200 TDI lacking low rev power

ok, thanks for the advice, is it worth mentioning that i potentiallt thraped the bolocks out of the engine about an hour previous to the power loss problem and the red warning light started flashing on the dash, this light has never flashed before and not flashed since, just seems coinsidental that ive done somethin to it maybee

forgot to mention that at the time of potential overheat, i was doing about 50 mph in third gear with half a ton of sand on the back end. maybey this didnt help

Which red warning light flashed??