Stuck throttle means replacing whole engine wiring loom

After breaking down twice in 10 days the C3 V3 1,2 was diagnosed as having a voltage deficiency from the "fuse box" (not actual a fuse box) in the engine bay. But this latest C3 version has that unit INTEGRATED (hard-wired) into the wiring, which means it ALL needs to be replaced - £1,600 repair and 2-3 weeks wait just to get the parts!!
Apparently the throttle bodies get dirty/carbon build up, which restricts movement of the flap, so it overloads the motor and then damages the supply distribution unit ("fuse box").
I would have thought it would be good practice to make these components separate and replaceable - like an old fashioned, actual, fuse for instance. So I am confused and disappointed.
The garage assures us that changing the wiring, including the failed distribution unit, WILL fix the problem.
Can anyone suggest any alternative diagnosis or similar problem? Even pointers towards an explanation, or options on whether this replacement WILL fix the issue?

First up I am not that familiar with the new crop of C3 and the pure tech engine and I am not a mechanic. However the engineering principals behind the construction wont really change. Having said that, if I am wrong I'm happy to stand corrected.

NRC3 wrote: ↑Thu May 19, 2022 7:00 am After breaking down twice in 10 days the C3 V3 1,2 was diagnosed as having a voltage deficiency from the "fuse box" (not actual a fuse box) in the engine bay. But this latest C3 version has that unit INTEGRATED (hard-wired) into the wiring, which means it ALL needs to be replaced - £1,600 repair and 2-3 weeks wait just to get the parts!!

The engine bay fuse box is named the BSM by the PSA group. The acronym is in French, so it's pointless for me to tell you what it stands for. I believe the BSM is not your problem. At least not according to the explanation given by the mechanic as you out lined below.
While I have no proof I find it hard to believe that the BSM is hard wired to anything. This would make assembly of the vehicle very difficult. The BSM is connected to lots of components in the front of the car not just the engine ECU (Electronic Control Unit), but I'm getting ahead of my self. I'll get to the engine ECU in a minute.

NRC3 wrote: ↑Thu May 19, 2022 7:00 am Apparently the throttle bodies get dirty/carbon build up, which restricts movement of the flap, so it overloads the motor and then damages the supply distribution unit ("fuse box").

Yes, while this sound plausible, the throttle body is not connected to the BSM!! The power and return (ground) for the throttle body come from the engine ECU. In fact all of the electrical connections to the throttle body servo go to the engine ECU. The engine ECU needs to monitor as many parameters of the throttle body as it can, including the current going to the throttle body servo motor. It is super important that the butterfly valve in the throttle body be under the control of the engine ECU or the engine could "run away"! If the current got too high, the engine ECU would stop powering the throttle body servo and do what ever it's programmed to do in that situation. So the explanation the mechanic gave you is not looking good so far.

NRC3 wrote: ↑Thu May 19, 2022 7:00 am I would have thought it would be good practice to make these components separate and replaceable - like an old fashioned, actual, fuse for instance. So I am confused and disappointed.

Car components are made with connectors in a modular fashion for many reasons. "Hard wiring" them would increase reliability but inhibit flexibility and therefor increase cost. I cant see it happening.

NRC3 wrote: ↑Thu May 19, 2022 7:00 am The garage assures us that changing the wiring, including the failed distribution unit, WILL fix the problem.
Can anyone suggest any alternative diagnosis or similar problem? Even pointers towards an explanation, or options on whether this replacement WILL fix the issue?

Unfortunately there is very little to go on here to form a diagnosis. Unless you miss understood what the mechanic said, I am not sure you could trust their diagnosis.
Taken on face value I would assume the mechanic polled the car with a scan tool and got a DTC (Diagnostic trouble Code) relating to the throttle servo motor or the TPS (Throttle Position Sensor). The moment that the engine ECU detects that it's not in control of the throttle it will shut down the engine.
Now, this is all supposition as I have extrapolated my best guess from what you are saying.
Is it possible the mechanic was talking about the engine ECU?
Burnt out wiring to the throttle body? Doubtful. The engine ECU will be monitoring current consumption.
1600 pound is a lot of money and just to be sure, I might get a second opinion.

Thank you for this insight. I think you are right to suggest a second opinion, especially given the costs likely. Also, it's probably right that they are working from the scan results. I'll have another conversation with them. Thank you very much for taking time to give this comprehensive view of the thinking.

If you are going to talk to the mechanic, see if you can get the DTCs that they are basing their diagnosis on.
If he is saying "Aw yeah, these cars always suffer that" run, don't walk away!

WARNING DANGER: there is one thing that WILL destroy the PureTech engine. The timing belt in the engine is submerged in oil. The oil and the timing belt must be changed at regular intervals. No more than 100,000Km or 60,000 miles for the timing belt and I personally recommend 10,000 Km or 6,000 miles (every 6 months) for the oil. I can not over state the importance of good clean oil in a modern engine!! Sorry but I do not believe in "extended oil change intervals".
When replacing the timing belt you MUST get a new one from Citroen only!! As I mentioned, it is not a regular timing belt.

One way or the other, please get back to us and let us know what happens.

Again, thanks for the good info. On the basis of these exchanges I'm going to see the garage this week. On the good side, they SEEM pretty competent, so let's see. The car has only done 10,000mi as was driven little before we got it, but I will check on their thoughts about the timing belt. Also, the C3 service schedule is annual irrespective of mileage, which I think is bit over the top if it is only doing 2,000-3,000 miles a year - but may be due to modern engines having components that get 'dated' in that time. I'll let you know how it goes.

The big problems with the PureTech engines seem to have been solved after 2017. So if your car was build after 2017 you really only need to keep an eye on the age of the timing belt, which is the same for most other cars.
My calculations are based on an "average" distance travelled, as 12,000 miles or 20,000 Km per year. If you do more or less than that, then adjust accordingly. I used to subscribe to the fact that you could "over service" a car...but not these days!
The more modern the engine, the more delicate it becomes. To squeeze the most out of the engines, for less, puts a great strain on them. The engines can not be neglected or they will get destroyed!

Ok. So I think we’ve a few years before a new belt but I’m keen to hear the garage’s story about the wiring. Thanks.