Thread: Error codes U0001, U0103 & U0101 - Dealer says trailer power drain

Hi all,
I am looking for a sanity check from the forum brain's trust on some errors thrown by my car, and the diagnosis by the dealer. Sorry if it is long, but I want to recite anything which might be relevant.

Background
Our car is a 2016 D4 Landmark, bought new in June. It is unmodified, other than a Redarc tow controller that was fitted by the local LR dealer.
We tow a 25ish year old car trailer with old-school bulb type lights, with 2 x blinkers, 6 x tail/running lights, and 2 x stop lights. It is connected to the LR black socket via a 7-pin flat-to-round adaptor. All up, trailer with race car on-board weights less than 1800 kgs.

Incident
About a week ago, my dearest and I headed down to Phillip Island for a race meet, towing our race car on a trailer. This was the first long trip for our new D4, and it had approx 2,000 kms on the odo at the start of the trip. The week before it had towed the same trailer to Eastern Creek and back, about 30 mins each way, with no issues.

We left Hills District of Sydney on Thu morning, and after about an hour on the M2/M7/Hume we stopped at a rest stop just south of Camden for a leg stretch and for SWMBO to get a coffee :-) When I restarted the car after about 15 mins, there was one or more alarm noises, and the message centre displayed a number of warnings, Gearbox error, Park brake auto-release not available, etc

We called LR Assist, and they sent an NRMA serviceman. After clearing the faults, he disconnected the trailer plug, and was able to induce the same errors again on restarting the car.

With his agreement, I captured a photo of the errors reported on his console. They were:
U0001 Bus for high-speed CAN communication
U0103 Gear shift module - Last communication
U0101 TCM Lost communication

He diagnosed it as a "computer glitch", with no apparent mechanical fault, and after again clearing the errors, he advised us to proceed with care.

The same fault occurred another 4 times over the following days of the trip, always on restarting the car after a stop while towing. Fortunately another member of our team, traveling in convoy with us, had an OBDII tool which allowed us to verify that it was a repeat of the same errors, before clearing those errors. In 2 cases, there appeared to have been even more errors reported on the message centre (eg ABS failure), and the car would not allow shifting from park and dropped to access height.
In all cases, after clearing the errors, we were able to proceed, with no apparent ongoing issue.

Diagnosis
I left the car with the local LR dealer today, and showed them the screen-shot from the NRMA serviceman's console.
They have rung and reported that their technicians say this error is caused by the trailer overloaded the electrical connection from the car.

Does this sound realistic given the trailer has only lights, and that the error did occur once with the trailer electrically disconnected?

Thanks for reading :-)

Just a quick report with some further information.

First a recap. Previously we had a bunch of faults thrown almost immediately after starting the car. This occurred at least 5 times on a trip from Sydney to Phillip Island and return, always on starting the car after a rest stop. We were towing our car trailer.

Last weekend we took the race car to Goulburn, with the same towing setup as previously. When taking a rest stop I would disconnect the trailer electrics from the car, and then leave it disconnected when restarting the car. Once the car was started and had been running for about 30 seconds, I would reconnect the trailer.

Out of 4 instances, there was no problem in 3 cases, but in one case the same errors were thrown immediately on reconnecting the trailer.

This suggests to me that the cause does lie with the trailer, as I can't it would be a voltage problem in that scenario, nor would a gear knob fault be the likely cause.

I plan to closely inspect all the wiring on the trailer, and probably replace some of the 8 tail/running lights with LEDs equivalents. I may also try disconnecting each of the powered light circuits in the trailer plug, leaving earth connected, then reconnecting them individually while trying to disconnect/reconnect the trailer to the car, and so narrow down which circuit is the likely culprit.

Any thoughts please?