"We decided that Oskarshamn nuclear power plant (OKG) should take nuclear reactor O2 offline immediately," the Radiation Safety Authority (Strålsäkerhetsmyndigheten) said in a statement.
The plant operator has to put one of its diesel generators through a 48-hour test run, the statement read. There was no immediate danger, it added.
"The power supply to the reactors is extremely important. This was one of the main problems at Fukushima," safety inspection chief Leif Karlsson told the TT new agency.
"If you don't have this system running, you cannot add water to the reactor."
The operator will not be allowed to switch on the reactor again before doing the test run and also see to that a second generator undergoes a maintenance check.
There are a total of three reactors at the site, which was Sweden's first nuclear power plant when it opened in 1972.
The reactors supply about 10 percent of Sweden's total annual electricity consumption, according to the operator's website.