The sport's dominant team said in a statement that a thorough investigation had been carried out at the factory last week with the energy recovery system stripped down.
"The issue is suspected to be associated with the insulation. The turbocharger will be replaced in addition to the oil pumps, after debris was found in the oil system," the statement said.
"With the repairs completed, this Power Unit will remain in the driver pool and travel to Sochi (for this weekend's Russian Grand Prix) as a spare."
Drivers are limited to five power units a season, with grid penalties incurred if they have to use more than their allocation.
Triple champion Hamilton started at the back of the grid in Shanghai on April 17 after experiencing a loss of power during the first phase of qualifying.
Mercedes then replaced the engine in his car for the race and he finished seventh.
The Briton is 36 points behind championship-leading team mate Nico Rosberg, winner of all three races so far this season, ahead of Sunday's fourth round in Russia. Hamilton has won the two previous Russian Grands Prix.
"There was plenty going through my head after China, as you'd expect. But, after all these years, experience has taught me to stay calm and keep pushing forwards when I get knocked back," Hamilton said in a team preview.