place. You may find it easier to pop the cover off the lock first, by prising the tabs on either side with the tip of a screwdriver. Also note that there's a plastic tab on the inside face of the lock that holds it in place once the screws are removed.

Lock, inside
Inside face of the lock, showing locating tab. The connector is at top right.

With a little bit work and a couple of thin bladed screwdrivers, you can get the plastic cover off the top of the lock and expose the motor and worm gear.  The motor is a press fit and can easily be removed, although I don't know where you'd get a replacement that pushes onto two pins like the ones in these locks do, but you may be able to re-grease the gearing if you think that's the problem, e.g. if you can hear the motor try to operate the lock when you press the button.

The release button

Boot release button
Boot release button

In this case I needed to replace the release button which is located between the two number plate lamps, and had bought a new, original Renault part for £19 on eBay - but nearer £40 is not an uncommon price for this. The symptom here was that all other central locking functions were OK, but when you pressed the button nothing happened - no noise from the lock at all and more importantly there was no feeling of a 'click' when the button was pressed. There's a microswitch inside the button and there should be a distinct 'click' that you can feel, if not hear, as it snaps from the open to closed position.

If you've got all the tailgate trim removed you notice that the wiring for this switch disappears into a space you can't easily get to. It seems that the entire section on the outside below the Renault logo that houses the number plate lamps and boot release button is a big plastic box stuck on the outside of the boot! There is a large drain hole at either edge of this 'box' and this is maybe a hint that it's prone to collecting water. More on this later.

Button, opened
The microswiitch and the outer part of the button separated.

Anyway, you don't need to take any trim off the tailgate at all to get this button out, as you can prise it out from the outside although it is a bit fiddly. I used a flat bladed offset screwdriver to lever up the two narrow edges a little bit at a time - there are tabs on each side that hold the button assembly in place so you're trying to press these in as you go. After a little work you'll have the button assembly maybe 5-10mm out and then you're likely to get 'stuck' because part of the body is catching on edges of the aperture and there's no angle you can wiggle it to that lets it slide out. At this point it's really helpful to know how the button is assembled. There are basically two parts: A rear part that contain the microswitch and has the connector moulded in and the front part that has the exposed bezel, with the rubberised skin and contains the push button and return spring. The two parts clip together, and it's actually OK to allow the front part to 'break off' from the rear part. Once they're separated the rear part is much easier to manoeuvre out of the aperture. However. you probably won't have very much wiring to play with behind the button.

Release the connector by prising up the tab on the side of the connector just above the body of the button assembly and pull the button away from the connector.  Being careful not to 'lose' the connector inside the hole, push the new button assembly onto the connector until it tab snaps into place. Now feed the button assembly into the aperture and press round the edges to make sure it is fully seated.

If you're reassembling the old button, note there's only one spring which is retained on the push button and shouldn't fall off - there's peg on the other side that presses on the large tab of the microswitch to operate it. It should be pretty obvious if you try to put it back together the wrong way round.

So what went wrong?

As an engineer, I always want to understand why something failed - its not enough just to know that it did fail. The fact that the button assembly came apart during removal was something of a bonus here as it became immediately apparent what was wrong.

With the two parts of the button assembly off the car I could there were small droplets of water on the inside of the casings. There is no seal of any sort between the two parts of the button case but the outermost part of the assembly is tall enough that even if some water gets into that box on the tailgate then it should never get inside the switch - that is, unless something manages to clog those drain holes I mentioned earlier. While I couldn't see any such drainage problem, it's not impossible that something temporarily blocked one or other at some point.


Bead of water was trapped in arrowed gap between contacts.

Pressing on the large tab on the microswitch should make the moving contact snap from the upper to the lower position with little effort, thereby making the contact with the silver coloured terminal.

Close inspection showed that a bead of water had actually got onto the contacts of the microswitch. The photo is after cleaning up so doesn't show the water itself, but a couple of things could happen here: Depending on how impure the water is, it could short, making the switch look like it was closed all the time. The central locking would then not respond to the button being pressed as it relies on the change from open to closed to initiate the release. The other thing that could happen is that in cold temperatures the water could easily freeze holding the switch open. This would cause the lack of 'click' I mentioned earlier and it was near freezing at the time I was looking at this problem. You can also get corrosion of the contact surfaces resulting in a poor quality contact, and indeed the lower side of the moving contact was significantly blackened (not visible in the photograph).

Recess on body with hole, arrowed.
Hole in contact, arrowed

There is another interesting "feature" of the button assembly that is perhaps questionable and is illustrated in the photographs on the right. There's a recess on the outside, underneath where the tab that secures the connector would sit, with a small, square hole. this seemingly passes right through the plastic body and emerges at the round hole near the end of the silver coloured static contact. I can't really see a reason for this hole to go all the way through, not even as an aid to automated assembly; it may be it's intended to be a vent to allow any condensation to evaporate off, but it seems like it would just lead any water that finds itself in that recess directly onto the contact and cause exactly the kind of problems I found. If you're replacing this button, then you may want to consider sealing that recess. I didn't, because I'd already fitted the new button before I found that hole.

One other thing I discovered after reassembling the old button was that cleaned up, it now clicked normally, but I could see a tiny bead of moisture appearing on the rubberised outer surface each time I worked the button. Very close inspection showed a tiny (about 1mm) split in the rubber and there was some water trapped between the membrane and the plastic part of the push button which was being squeezed out with the pressure. However this would normally be downward facing and was so small that I don't imagine this was where water was getting in, but a larger tear might be a problem.

Refurbished, and with a spot of Loctite silicone on that tiny tear, the old switch is now serviceable, so I'll hang on to it as a spare, although I suspect the new switch will likely outlive the car!