Disco 3 (LR3) Dreaded Limp Mode - Suggestions?

Got a 2008 Discovery 3 - For context it really only does short journeys, towing my horse trailer etc. Was towing horse trailer (third day in a row) and noticed when towing to venue I had no side lights on trailer, but all other lights working. Got there and noticed there was a snapped wire in the plug - obviously the side lights.
Drove home and car goes into limp mode, after displaying almost every warning light you could think of. Restart the car and it clears, but happens again after about half a mile.
Since then, car does it alone without the trailer when going over 50mph.
Been into garage, had diagnostics and nothing really shows (no faults on dash at all). Only thing showing was about fuel pressure. Changed all filters and hasn’t fixed the issue. Garage think potentially looking at the fuel pump or injectors next.
I’m not totally convinced as it doesn’t really do it on harsh acceleration.
Could the faulty trailer plug have been the cause, or is that just a huge co-incidence?!? It’s such a minefield to work out what’s wrong and where to start (that’s Land Rovers I know!).

Its not the tow electrics, its the fact that you were towing, increasing the engine load.
Change the fuel filter, if not done so already, next step would be to check the in-tank pump pre-filter, I've seen a few of these blocked with 'tank bloom' and other cr@p from cheap diesel. It also doesn't help if you're in the habit of running it with little fuel in the tank as this allows condensation to form.
If the pump in the tank is noisy, you may well find cleaning the pre-filter is too late and a replacement pump module is required.

Does changing the electrical load on the on the car make any difference. For example, if you put the heavy current load windscreen and rear demisters (not the fan, the wire in glass demister) on? Take note of the weather too and if the level of humidity makes a difference. I ask all this because it sounds to me like a bad earth connection somewhere. The -ve black wire battery-to-chassis would be a suspect. The additional load of the trailer might have just pushed the poor connection over the edge.

Electrical load / poor ground connection issues would cause more DTC's than (I'm guessing at) P2290 for low primary fuel pressure. If it were caused by low voltage / high ground connection resistance, I'd expect plenty of 'U' codes and 'B' codes to confirm it.

Rickspark said:

Does changing the electrical load on the on the car make any difference. For example, if you put the heavy current load windscreen and rear demisters (not the fan, the wire in glass demister) on? Take note of the weather too and if the level of humidity makes a difference. I ask all this because it sounds to me like a bad earth connection somewhere. The -ve black wire battery-to-chassis would be a suspect. The additional load of the trailer might have just pushed the poor connection over the edge.

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Thanks! When it happened the first time, I think I had aircon on and the radio. As soon as it happened i turned everthing additional off, and it did happen again.
Getting it back as they can’t change the fuel pump for a couple of weeks anyway, so will do some more testing of our own.

Graculus said:

Its not the tow electrics, its the fact that you were towing, increasing the engine load.
Change the fuel filter, if not done so already, next step would be to check the in-tank pump pre-filter, I've seen a few of these blocked with 'tank bloom' and other cr@p from cheap diesel. It also doesn't help if you're in the habit of running it with little fuel in the tank as this allows condensation to form.
If the pump in the tank is noisy, you may well find cleaning the pre-filter is too late and a replacement pump module is required.

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The filters made no difference so they’re now looking at the pump. They can’t fit it in for a couple of weeks, so will bring it home then return it. Fingers crossed that’s the issue!

Before removing anything, it's best to check the low pressure side at the test point / bleed valve adjacent to the oil filter housing.
If the LP side is OK under load (i.e. a test drive, with an analogue gauge attached) and the in-tank pump isn't noisy, then the next stage would be to check & monitor the HP fuel pressure, the described symptoms indicate a LP fuelling issue but sometimes this can be due to the HP pump 'passing' too much fuel back to tank and failing to achieve the required pressure at the rail.
It would be helpful to know any engine DTC's set.