Mike,
It appears to be a known fault, but there appears not to be a known fix – as usual. In short it is nothing to worry about. If you were in ECO mode, take it out and it probably wont happen again.
This is an interesting read, taken from a VW forum.
Incident 1: 27 July 2019 about 10:40.
I was on my return journey home from picking up my brand new GOLF 5 DOOR HATCH 1.5 TSI EVO 130PS DSG.
About 20 mins after leaving dealer (up-hill and down dale), there was drizzle, it was getting overcast and muggy. The wipers were on intermittent, the blower on low and the lights on Auto. It was not dark enough for the headlights to be on. There were about 30 miles on the clock.
I was climbing as gentle hill and went over the top and started to descend. I took my foot off the accelerator
The car started to COAST and the rev counter went to zero. My previous car was a Skoda 1.4tsi with DSG and in that car, it would coast but the rev counter would go to 600rpm. Being new to the Golf, I can only assume the engine actually stopped!
After about 200yds, the drizzle became heavy, the hill went into a dark wood. The headlights came on
The wipers went to fast wipe and because the screen misted over, I may have moved the control to Demist. The blower went to Full and the aircon may have come on. The screen cleared very quickly.
Hey Presto: A message “Error: vehicle power system. Please visit workshop” came up”.
I stopped as soon as I could and turned off the ignition. The message disappeared when engine turned off.
We called the Dealer and he said “It’s a new car and probably a glitch”.
Incident 2: 31 Jul 2019 about 19:25 with 115 miles on the clock.
After driving 4 miles from start, it got very dark under trees so I switched the headlights on (3 o’clock position of switch). I was coasting down a slight hill, the car went to COAST. The rev counter went to zero, the engine stopped. After about 100yds, the message “Error: vehicle power system. Please visit workshop” appeared. I stopped but left the engine running so that I could photograph the display. It was the same as one posted earlier.The message disappeared when the engine was turned off.
Visit to the Dealer 1 Aug 2019.
I gave the service manager a copy of what I had written and also some prints from screen shot of the Trocforum topic that showed it was happening to others. It was like going to the Doctors with a Google diagnosis of a health problem! After about 30 mins, he came back and was complimentary that I had taken the time to describe the fault. The technicians had been able to recover the fault log for both times and it correlated with my description. It was under “ECO COAST MODE” errors. I appeared that in the Eco Coast mode, things were not happening as expected hence the fault. He didn’t have a fix so submitted a “Technical assistance Query” back to VW.
My theory
I was an engineer before retiring and tried to understand the sequence of events. My theory is as follows (using Incident 1):
Going up the hill the accelerator position was a “Demand” sent to the EMU (Engine Management Unit). The EMU then set the Fuel input to the injectors for tractive power for gradient along with the DSG gear needed. It also noted the Electrical Power needed for wipers (intermittent), the blower (low) but none needed for the headlight (on Auto). Power was not needed for the Aircon.
Then, going over the top, with my foot off the accelerator, the need for tractive power went to zero. The electrical need was still low so could come from the battery. When this all went into the EMU, the EMU decided that all the parameters for “ECO COAST” were satisfied. The DSG disengaged both clutches and the fuel was cut to zero, The Engine stopped.
After about 200yds, there was a massive and quick change to electrical power demand: Wipers full and On, Headlights on, Blower on Full. The battery would have to supply ALL this power. (Also, if the aircon came on then it needed the engine to be rotating).
The EMU was hit with all these demands at once. It needed to start the engine with even more drain on the battery. The system probably didn’t react and produce enough power quickly enough and flagged up a “vehicle power system” error.
Conclusion:
With all the “ECO” demands placed on car makers and VW in particular, the Engine Mapping Software could be “too keen” to go into ECO mode. The solution could be to “slug” the fault indicator so it accepted the temporary overload condition until (say) a few seconds to allow the power generation system to be up to full. A simple timer could be set so that if the if temporary overload condition lasted (say) more 5 seconds, then, and only then, would it raise the flag “vehicle power system” error.