Thanks again Mike. I'll try that, but it may take a couple of days before I can get around to it. I'll report back. Thanks again.

On 29/03/2025 at 14:20, MikeTheThinker said:

I've just had a look at the wiring diagrams from Mr Haynes fine manual (!) and note that all the courtesy lights are on the same power feed, so if the cabin interior lights are OK the problem is downstream.

Can you check the feed wire to one of the boot lights? It should be Red/Brown and with the interior lights on (to show power is getting there from the control relay) you should show 12v to earth. If that's OK, try earthing the earth side of the interior light. If it lights your problem is downstream wiring to the switch, the switch or the switch earth. If no light, the bulb or the light fitting are faulty.

I'll ferret a bit more later to see if there's anything helpful regarding the reversing sensors.

Hi Mike,

Just got around to this - sorry for the delay.

The boot light feed does indeed show 12V to earth when the cabin interior lights are on. And, lo and behold, the boot light does illuminate when the earth side is earthed. So I understand that the problem is downstream (i.e. down the earth side wiring). I am a bit suspicious of the wiring bundle in the compartment behind the panel on the nearside of the boot, because it has flooded in the past, and all the wiring bundles run along the floor of that compartment. There is nothing visibly wrong with any of them, but I don't really know how to test. And then they just disappear into the bodywork around the wheel arch, on their way to the front of the car.

Can you suggest how I might go forward from here?

Cheers,

scandalxk

Check the wiring at the ground stud behind the trim on the left side of the boot. It is a common problem when water has flooded the area (which you also reported). In fact it is usually due not to rainwater but caused by having a washer solution that is too weak during the winter. On cold nights the water in the washer hose behind the trim freezes and the ice pushes off the coupler in the pipe. Next time you use the rear washer, the wash water is pumped into the boot. This then corrodes the connections at the earth stud and gives the result you found.

Just clean up the connections with a fine emery cloth or similar and reconnect.

On 17/04/2025 at 09:54, pikpilot said:

Check the wiring at the ground stud behind the trim on the left side of the boot. It is a common problem when water has flooded the area (which you also reported). In fact it is usually due not to rainwater but caused by having a washer solution that is too weak during the winter. On cold nights the water in the washer hose behind the trim freezes and the ice pushes off the coupler in the pipe. Next time you use the rear washer, the wash water is pumped into the boot. This then corrodes the connections at the earth stud and gives the result you found.

Just clean up the connections with a fine emery cloth or similar and reconnect.

Thanks, I will do that, and report back. As usual, it may take a few days...

Regarding the flooding, exactly that happened to my previous Octavia: shortly after I bought it I noticed that the washer didn't work. Then I noticed that the boot smelled of washer fluid, and I found that the spare wheel was swimming in the stuff, overflowing from the compartment behind the panel - caused by the pipe popping off the coupler, as you described.

On my current Scout the coupler is much more substantial - a screw fit, not a push fit - and I am careful to keep the antifreeze strong. However, there are at least two other routes for water to get in. One is through the cabin pressure relief valve, which is a large rectangular rubber flapped valve fitted to the bodywork inside that panel: if it comes adrift, water can get in there. The other is if you leave the tailgate open in the rain, water can drip (in fact flow) off the top left corner of the tailgate, onto the strut, and into the bodywork through the strut fitting, then down into the said compartment.

And finally, there is a drain hole near the front of the compartment, which gets blocked with crud. So, overall, not very well designed.

23 hours ago, scandalxk said:

Thanks, I will do that, and report back. As usual, it may take a few days...

Regarding the flooding, exactly that happened to my previous Octavia: shortly after I bought it I noticed that the washer didn't work. Then I noticed that the boot smelled of washer fluid, and I found that the spare wheel was swimming in the stuff, overflowing from the compartment behind the panel - caused by the pipe popping off the coupler, as you described.

On my current Scout the coupler is much more substantial - a screw fit, not a push fit - and I am careful to keep the antifreeze strong. However, there are at least two other routes for water to get in. One is through the cabin pressure relief valve, which is a large rectangular rubber flapped valve fitted to the bodywork inside that panel: if it comes adrift, water can get in there. The other is if you leave the tailgate open in the rain, water can drip (in fact flow) off the top left corner of the tailgate, onto the strut, and into the bodywork through the strut fitting, then down into the said compartment.

And finally, there is a drain hole near the front of the compartment, which gets blocked with crud. So, overall, not very well designed.

So I checked, and I realise that I have checked that ground stud before. It is fine: I can make the boot courtesy lights come on by earthing them via the stud itself, or via the swages of the other (both brown) cables that are attached to it. It is not clear whether the courtesy lights are actually connected to that stud; the blue and white earth cable from the light unit joins a big bundle of cables containing other blue and white ones, and without a wiring diagram (or the knowledge to understand one!) I don't know where they go, or where they finally earth to ground.

This particular ground stud is well above the level of any possible flooding, mounted on the rear bodywork and not on the floor of the compartment. There is another stud, identical but unused, on the floor of the compartment: I guess that may be the one which was prone to corrosion due to flooding, and perhaps that is the reason it is no longer used...?

But anyway, the stud is fine so I need to look elsewhere downstream to find the problem.

Edited 19 April, 20251 yr by scandalxk

On 29/03/2025 at 14:20, MikeTheThinker said:

I've just had a look at the wiring diagrams ...

Mike, any further thoughts following the investigations described above?